Recommended Priorities for NASA’s Gamma-Ray Astronomy Program i

TABLE OF CONTENTS

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

1. The Role of Gamma-Ray Astrophysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. Scientific Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1 The Origin of the Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Prompt Emission From Supernovae & Novae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

2.1.2 Galactic Nucleosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

2.1.3 Iinterstellar Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Nature of Black Holes & Neutron Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.1 Black Hole Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

2.2.2 Accreting Neutron Stars: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.3 White Dwarfs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2.4 Spin-Down Pulsars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.3 Extragalactic Astrophysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.3.1 Seyferts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3.2 Blazars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3.3 Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.3.4 The Diffuse Gamma-Ray Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.4 The Mystery of the Unidentified High-Energy Gamma-Ray Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.5 The Gamma-Ray Burst Enigma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.6 Solar Gamma Rays: Exceptional Photons From An Unexceptional Star . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3. Current Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.1 Compton Gamma Ray Observatory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.2 Integral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

3.3 Complementary X-Ray Missions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.4 Gamma-Ray Burst Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

3.5 HESSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.6 AGILE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.7 TeV Telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

3.8 Ultra Long Duration Balloon Program for High-Energy Astrophysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.9 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

4. Future Missions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.1 Missions in the 1998 OSS Strategic Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.1.1 Glast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.1.2 Constellation X . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.2 Medium Range New Missions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2.1 High-Resolution Spectroscopic Imager (HSI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2.2 Energetic X-ray Imaging Survey Telescope (EXIST) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.2.3 Advanced Compton Telescope (ACT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.3 Long Range New Missions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.3.1 Next Generation GRB (NGGRB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.3.2 Next Generation High-Energy Gamma Ray (NGHEG) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.4 Suborbital Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.5 Space Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

5. Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.1 Imaging Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.1.1 Multilayer Mirrors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.1.2 Coded Aperture Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.1.3 Compton Telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.1.4 Pair Production Telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.2 Detector Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.2.1 Strip and Pixel Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.2.1.1 CdZnTe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.2.1.2 Germanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.2.1.3 Silicon Strips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.2.1.4 Liquid Xenon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.2.2 VLSI/ASIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.3 Computational Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

6. Supporting Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6.1 Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6.2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6.3 Ground Based . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

7. Education and Public Outreach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

8. Recommended Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

8.1 1997 GRAPWG Missions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

8.2 Top Priority Science Topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

8.3 Top Priority Mission Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

8.4 Gamma-Ray Bursts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

8.5 Other Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
ii
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program iii

FOREWORD

This document provides a review of the

recent discussions of the Gamma-Ray

Astronomy Program Working Group

(GRAPWG). The GRAPWG is an ad-hoc committee

formed by NASA to provide recommendations

about the future of gamma-ray astrophysics. More

information on the GRAPWG can be found at

http://universe.gsfc.nasa.gov/grapwg.html.

This publication updates the original GRAPWG

report published in 1997. Based on that input and

GRAPWG Current Membership

others, NASA created a Strategic Plan in 1998.

In it were two missions of particular interest to

high-energy astrophysics and gamma-ray astronomy:

The Gamma Ray Large Area Space Telescope

(GLAST) and a Hard X-ray Telescope (HXT) on

Constellation-X. The charge for the current meetings of the GRAPWG was to look at gamma-ray

science opportunities beyond those two major

missions and an assumed gamma-ray burst

Explorer.

Elena Aprile (Columbia)

Jerry Fishman (NASA/MSFC)

Neil Gehrels (chair, NASA/GSFC)

Josh Grindlay (Harvard)

Fiona Harrison (Caltech)

Kevin Hurley (UC Berkeley)

Neil Johnson (NRL)

Steve Kahn (Columbia)

Vicky Kaspi (MIT)

Mark Leising (Clemson)

Rich Lingenfelter (UC San Diego)

Peter Michelson (Stanford)

Mike Pelling (UC San Diego)

Roger Romani (Stanford)

Jim Ryan (UNH)

Bonnard Teegarden (NASA/GSFC)

Dave Thompson (NASA/GSFC)

Trevor Weekes (Whipple/CfA)

Stan Woosley (UC Santa Cruz)

Daryl Macomb (secretary, NASA/GSFC)

Alan Bunner (ex-officio, NASA)

Paul Hertz (ex-officio, NASA)

Lou Kaluzienski (ex-officio, NASA)
iv
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program v

The time is ripe for new initiatives and missions in gamma-ray astronomy. The results

from the current missions, CGRO, RXTE and

BeppoSAX, have stimulated tremendous interest in

the field and have demonstrated the importance of

hard X-ray and gamma-ray observations to our

understanding of the universe. Looking ahead to

the next decade, further discoveries are expected

from these missions and from ESA’s INTEGRAL mission (launch in 2001). However, there are currently

no future approved missions beyond INTEGRAL to

advance the field. With this in mind the GRAPWG

made recommendations in 1997 (GRAPWG Report;

April 1997) for instruments to fly in the 2000 to

2010 time frame. We are delighted that some of

the recommended missions are now part of NASA’s

Office of Space Science strategic plan. With NASA

in the process of updating that plan, we have

reassessed and expanded on our recommendations

and present this update.

1997 GRAPWG MISSIONS

In the previous GRAPWG report, the top-priority

was given to the GLAST high energy gamma-ray

mission to follow on the discoveries of the EGRET

instrument on CGRO. Other high priority missions

were a focusing hard X-ray telescope and a nextgeneration nuclear line and MeV continuum mission. For Explorer-class missions the top scientific

opportunities were found to be for gamma-ray

burst observations and hard X-ray surveys. Some of

these missions have now been started by NASA:

GLAST is in the OSS strategic plan for new start in

2002; the Swift gamma-ray burst MIDEX has been

selected for Phase A study with final selection of

two missions to fly out of five studies to be made in

September 1999. Also, the OSS strategic plan

contains the top mission of the X-ray community,

Constellation X, which has a focusing hard X-ray

telescope (HXT) onboard that achieves some of the

objectives identified by the GRAPWG for a focusing

hard X-ray mission.

The GRAPWG finds that the scientific case for

GLAST, Constellation-X HXT and Swift has grown

since 1997. The GRAPWG continues to give its

STRONGEST ENDORSEMENT to these mission,

which are the backbone of NASA’s future program

in hard X-ray and gamma-ray astronomy.

1999 SCIENCE PRIORITIES

The GRAPWG identifies the following PRIORITIZED list to be the most compelling science topics

that future hard X-ray and gamma-ray missions

can address beyond those covered by GLAST,

Constellation-X HXT and Swift. These are areas in

which hard X-ray and gamma-ray astronomy offers

unique capabilities for advancing our understanding of the universe. Each science topic is followed

by a list of areas in which key contributions are

expected.

The HIGHEST PRIORITY science topic is:

1) NUCLEAR ASTROPHYSICS AND

SITES OF GAMMA RAY LINE EMISSION

Gamma-ray astronomy holds the promise of revolutionizing studies of nucleosynthesis in our galaxy

and beyond. Through the detection of nuclear lines,

sites of nucleosynthesis can be studied and elemental abundances can be measured. In addition, the

configuration and dynamics of the emitting gas can

be determined. Topics for future missions include:

EXECUTIVE SUMMARY
vi

• Abundance yields of explosive nucleosynthesis

• Mass cut between SN ejecta and core

• Supernova and nova explosion physics and

dynamics

• Sites of nucleosynthesis in the Galaxy and universe

• Cosmic nucleosynthesis rate from redshifted

SN Ia lines

• Supernova rate in the Galaxy

• Better understanding of SN Ia cosmological

distance scale calibration

• Cosmic ray interactions with interstellar gas

• Positron diagnostics of compact objects

Other high priority topics are:

2) GAMMA RAY BURSTS

Appropriate to their nature, gamma-ray burst

studies continue to change quickly and dramatically. The increasing number of counterparts at

lower energies when coupled with the impressive

BATSE database are leading to a new era in GRB

studies. Aside from the intrinsic astrophysics of

GRB’s, bursts will become an important probe of

the early universe. Topics for future missions

include:

• Links to star formation

• Evolution and populations of massive stars

• Possible sites of black hole formation

• New GRB populations and mechanisms

• Probes of dusty matter in distant galaxies

• Probes of the intergalactic medium out to high

redshift

3) HARD X-RAY EMISSION FROM ACCRETING BLACK

HOLES AND NEUTRON STARS

Hard X-ray and gamma-ray studies of accreting

sources are becoming increasingly critical for full

understanding of these objects. Detections of

galactic and extragalactic black hole systems at

high energies provide a laboratory for studying

black holes across a wide range of masses. Topics

for future missions include:

• First population study of absorbed Seyfert 2’s

• Constraints on blazar spectra and diffuse IR background

• Non-thermal components in galactic transients

• Jets associated with galactic BH’s and AGN

• Black hole parameters (spin, mass)

• Accretion physics

4) MEDIUM ENERGY (500 KEV–30 MEV) EMISSIONS

Distinct from nuclear lines, the continuum emission in the medium energy range has been shown

to be important for understanding nonthermal

emission from objects such as pulsars and AGN and

sites of cosmic ray interaction with gas. This relatively unexplored band ties together studies at MeV

and GeV energies. Topics for future missions

include:

• Search for MeV blazars and spectral studies to

understand emission

• Pulsar physics through broad-band spectral

studies

• Components of diffuse galactic emission

• Extragalactic diffuse emission in poorly measured

MeV band

• Nonthermal components from accretion-driven

sources

• Cosmic ray interactions with the ISM

1999 MISSION RECOMMENDATIONS

Figure 1 shows the mission roadmap that the

GRAPWG has developed for hard X-ray and

gamma-ray astronomy. In addition to the three

missions mentioned above, the GRAPWG finds

three near-term missions and two long-term concepts to be the most exciting for addressing our

top-priority science topics. These are as follows.

ADVANCED COMPTON TELESCOPE (ACT)

The HIGHEST PRIORITY major mission recommended by the GRAPWG is ACT, a high-technology

MeV line and continuum Compton telescope mission operating in the 500 keV to 30 MeV range.

With a factor of 30 improvement in sensitivity compared to CGRO and INTEGRAL, it promises detailed

studies of sites of nucleosynthesis in the universe

and a deep survey of continuum sources. The

optimum configuration of large imaging detector

arrays based on either semiconductor or high density rare gases is being studied to enable a mission in

this challenging energy band. The mission addresses

science areas (1), (2) and (4) in the above list.
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program vii

Two other high-priority missions are of particular

interest in the coming decade for accomplishing

our science goals.

HIGH-RESOLUTION SPECTROSCOPIC IMAGER (HSI)

A high priority intermediate or enhanced MIDEX

class mission recommended by the GRAPWG is HSI,

a focusing optics telescope operating in the 10 to

170 keV range. With a factor of 100 improvement

in sensitivity compared to RXTE, this mission will

answer key questions on the nature of accretion

onto neutron stars and black holes and will allow

detailed studies of sites of nucleosynthesis in the

universe. New multilayer mirror technology will

enable the upper energy bound of the mirrors to

be as high as 200 keV. The mission addresses science areas (1) and (3).

ENERGETIC X-RAY IMAGING SURVEY TELESCOPE (EXIST)

A high priority intermediate or enhanced MIDEX

class mission recommended by the GRAPWG is

EXIST. Factor 100–1000 improvement in sensitivity

compared to the only previous all-sky hard X-ray

survey (HEAO-1) will allow discovery of the predicted, but so-far unobserved, class of absorbed

Seyfert 2’s that are thought to make up at least half

of the total inventory of AGN’s. A large area array

of new-technology solid state detectors, used in

conjunction with a wide field-of-view coded aperture, will cover the 5–600 keV region and address

science areas (2) and (3) as well as significant portions of (1). The International Space Station is a

possible platform for such an instrument.

Complementing these missions will be projects

which will extend and improve upon those already

in the strategic plan.

NEXT GENERATION GAMMA-RAY BURST MISSION

(NGGRB)

The GRAPWG believes that gamma-ray bursts

will continue to be one of the most important and

fascinating areas of astronomical research for tens

of years to come. A mission will be needed in the

post HETE-II and Swift era to further this field.

Emphasis in that time frame may involve observations of nonelectromagnetic radiation such as

gravitational waves and neutrinos and will certainly

involve multiwavelength electromagnetic instrumentation. To correlate these data with known

properties of bursts and to monitor the sky for

infrequent special events, it will be essential to have

a continuous gamma-ray burst monitor in space.

The GRAPWG recommends that such a mission,

NGGRB, be identified in NASA’s program.

NEXT GENERATION HIGH-ENERGY GAMMA-RAY MISSION

(NGHEG)

The discoveries of GLAST will produce strong

interest in the astronomical community in high

energy gamma-ray phenomena and will undoubtedly raise new fundamental questions. The band

width of the high energy range is huge, from 30

MeV to 300 GeV, and overlaps with the growing

number of very high energy (TeV and PeV) groundbased observatories. The GRAPWG recommends

that a mission called NGHEG be identified in

NASA’s program to follow on GLAST.

GAMMA-RAY BURSTS

The GRAPWG is particularly intrigued by the

gamma-ray burst problem and the promise that

bursts offer for fundamental studies in astrophysics.

There are multifaceted implications that bursts have

on many future missions. Below are listed some

topics and recommendations on gamma-ray burst

astronomy. The GRAPWG recommends that:

• HETE-II should be flown on schedule.

• The Swift GRB MIDEX mission, now in Phase A,

should have high priority for flight.

• Support should continue for the Interplanetary

Network as an effective means for deriving

arcminute GRB locations.

• Support should continue for BATSE and the

Gamma-ray burst Coordinate Network (GCN).

• The GRB monitor currently planned for GLAST

will greatly enhance its GRB capabilities.

• Synergism between space-borne GeV GRB observations and ground-base TeV observations should

be recognized and exploited.

• A global network of small, dedicated GRB robotic

telescopes should be developed.

• It is highly desirable to establish a network of

coordinated 1–3m telescopes to monitor lightcurves and (for the brightest events) spectra

over the first few days.
viii

• Time on major ground- and space-based observatories should continue to be provided for GRB

follow-up observations.

OTHER RECOMMENDATIONS

Gamma-ray astrophysics is a broad enterprise

covering many efforts. The GRAPWG recommends

that the following items receive special

consideration:

• Technology Development: Many exciting new

technologies are arising in gamma-ray astronomy,

including multilayer mirrors, Laue lenses (Bragg

concentrators), complex coded masks, solid-state

pixel and strip detectors, rare gas and liquid

detectors, and VLSI electronics. These form the

backbone and future of our field. The GRAPWG

strongly recommends a vigorous program of

technology development for hard X-ray and

gamma-ray astronomy.

• TeV Astronomy: Aside from their independent

successes, ground-based observatories will provide an important complement to future high

energy gamma-ray missions such as GLAST. The

GRAPWG endorses the continued development of

TeV telescopes with low energy thresholds.

• Balloon Program: The ultra-long duration balloon

(ULDB) program offers great potential for both

instrument development and significant science

in gamma-ray astronomy. The GRAPWG recommends strong NASA support for LDB’s and ULDB’s.

• International Space Station: The GRAPWG

views the ISS as an opportunity for hard X-ray

and gamma-ray research. It is particularly well

suited for wide-field instruments and long-term

monitors.

• Optical Telescope Support: Many areas of

gamma-ray astronomy research, particularly

gamma-ray bursts and AGN studies, benefit from

a multiwavelength approach. In particular, optical

telescopes can provide important monitoring

capabilities which are difficult to achieve at other

wavelengths. The development of a network of

optical telescopes capable of near-continuous

observation of gamma-ray transients is supported

by the GRAPWG.

• Data Analysis and Theory: Making the most of

the rich databases expected from future missions

is an important concern of the GRAPWG.

Adequate support for data analysis and theory is

a cost effective way of maximizing the return

from current and future experiments. 
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program ix

High

Energy

Gamma

Rays

Nuclear

Astrophysics

and

MeV

Gamma

Rays

Hard

X-rays

Gamma Ray Astronomy Roadmap

TRL 5 TRL 7

TRL 1 TRL 3 TRL 5 TRL 7

TRL 3 TRL 5 TRL 7

TRL 5 TRL 7

TRL 3 TRL 5 TRL 7

'98 '00 '02 '04 '06

'Year

'08 '10 '12 '14

GLAST

TRL 7 AGILE - Italy

INTEGRAL - ESA

ACT

TRL 3 TRL 5 TRL 7 HSI

EXIST

N.G.H.E.G.

Con - X HXT

Gamma

Ray

Transients

TRL 5 TRL 7

Beppo SAX

HETE - 2

N.G. GRB

Swift

RXTE/HEXTE
x
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 1

CHAPTER

1

The Role of Gamma-Ray Astrophysics

The place of gamma-ray measurements in

astrophysics underwent a fundamental

change in the Compton Gamma-Ray

Observatory (CGRO) era. In the field’s infancy,

attention focused on the penetrating power of

cosmic gamma rays and their production in

familiar processes, especially radionuclide decay.

Experimental techniques in the PI-class missions of

the 70’s and 80’s drew heavily from high-energy

physics programs of the previous decades. A principal goal was a “discovery” level opening of the

MeV-GeV electromagnetic spectrum, adopting

established high-energy techniques. Missions

focused on identifying the source of the diffuse

background emission, separating point sources and

localizing transients. These missions led to a census

of astrophysical sites where nonthermal gamma-ray

processes occur. CGRO and its predecessors have

been very successful in providing this overview of the

high-energy sky. Many of

the anticipated high-energy

processes have been confirmed and gamma-ray

emission has proved a

robust signature of the most

violently active sources in

the universe. Moreover, the

CGRO era has moved

gamma-ray astronomy to a

central role in mainstream

astrophysics. Several of the

most puzzling phenomena

of modern astrophysics figure prominently in the

gamma-ray band. The

strong guest investigator

program of CGRO and the wide participation in

follow-on programs at other wavelengths underlines

the impact of gamma-ray studies on a range of

astrophysical problems. We can best illustrate this

impact by summarizing a few key problems brought

to light by CGRO observations and a few new

scientific directions inspired by such results. More

complete descriptions can be found in chapter 2.

GAMMA-RAY BURSTS

Before 1991, descriptions of the gamma-ray

burst problem acknowledged the wide range of

feasible models, but focused with increasing confidence on models producing bursts from a nearby

population of galactic neutron stars. In a dramatic

development, the presentation of the initial BATSE

spatial and flux distributions at the 1st Compton

Symposium abruptly overturned established thinkFigure 1.1. This BeppoSax observation of GRB 970228 detected the first ever fading X-ray source in a

GRB error box.

BeppoSAX observation of GRB970228 field

SAX MECS 1997 Feb 28 Exposure: 14334 s

5h02m368 5h02m099 5h01m428 5h01m158

BeppoSAX observation of GRB970228 field

SAX MECS 1997 Mar 3 Exposure: 16272 s

5h02m368 5h02m098 5h01m428 5h01m159

250

250 300 350 400 200

+12 ̊00'00"

+11 ̊54'00"

+11 ̊48'00"

+11 ̊42'00"

+11 ̊36'00"

+12 ̊00'00"

+11 ̊54'00"

+11 ̊48'00"

+11 ̊42'00"

+11 ̊36'00"

X Pixels X Pixels

Y PixelsY Pixels

0 4 9 29 73 1 5 9 10 10

250 300 350

300 350 400

200 250 300 350
2

ing and gave strong support to the idea that bursts

represent titanic energy releases at cosmological

distances. This paradigm was confirmed with the

detection of X-ray, optical and radio counterparts

in 1997/1998 and the measurement of several

cosmological redshifts (see Table 2.1). One should

remember, though, that these results only intensify

the burst puzzle from a physicist’s perspective —

how is a supernova’s worth of energy released in a

highly relativistic form and how does this process

give rise to the great variety of gamma-ray burst

time structures and spectral shapes? As described in

the body of this report, progress on the problem

will require a significant improvement in burst sensitivity, particularly at high energies, and careful

coordination with other wavebands. Several missions in the post-GRO era present exciting opportunities for advancing our understanding of these

enigmatic events. 

Figure 1.2. Gamma-ray observations such as those shown here

for MRK 501 have become an integral part of multiwavelength

monitoring of AGN.

ACCRETING BLACK HOLES

Another important result of the CGRO mission

has been the observation of strong, non-thermal

emission associated with jet sources. A spectacular

example is the detection of nearly 100 “blazars” by

the EGRET experiment, often with rapid flux variations and hard photon spectra extending above a

GeV. Several of these AGNs have been detected by

ground-based telescopes with energies up to 5

TeV and doubling time variability as short as 15

minutes. These AGN are objects of intense study

at lower energies; in particular, many GeV blazars

exhibit “superluminal” motions at VLBI scales

underscoring the important connections of groundbased studies with gamma-ray observations.

Related discoveries have resulted from BATSE monitoring of X-ray outbursts along the galactic plane.

Several sources show hard spectral tails extending

to roughly an MeV. Some sources showing strong

outburst activity, such as GRS 1915+105 and GRO

J1655-40, were recently discovered to show evidence of relativistic jet outflows in the radio band.

These observations suggest that non-thermal emission, starting in the hard X-ray/soft gamma-ray and

extending to TeV energies, is closely connected

with the formation of relativistic jets. Study of the

high energy emissions, especially in correlation

with multiwavelength variability, promise to provide important new insights into the physics of the

‘central engines’, the accreting black holes believed

to be the core of AGN and these transient Galactic

X-ray sources. Hard X-ray observations have now

shown the excess of obscured AGN and may provide the most direct way to inventory the stellar

black hole content (in X-ray transients) in

the Galaxy.

UNIDENTIFIED SOURCES

When a new wavelength range is opened, it

is most exciting when a class of sources not

prominent at other energies dominates the sky.

At MeV-GeV energies, SAS-2 and COS-B showed

that there is a population of Galactic sources not

identified with previously known objects. CGRO,

with improved sensitivity, energy resolution and

background modeling has surveyed this population. Of the 169 objects detected, 79 are located

within 10 degrees of the galactic plane. Since

Geminga, the brightest of these sources, is now

identified as a radio-quiet pulsar, it seems likely that

many others will be spin-powered pulsars. This

provides an important new window on the neutron

star population in our galaxy. Equally exciting is the

possibility that other classes of gamma-ray stars will

be discovered; the detective work of identifying

the galactic plane population will be a theme in

follow-on work to CGRO.

10

–10

10

–11

10

–12

10

–13

10

–14

1091011101310151017101910211023102510271029

(Hz)

F(erg cm–2 s–1)

10

–9

0.1 e

Microwave

1 e

IR

1 keX-ray

1 Me-ray

1 Ge-ray

1 Te-ray

1995

1996

1997 April 9–15

1997 April 16

1997 April 9

Archival

1997 Average
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 3

NUCLEOSYNTHESIS

In the technically challenging nuclear line

regime, CGRO has uncovered only the tip of the

expected MeV line emission science at sensitivities

of a few times 10–5 photons cm–2 s–1, but already

some surprising results have appeared. For

example, distribution of radioactive 26A1 mapped

by COMPTEL (see Figure 2.2) argues for production

dominated by massive star death, although surprising excesses in the Vela region are unexplained.

Also, the strong COMPTEL detection of 44Ti from

Vela (Figure 2.1) implies substantial synthesis of

56Ni. This makes the low luminosity of this Type II

supernova event a mystery. While the INTEGRAL

will provide a significant improvement in sensitivity,

efforts to develop new technology in this area will

be needed for the field to reach its full potential.

NEW DIRECTIONS

We should also note some new research directions in gamma-ray astronomy spurred by CGRO

observations. The impact on such problems provides a good measure of the power of future missions. One new area is the association of nonthermal spectra with black hole accretion. In addition

to the GeV emission and rapid variability of the

blazars, the suprathermal hard X-ray/soft gamma

tails in some sources are important diagnostics for

disk accretion and processes

at the disk inner edge.

GRANAT, BATSE, and OSSE

measurements above 30 keV

show such spectral components in Seyfert AGNs.

Intriguingly, when similar

features appear in galactic

X-ray binaries, dynamical

studies have shown the

sources to be excellent candidates for black hole accretors. Coupled with this,

recent work on accretion

disk solutions (e.g., advection dominated disks) suggest that disk inner edge

conditions are crucial in producing the optically thin

regions that generate such

non-thermal spectra. Thus,

we can hypothesize that the

perfectly absorbing boundary of an accreting black

hole is central to the formation of an optically thin

electron population responsible for the hard X-ray

mission and possibly to the acceleration of relativistic jets. Thus the gamma-ray regime provides a

unique window on this problem. A second arena

where gamma rays draw our attention to the most

exotic sources lies in the mult-GeV emission of

gamma-ray bursts. CGRO sensitivity limited detection of this emission to a handful of the brightest

bursts, but it may be a common feature of the

burst process. Conditions required for the production of this multi-GeV flux, which in some cases

may dominate the total burst energy, are extreme.

We may learn more about burst physics from these

radiations than from the more chaotic low-energy

emission. This theme of the highest energy providing the sharpest diagnostics is echoed in the study

of spin-powered pulsars. While intensively studied

in the radio through X-ray bands for over 25 years,

the physics of the pulsar magnetosphere is still

poorly understood. CGRO has taught us that the

familiar radio pulsar emission is a small fraction of

the bolometric photon luminosity: the power spectrum of their photon emission peaks in the GeV

range for many young pulsars. A final example

illustrates the ability of new gamma-ray data to

address fundamental problems in mainstream

Figure 1.3. Sources from the 3rd EGRET catalog. One of the most obvious features continues to be

the large number (169) of unidentified sources.`
4

astrophysics. The detection of several nearby AGNs

by ground-based air Cerenkov telescopes with

~300 GeV thresholds shows that blazar emission

can extend well beyond the present sensitivity limit

of space missions. Since this gamma-ray flux is

attenuated by the ambient cosmic photon fields,

measurements of absorption of the GeV-TeV spectrum may be used to fix the soft IR-optical radiation field at high redshift. This new cosmological

tool helps to constrain galaxy formation and environments in the early universe. 

Figure 1.4. The number of known gamma-ray point sources

(detected at energies above 50 keV) during the 30-year history

of gamma-ray astronomy through early 1999.

While there are great opportunities for future

gamma-ray discoveries, the prospects in many

other areas of astrophysics are strong as well. It is

therefore important to highlight the unique potential of gamma-ray observations. Above all, gammaray astronomy zeros in on some of the most exotic,

violent, and fascinating sources: black holes, neutron stars, and explosive sources of nucleosynthesis

and particle acceleration. Also, the field is relatively

new. With CGRO, gamma-ray measurements

have just reached the sensitivity that attract wide

theoretical attention and correlative investigations.

The rapid development spurred by this synergy

should be encouraged over the next decades.

Finally, it is a field where new detector development and adoption of technologies used for related

terrestrial applications offer the opportunity for

dramatic gains. With directed resources for future

development and rapid promotion of new technologies to space payloads, certain areas of

gamma-ray astrophysics can expect great leaps in

sensitivity. These opportunities will be described in

the following sections. Thus, with novel technology

greatly advancing our observational capabilites,

gamma-ray astronomy will continue to provide

new physics insight by probing the most exotic

objects in the universe. The opportunities for this

field over the next decade and a half are

particularly exciting.

Gamma-Ray Source Totals

number of known sources 500

400

300

200

100

1970 1975 1980 1985 1990 1995 2000

0
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 5

CHAPTER

2

Scientific Objectives

The legacy of the past and current generation of gamma-ray missions is a diverse and

compelling picture of the high-energy universe. From Solar physics to the endpoints of stellar

evolution, from the diffuse glow of the Milky Way

to the extragalactic diffuse emission, gamma-ray

observations continue to make fundamental contributions. This chapter describes the status and direction

of gamma-ray research for all of these topics. 

2.1 THE ORIGIN OF THE ELEMENTS

2.1.1 PROMPT EMISSION FROM SUPERNOVAE & NOVAE

Because the sites of explosive nucleosynthesis — novae and supernovae — are optically thick to gamma rays, only the delayed

gamma-ray line emission from the decay of synthesized radionuclei can be observed. Furthermore,

this is possible only for sites that become at least

partially transparent on time scales comparable to

the radioactive decay mean lives. The most luminous lines from individual events are the 56Ni and

56Co lines of Type Ia supernovae. Such supernovae

are required to make ~0.6 Mo of 56Ni during their

explosion to provide both the energy to unbind

the white dwarf and to power the light curve. The

ejecta from these supernovae also have higher

velocities than Type II’s because the characteristic

1051 ergs of kinetic energy is distributed within an

object about 10 times less massive. The flux at

maximum in the prominent lines of 56Co in a typical Type Ia supernova is about 3x10–5 (10 Mpc/D)2

photons cm–2 s–1 occurring about 50 to 150 days

after the explosion. These lines are quite broad,

however, with typical velocities of about 5000

km/s. The full width is thus about 30 keV. An

enduring goal has been to measure Type Ia supernovae in the Virgo cluster at about 20 Mpc where

the event rate is high. To study these supernovae

and learn anything save the well-known fact that

they made some 56Ni, one needs broad line sensitivities no worse than a few times 10–6 photons

cm–2 s–1. Lacking adequate sensitivity to do this,

one must await the occasional nearby event. Ideally

one would like not only to see the lines, but to

resolve their velocity structure and get the velocity

distribution and mass of 56Ni made in the explosion. This constrains the explosion mechanism (detonation or deflagration) and provides information

on mixing of the inner and outer layers of the

supernova. Similar information can be obtained by

watching the time dependent transparency of the

event, but to do this one must begin to measure

the flux quite early and continue measuring it for a

long time. SN 1987A was typical of Type II supernovae, though about a factor of 10 brighter at

maximum in the gamma-ray lines of 56Co (because

it was a blue supergiant instead of a red one). The

peak flux at 55 kpc was about 10–3 photons cm–2

s–1. This means that observations of Type II supernovae will be restricted, for the next decade or so,

to improbable occurrences in the local group of

galaxies. Type Ib supernovae are also massive stars,

but lack hydrogen envelopes. They produce about

4 times less 56Ni than Type Ia, but expand almost

as rapidly. Thus their signal is intermediate between

Types II and Ia, about 5x10–6 (10 Mpc/D)2 photons

cm–2 s–1 and the lines are a little narrower. Thus a

mission having broad line sensitivity of a few times

10–6 photons cm–2 s–1 might detect an extragalactic

Type Ib supernova during a mission lifetime of
6

several years. Any supernova in our galaxy would

be very bright in the decay lines of 56Co and thus

CGRO provides a fail-safe against missing the next

galactic event.

Figure 2.1. COMPTEL detection of 1.157 MeV 44Ti line emission

from Vela.

The other isotopes detectable from individual

supernovae are 44Ti, 57Co, and 60Co. Models for

Type II supernovae predict a 44Ti mass from 0 to

2x10–4 Mo. Since this isotope comes from the deepest layers ejected in the supernova, its ejection is

sensitive to the uncertain explosion mechanism and

to the details of the fall back (e.g., whether the

supernova makes a black hole), which makes detection of this isotope very interesting. COMPTEL has

reported the detection of the 1.157 MeV line from

the 44Ti–44Sc decay from the Galactic supernova

remnants, Cas A and Vela (Figure 2.1). For Cas A,

using current estimates of the distance and a new

half-life determination for 44Ti, the implied yield is

about 2x10–4 Mo, consistent with models, but it

remains a mystery why Cas A was not a brighter

supernova given that 44Ti ejection implies 56Co ejection. Assuming a comparable 44Ti yield in other

Type II supernovae, the planned INTEGRAL mission

should discover several other young remnants in

our galaxy. However, it should be noted that 44Ti

decay also produces comparable fluxes in lines at

67.85 keV and 78.38 keV. It may be that hard X-ray

instruments can be built with greater sensitivity. SN

1987A is also expected to have made ~0.5x10–4 Mo

of 44Ti (highly uncertain) implying a flux for the

next few decades of about 2x10–6 cm–2 s–1. A deep

galactic plane survey for obscured SNR using the

hard x-ray (68, 78 keV) lines from 44Ti will provide

important new constraints on the SNR rate in the

Galaxy with its implications for both galactic

nucleosysthesis and the pulsar birthrate problem.

Gamma-ray lines of 57Co (T1/2 = 271.8 d) were

detected from SN 1987A by OSSE implying a ratio

56Fe/57Fe of about 1.5 times the solar value, an

interesting constraint on both the star’s evolution

(i.e., the neutron excess in the silicon shell) and

galactic chemical evolution. However, the signal of

this isotope and 60Co (T1/2 = 5.27 y) are such that

they are only likely to be detected from fortuitous

supernovae in the local group. The most prominent

radioactivity expected to produce gamma-ray lines

from classical novae is 22Na. The synthesis of this

species is highly uncertain and is sensitive to the

nature of convection during the explosion and

whether the nova event occurs on a carbon-oxygen

white dwarf or a neon-oxygen white dwarf (the

signal is much stronger from the latter).

2.1.2 GALACTIC NUCLEOSYNTHESIS

Among the several windows we have on star

formation, evolution, and death, and the associated

generation of the elements, gamma-ray spectroscopy provides unique information. Nature

provides us with radioactive tracers of current

nucleosynthesis across the entire Galaxy. This gives

insight into the global star formation rate and the

mass spectrum of stars, especially at the massive

end. As we can see the production of specific isotopes, we are in effect looking right into the zones

of stars where they are produced.

The prototype isotope for these studies is 26Al

(mean lifetime 1.1 My.) We see a million-year snapshot of core-collapse supernovae (and their massive

star progenitors; how much each produces will be

clear only with better observations) across the

Milky Way. It traces the thin disk with apparent

enhancements in nearby regions of increased activity, possibly the Galaxy’s spiral arms. A handful of

individual objects are close enough that we can

already begin to assess their nucleosynthesis yields

directly. The Large Magellanic Cloud might be

detectable to a mission having narrow line sensitivity better than 10–6 cm–2 s–1 in a field of view

PARM NUMBER 0.500000- 1.50000

14

12

10

Galactic Latitude

Galactic Longitude

LOG LIKELIHOOD RATION + PARMS

3

6

4

2

0

–2

–4

–6

–8

–10

–12

–14

380

0 5 10 15 20 25 30 35

378 376 374 372 370 368 366 364 362 360 253 256 254 252 250
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 7

that encompassed the LMC. At 10–7 cm–2 s–1 one

might even begin to see 60Fe in the LMC and 26Al

in the SMC. 

From some of the same regions where 26Al is

produced, and probably different sites as well, we

also expect observable quantities of 60Fe (mean lifetime 2.2 My). This will not only clarify core-collapse

supernova nucleosynthesis but also possibly teach

us how the nuclear flame begins in thermonuclear

explosions of white dwarfs.

Another bright line, at 511 keV from electronpositron annihilation, also appears to trace disk

nucleosynthesis of at least a few isotopes, including

26Al, 44Ti, and 56Ni. It is additionally very bright

from the Galaxy’s central region, roughly tracing

the nuclear bulge. How much of this is from radioactive decay or compact objects such as pulsars

and black hole binaries, especially those with jets,

will be determined only with more and better data.

The potential of these types of observations

remains unrealized mainly because of the limited

sensitivity and spatial resolution of current instruments. With much improved sensitivity and good

spatial resolution, some of these “diffuse” emissions

would resolve into individual massive stars, supernova remnants, star clusters, and associations. We

would gain unprecedented understanding of each,

seeing perhaps the variation of nucleosynthesis

yields, explosion dynamics, and the efficiency of

massive star formation, as well as their mean

values. We could hope to see relic emission of past

starburst episodes at the Galactic center. The detection of point sources of positron annihilation radiation would clue us in to the contributions of those

sources to the diffuse emission as well as to the

nature of their central engines.

Some of these objectives will be addressed by

the forthcoming INTEGRAL spectrometer SPI, but

not most. Its coded-mask technique is optimized

for point sources, so its sensitivity to extended

sources degrades rapidly with source extent. It will

study well the brightest concentrations of 26Al emission and probe the physics

of the central Galaxy’s

positron annihilation medium with its good energy

resolution, but it will be

hard pressed to detect

60Fe unless its distribution

has sharp peaks. 

2.1.3 IINTERSTELLAR

PROCESSES

The detection of nuclear

de-excitation lines, such as

those at 4.44 and 6.13 MeV

from excited states in 12C

and 16O, produced by accelerated particle interactions

with interstellar gas and

dust can provide the first

direct measure of the intensity of low energy cosmic

rays in the Galaxy. Such particles with energies < 100 Mev/nucleon could make

a significant contribution to the galactic nucleosynthesis of the light elements, Li, Be and B, but these

low energy particles can not be detected in the

inner solar system because they are excluded by

the solar wind. Thus a search for nuclear de-excitation lines both from the diffuse interstellar gas and

from star formation regions around giant molecular

clouds can provide unique information on the

intensity, spectrum and spatial distribution of

cosmic rays at low energy, and offer clues to their

origin and acceleration. The reported discovery by

Figure 2.2. The intensity of 26AI emission as observed with COMPTEL shows interesting structure

along the galactic plane.

CGRO / COMPTEL 1.8 MeV, 5 Years Observing Time

0.00 0.16 0.33 0.49 0.65 0.82 0.98 1.14 1,31 1.47 1.63 1.80 1.96 2.12 2.29 2.45 2.61

Uwe Oberlach et al.

May 22, 1997

Intensity [ph cm2 s1 sr1] 108
8

COMPTEL of such gamma-ray emission lines from

the Orion giant molecular cloud complex has

turned out to be only an artifact of the data analysis. However, the announcement reawakened

awareness of the importance of such measurements

and demonstrated the need for new observations

with much more sensitive detectors than those

planned for the near future.

The galactic diffuse gamma-ray continuum

emission is the dominant feature of the highenergy (> 10 MeV) gamma-ray sky. This diffuse

emission is produced primarily by cosmic-ray electron and proton interactions with the matter (via

Bremsstrahlung and nucleon-nucleon interactions)

and photons (via inverse Compton interactions) in

the interstellar medium. A high-energy gamma-ray

telescope with better angular resolution will permit

more detailed searches for cosmic-ray gradients

including variations in the electron to proton ratio,

cosmic ray contrast between the galactic arm/interarm regions, and evidence for regions in which the

cosmic-ray spectrum differs from the local observed

spectrum. Increased sensitivity coupled with

improved angular resolution will also allow the flux

from fainter gamma-ray point sources to be more

accurately separated from the galactic plane diffuse

emission. The gamma-ray emission from molecular

clouds arises from the same cosmic-ray interactions

with matter which produce the general galactic

diffuse emission. Molecular clouds provide a means

to study these processes and the galactic cosmic

rays in localized regions of the galaxy.

Nucleosynthesis Objectives:

• Measure gamma-ray line emission from nearby

extra-galactic supernovae.

• Search for nuclear de-excitation lines from both

the diffuse interstellar medium and from star formation regions in the galaxy to determine the

intensity, extent and origin of low energy cosmic

rays and their role in light element production.

Nucleosynthesis Requirements:

• Flux sensitivity at least a factor of 10 better than

INTEGRAL with comparable energy resolution

and angular resolution of less than 1 degree.

2.2 NATURE OF BLACK HOLES &

NEUTRON STARS

2.2.1 BLACK HOLE SYSTEMS

Less than a decade ago, the only black holes

suspected were in massive binaries such

as Cyg X–1. The situation has changed

dramatically with the discovery of highly-transient

compact binary systems with a low-mass stellar

companion and a high-mass compact primary

(almost certainly a black hole based on the dynamical mass). The estimated total number of these

systems in the galaxy may be hundreds or more,

and thus they could be the dominant class of X-ray

binaries. Black hole systems with a high-mass companion have persistent hard spectra, often extending out to 200 keV, and low-mass companion

transient systems have spectra extending out to

beyond 100–200 keV, with transient broad and

redshifted 511 keV line emission reported for at

least one system (Nova Muscae 91). Spectra of

samples of black holes will allow detailed tests of

emission models and comparisons with neutron

stars. Further comparisons with spectra of AGNs,

believed to contain super-massive black holes,

could then be made to determine the self-similarity

of accretion flows onto black holes over a wide

range of mass scales. These similarities are of growing interest as galactic black hole candidates exhibit relativistic outflows of radio emitting matter.

Sources such as GRO J1655–40 and GRS 1915+105

which have been detected at energies as high as

800 keV provide a local laboratory for black hole

studies.

2.2.2 ACCRETING NEUTRON STARS:

X-RAY BURSTERS AND PULSARS

In low magnetic field (B < 108–109 G) neutron

star systems, the weak field cannot channel the

accretion flow onto the neutron star. When in a

state of low accretion, these systems appear to

exhibit hard X-ray power law components (hard

tails) extending out to usually only ~60–100 keV.

Spectral measurements to determine the self-similarity of the photon index (typically 2.5–3.0) and

cutoff energies are of primary interest for understanding the accretion flows onto these systems

(versus black holes). 
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 9

Figure 2.3. Recent CGRO and RXTE observations have provided

important insights into accreting binary systems. 

Studies of X-ray bursters with BATSE, RXTE and

BeppoSAX as well as studies of individual systems

(e.g., 4U 0614+09 and 4U 1915–05) have suggested common characteristics of the hard spectra from

neutron stars and the differences (primarily hard x-ray

luminosity) between neutron star and black hole

hard X-ray spectral components. These suggest many

observational follow-up studies for future missions

with much higher sensitivity and resolution.

Figure 2.4. An artist’s view of a magnetar. The superstrong

magnetic fields of these objects provide a unique laboratory.

Accreting neutron stars with high magnetic fields

are observed as X-ray pulsars. The detection and

detailed study of cyclotron lines in their hard X-ray

spectra are the best and most direct method of

determining neutron star magnetic fields. Indirect

arguments invoking spin-up or spin-down near the

equilibrium spin period often indicate rather high

magnetic fields (~1014 G in the case of GX 1+4).

Recent measurements of a cyclotron feature at 110

keV and a possible feature at 55 keV in A 0535+26,

implying B ~1013 G, have strengthened the case for

high magnetic fields for some accreting pulsars.

High fields are similarly inferred for other Be binaries. Magnetic dipole spin-down remains a possibility although the implied fields approach 1014 G in

several cases. Because of the rapid spin-down to the

radio pulsar death line, such ultra-high field neutron

stars may be best observed in the X-ray regime.

Figure 2.5. The emission history of earliest known SGR’s.

It is crucial to maintain a hard X-ray monitoring capability to

understand these objects.

Cyclotron line features and high quality continuum spectra of such sources would probe the

strongest magnetic fields in nature and should give

important evidence of new quantum effects

expected near 1014 G. The persistence of such high

fields may be related to the accretion history of

these objects. If so, relatively low average accretion

rates may be important such as those seen in the

Be systems, implying either transient X-ray sources

or low steady luminosities. Accordingly, studying

these unique high field sources presents several

observational challenges: the sources will be transient or faint and the need to obtain high sensitivity, high resolution spectra covering two

cyclotron harmonics requires sensitivity to energies

as high as 500 keV–1 MeV. The INTEGRAL should

give important results on some brighter systems,

but future large area imaging experiments will be

needed to probe the physics of ultra high-field

accreting neutron stars.

A recent triumph in neutron star studies comes

from the solution to a long-standing puzzle. The

Soft Gamma Repeaters (SGR) are sources of spo1975 Events (per half-month) 50

40

30

20

10

0

Events (per half-month) 5

4

3

2

1

0

Events (per half-month) 5

4

3

2

1

0

1980 1985 1990 Year

SGR 1806-20

SGR 1526-66

SGR 1900+14

Year

1995 2000

1975 1980 1985 1990 1995 2000
10

radic, varying bursts of hard X-ray emission. The

evidence that SGRs are associated with supernova

remnants followed by the detection of pulsed emission confirmed the neutron star origin of these

sources. For two of the five known SGRs, SGR

1806-20 and SGR 1900+14, detection of pulse

periods and spin-down rates allow estimates of

magnetic field strengths above 1014 G. This establishes the magnetar origin of SGRs. Further observations will reveal what fraction of neutron stars are

born as magnetars and explore their relationship to

the “anomalous X-ray pulsars”. Furthermore, the

question of whether SGR’s are really being powered

by accretion, like the closely related “bursting pulsar”, is still open. Not only will neutron star physics

benefit from further SGR studies, but the possibility

exists that SGR studies may provide insight to the

gamma-ray burst mystery

2.2.3 WHITE DWARFS

Since the proton accretion free-fall energy onto

a white dwarf is ~200 keV, accreting white dwarfs,

or cataclysmic variables (CVs), are natural hard

X-ray emitters. The magnetic CVs, or AM-Her and

DQ Her systems (strong and moderate magnetic

fields, respectively) may have accretion flows closest to free-fall since their disks are nonexistent or

marginal (respectively). Much more sensitive hard

X-ray observations would allow the first broad comparison with the ROSAT Survey, which has greatly

extended (to more than 40) the known sample of

AM Her systems. These are “ultra-soft.”The higher

spectral resolution of future hard X-ray missions

would allow (for example) a systematic search for

Figure 2.6. This map of 2.2 MeV radiation shows evidence for

transient line emission from the white dwarf RE J0317-853..

the expected change in hard X-ray cutoff energy

vs. mass of the white dwarf (due to changing M/R)

as might be observable in “new” >200 MG AM

Her systems. The recent announcement that

transient 2.2 MeV line emission was possibly

detected by COMPTEL from the strongly magnetized white dwarf, RE J0317-853 could be the

beginning of a new era in exploring white dwarfs.

2.2.4 SPIN-DOWN PULSARS

Rotation-powered pulsars — rotating, magnetized neutron stars — are among the brightest

persistent sources in the gamma-ray sky, and are

among the few sources that have been unambiguously identified in high-energy gamma-rays.

By virtue of their enormous magnetic and gravitational fields coupled with their rapid rotation,

pulsars are today well-established, mainly from

gamma-ray observations, as being powerful particle

accelerators. Pulsars offer a uniquely direct way of

being identified with a gamma-ray source, because

of their signature pulsations at a predictable pulse

period.

Thanks to the successes and timing capabilities

of past and present gamma-ray telescopes, today

there are certain associations between seven

gamma-ray sources and rotation-powered pulsars:

the Crab and Vela pulsars, Geminga, PSRs

B1055–52, B1509–58, B1706–44, and B1951+32.

Tantalizing evidence exists for high-energy gammaray pulsations from two more: PSRs B0656+14 and

B1046–58.

These identifications have demonstrated that

gamma rays hold the key to understanding pulsar

emission. The ultimate source of energy for a pulsar

is its rotational kinetic energy; gamma-ray observations have proven that pulsars are efficient

machines for converting this mechanical energy

into non-thermal high-energy gamma-ray photons,

with typical conversion efficiencies around 1%, and

an efficiency of nearly 10% reported in one case.

Understanding how this energy conversion is

achieved may well have implications for other

astrophysical objects, such as black holes and active

galactic nuclei. Yet an understanding of the physics

involved has been elusive.

There are two possible locations for the gammaray emission regions in pulsars: near the polar cap

COMPTEL 2.2 MeV All-Sky Map

+90

+180 –180

–90

Galactic Coordinates

0.0000 0.0005

Intensity

0.0010 0.0015 0.0020
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 11

and in the outer magnetosphere near the light

cylinder. Models invoking these different regions

predict different high-energy pulse morphologies,

spectra, pulse phases as a function of energy, and

angular extents of the gamma-ray beams. Though

the handful of identified gamma-ray pulsars has

placed significant constraints on such models, the

issue of where the emission originates has yet to be

laid to rest.

Determining which model is correct is important

not only for understanding the pulsar emission

mechanism, but also because gamma-ray pulsars

can provide a window on otherwise ‘invisible’ pulsars in our Galaxy. Their contribution to the diffuse

Galactic gamma-ray emission, their connection to

the unidentified EGRET sources (see also Section

2.4), and their total number and hence birth rate in

the Galaxy, depend on how many are visible, that

is, on their gamma-ray beam size, luminosity and

spectrum.

More and more, gamma-ray observations have

come to play integral parts of multi-wavelength

studies of pulsars. In essence, gamma-ray observations clarify the fate of the spin-down energy and

its availability for other processes such as a wind to

power synchrotron nebulae, through detection of

pulsed and unpulsed emission. For example, the

underlying physics of all mechanisms for production of X-rays is ultimately tied to processes that

involve gamma-rays in a fundamental way: magnetospheric emission luminosity peaks at gamma-ray

energies, and comparisons of X-ray and gamma-ray

pulse morphologies, relative

pulse phases, and phaseresolved spectra are basic

diagnostics; polar cap

reheating, a source of soft,

thermal X-rays, is likely a

result of higher energy

processes at higher altitudes

that may be responsible for

the gamma-ray emission;

and pulsar wind nebula

emission, which, at least in

the Crab nebula (the only

one to have yet been studied in significant detail)

extends across the electromagnetic spectrum up to

TeV energies and signals a

population of wind electrons having energies as

high as 1016 eV.

Aside from the classical pulsar problems, the

study of isolated neutron stars in general is undergoing a renaissance, with the realization that these

objects can embody even more extreme properties

than has been appreciated. In particular, magnetic

fields as high as 1015 G have been suggested in

some unusual sources. Gamma rays are crucial to

existing and upcoming multiwavelength efforts to

understand generalized isolated neutron star

physics, and to unify the population.

The strategy for future observations has been

made clear by the foundation laid by past gammaray missions. The debate regarding the origin of

gamma ray emission in the magnetosphere must

be settled by obtaining a significant population of

gamma-ray pulsars to study. The gamma-ray

properties of pulsar wind nebulae themselves

must also be established beyond the single data

point of the Crab. The nature of the unidentified

Galactic plane gamma ray sources (see also section

2.4) must be better determined, to establish

whether they are indeed candidates for young

rotation-powered pulsars, and what the implications are for their birth rate. Improvements on the

positions will permit searches for either pulsed or

unpulsed counterparts at lower energies.

Continued monitoring for soft gamma-ray outbursts is necessary for reliable population estimates

and magnetar identification.

Figure 2.7. These multi-wavelength pulse profiles of the known gamma-ray pulsars show the diverse

shapes and energy dependencies which a complete theory of magnetospheric emission must describe. 
12

Significant progress in related areas is being

made already and begs for gamma-ray follow-up:

in the past year, over 250 new radio pulsars have

been discovered in the plane of our Galaxy; many

of these sources could be high-energy detectable.

In addition, several discoveries of extremely young

and energetic pulsars have recently been made at

X-ray energies and will be key sources for gammaray follow up. Planned radio observatories and X-ray

missions will undoubtendly continue to make major

neutron-star discoveries whose gamma-ray followup is an obvious next logical step; the reverse could

easily be true if instrumentation permitted.

Figure 2.8. This color coded view of the gamma-ray emission

regions of a pulsar magnetosphere shows how the pulse profile can

eventually be mapped back to the source. Sensitive tests of emission

mechanisms is the result.

The properties that future gamma-ray instruments must have in order to effectively address the

major issues in pulsar and exotic neutron star

science are (i) high angular resolution, (ii) large

collecting area, and (iii) photon time tagging at the

tens of microseconds level. The objectives and

requirements for compact object studies are:

Compact Object Objectives: 

• Identify a larger gamma-ray sample of accreting

black holes and neutron stars. 

• Monitor the sky for transient hard X-ray emission

to identify new active accreting black holes and

neutron stars. Constrain the BH population of the

Galaxy.

• Measure temporal and spectral variations of emission from accreting black holes and neutron stars.

• Search for e+/e– anihillation line emission in the

flaring 10 keV to 1 MeV spectra of accreting

black holes.

• Identify a larger gamma-ray sample of both radio

pulsars and radio-quiet pulsars. 

• Measure phase-resolved energy spectra for many

pulsars over a broad range. 

Compact Object Requirements:

• Effective area for a high-energy telescope at least

5 times that of EGRET, for high counting rate;

substantially increased FOV for total pulse

statistics.

• Moderate spectral resolution from 10 MeV to 100

GeV; excellent calibration at low energies to allow

comparison with Compton/Scintillator detectors.

• Source locations one arcmin or better to allow deep

x-ray, optical/IR, and radio searches for pulsars

and black holes in newly discovered transients. 

• All-sky monitoring of hard (10–600 keV) X-rays

with sensitivity at least two orders of magnitude

better than HEAO–1. 

2.3 EXTRAGALACTIC ASTROPHYSICS 

The Compton Gamma Ray Observatory

(CGRO) has contributed substantially to

our understanding of Active Galactic

Nuclei. Perhaps the most obvious and widely

known result is that there are clearly two classes of

gamma-ray-emitting AGN in the sky: these are the

jet-dominated blazars, and the more ordinary,

radio-quiet Seyferts and quasars. The marked

differences between the two classes are seen even

in the radio and optical data: blazars are dominated by compact, milliarcsecond cores, and the

radio and optical emission is strongly polarized.

The compact radio cores show superluminal

expansion. With the rapid and large-amplitude

variability observed in the GeV gamma rays, we

now think that the entire emission arises in a jet

pointing close to the line of sight, where the emission is strongly enhanced due to the relativistic

Doppler effect. The radio and optical emissions are

most likely due to the synchrotron process, while

the hard X-rays and gamma rays are due to

Comptonization, of either the internal synchrotron

radiation, or photons external to the jet. The blazar

Polar Cap

Outer Cap
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 13

luminosity often peaks in the gamma-ray band

above 1 MeV and all CGRO instruments have

detected the high energy emissions from several

blazars. In the case of radio quiet objects, the

overall electromagnetic spectra are quite different.

Within the current sensitivity, these spectra do not

extend beyond a few hundred keV. Instead, they

appear to cut off, such that there are no observed

photons at or beyond the 511 keV annihilation line.

In many cases, the hard X-rays and soft gamma

rays vary on a relatively short time scale, indicating

a compact emission region. This implies that they

must be produced very closely to the central

engine, and are probably the primary form of

radiative power in radio-quiet AGN.

2.3.1 SEYFERTS

The observations by OSSE on CGRO showing a

break or an exponential cutoff in the high energy

X-ray spectrum of Seyfert galaxies have had enormous impact on our theoretical understanding of

the X-ray and gamma-ray emission processes. They

rule out purely non-thermal e± models and favor

thermal or quasi-thermal Comptonization as a

mechanism for the X-ray to gamma-ray continuum.

A view of the Seyfert nucleus which is consistent

with X-ray and gamma-ray observations of CGRO,

ROSAT, ASCA, RXTE and BeppoSAX indicates that it

contains optically-thin, mildly-relativistic, plasma

which emits, via Comptonization, a power-law

spectrum in X-rays. The plasma is above an accretion disk, which Compton-reflects this radiation

and gives rise to a broad continuum peak at ~30

keV and an emission line at ~6.4 keV. The nucleus

is embedded in a thick molecular torus. The spectrum observed in the polar directions relative to the

torus is the intrinsic power law and Comptonization of the nucleus, identifying Seyfert 1 objects.

Viewed from the plane of the torus, the intrinsic

spectrum is strongly absorbed by the neutral torus

material with column depths of 1022 – 1025 cm–2.

The X-ray emission in these cases are highly absorbed,

identifying Seyfert 2 objects; however, the intrinsic

power law spectrum and break should be the same

as for Seyfert 1 objects. Recent BeppoSAX observations

show that obscured AGN may dominate the soft x-ray

(ROSAT) source counts by factors of at least 2–4.

Hard X-ray and gamma-ray observations of

Seyfert objects thus provide key information in the

understanding of Seyfert AGN:

Figure 2.9. The broad-band spectrum of NGC 4945 using

CGRO/OSSE and ASCA data shows the complex behavior including

the expected obscuration of low-energy emission. Gamma-ray

observations are crucial for fully understanding active galaxies. 

• Broad band coverage to several hundred keV,

with response extending to above 511 keV, is

required to identify the character of the intrinsic

nuclear spectrum and characterize the thermal

plasma.

• They determine the extent of non-thermal

contributions to the nuclear emissions and the

contributions of e± pairs.

• They offer the opportunity to discover many new

Seyfert 2 AGN which have not been detected in

X-ray surveys due to the large absorption in that

lower energy band.

2.3.2 BLAZARS

One of the major accomplishments of gammaray astronomy in recent times has been the detection of high-energy gamma rays from a class of

active galaxies termed “blazars.” The observed

luminosity of some of these at gamma-ray energies

exceeds that at other wavebands by as much as

two orders of magnitude; energy considerations

demand that the gamma-ray emission be beamed.

The gamma rays are thought to be produced in

jets containing highly relativistic plasma, moving

with roughly the same Lorentz factors required to

explain the superluminal motions detected in many

sources. The timescale of variability of the gammaray emission is shorter than at radio frequencies,

implying that the gamma rays originate in the portion of the jet that lies between the central engine

and the radio jet imaged with VLBI. Very little is

channel energy (keV)

keV/cm2 s keV

1

10–6 10–5 10–4 10–3 0.01

10 100 1000
14

known about this portion of the jet. Yet it is precisely the region where the most important physics

occurs: the formation of the jet, the acceleration of

the energetic particles, the collimation of the flow

into a narrow cone, and the acceleration of the

flow to Lorentz factors up to 20 and possibly as

high as 100. 

In some cases, the gamma rays are detected

during periods of enhanced activity at other wavelengths. In a few well-monitored objects, there is a

close association between flares seen at millimeter

to optical wavelengths and high gamma-ray states

(e.g., PKS 0528+134 and 3C 279) and between

UV to X-ray and very high-energy gamma-ray

emission (e.g., MKN 421). The particle injection/

acceleration process, which appears to fluctuate

rapidly, can only be studied directly by observing

the GeV-TeV gamma rays. With an improvement in

sensitivity of a factor of 10 in a GeV space telescope (a combination of larger effective area and

better angular resolution), the number of blazars

visible to a future instrument should increase by a

factor of at least 30, from the present 93 to several

thousand, which would (if there are not basic

differences) encompass all the known blazars. A

similar improvement in sensitivity in ground-based

observatories will permit the detection of weaker

nearby blazars as well as long-and short-term

monitoring programs. Simultaneous multiwaveband monitoring will result in exciting inferences

regarding relativistic jets and energetic particle

acceleration in blazars. The ability to follow flares

smaller than those which can be detected with

EGRET, and to obtain better time resolution will

lead to the derivation of the geometry and physical characteristics of the inner jet by observing

time delays at different frequencies as the flare

propagates along the jet. 

Of particular importance will be the measurement of the energy spectra of gamma-ray blazars

as a function of redshift, since the ability of highenergy gamma rays to traverse large cosmic

distances is limited only by photon-photon pair

production off ambient photons. For the closest

blazars this attenuation will begin to occur at energies above 4 TeV but for the most distant objects it

will begin to occur above 30 GeV. To complete this

study, observations with high sensitivity will be

required from both space-and ground-based

gamma-ray telescopes. Observations of the highenergy cutoff to the gamma-ray emission from

blazars will therefore allow an inference of the

ambient photon field and may define the epoch of

galaxy formation.

2.3.3 CLUSTERS

The detection of hard X-ray emission from

inverse Compton scattering of 2.7 K background

photons from high-energy intracluster electrons

would provide a very direct way of investigating

intergalactic magnetic fields. The presence of electrons is inferred from the observation of cluster

radio halos. The Coma cluster is the best known

example, but others include A 2255, A 2256, A

2319, and A 1367. The origin of the magnetic

fields in galaxy clusters is not understood, nor is

the mechanism by which the electrons are accelerated. Since we cannot be confident of equipartition, the magnetic field cannot be estimated from

the radio observations alone. Observation of the

inverse Compton X-rays together with the radio

observations would provide a much more direct

method of establishing the magnetic fields, but

requires X-ray observations above about 20 keV

to avoid confusion from thermal emission. An estimated flux sensitivity of ~3 x 10–6 photons cm–2 s–1

keV–1 in the ~20–60 keV range is needed to make

the crucial measurement. The hard X-ray detector

(PDS) on BeppoSAX has recently discovered a significant hard excess (>25 keV) above the thermal

emission (kT ~ 8 keV) for the Coma galaxy cluster.

This implies a cluster magnetic field of ~0.14

microGauss, the first such determination. Higher

sensitivity and spectral resolution observations are

needed as well as imaging to map the emission

and rule out AGN contributions. Focusing hard

X-ray telescopes, with high resolution detectors, are

particularly well suited to this important problem.

2.3.4 THE DIFFUSE GAMMA-RAY BACKGROUND

Data on the cosmic diffuse gamma-ray background have been obtained by over 20 balloon

and satellite borne instruments over the past 30

years, but only recently have good spectral measurements been made. Many questions remain

unanswered though, such as the spectral shape in

the MeV region, the presence or absence of nucleosynthetic lines, the angular distribution, and the

origin of the radiation. 
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 15

The low-energy portion of the cosmic diffuse

spectrum (10 keV to 60 keV) is characterized by a

bremsstrahlung spectral form that can be approximated by a power-law segment of energy index

~0.4. The energy spectrum transitions to a power

law of index ~1.6 above 60 keV. At one MeV, there

is still uncertainty as to the shape. Prior to 1995,

the spectrum was thought to have a hump at ~2

MeV as detected by instruments on balloons,

HEAO–1, and Apollo 16/17. 

Figure 2.10. The diffuse background from soft X-rays to highenergy gamma rays. 

Recent measurements, however, by the COMPTEL instrument on CGRO in the 0.8–30 MeV range,

and a careful reanalysis of data obtained with SMM

and HEAO–1 have not confirmed the presence of

the hump. Above several MeV the spectrum has an

energy index of ~1.0 as seen by recent measurements by EGRET.

Various theoretical attempts have been made to

model the source of the diffuse background as

unresolved AGN. It is generally possible to fit the

spectrum with dominant contributions from

absorbed Seyfert 2’s between 10 and 400 keV

and blazars between 3 MeV and 10 GeV. There

may be an excess emission at ~1 MeV above the

AGN models, although the data quality is not good

in this range. The origin of such an excess could

be “MeV Blazars” or possibly gamma-ray line and

continuum emission from unresolved Type 1a

supernovae.

Extragalactic Objectives:

• Detection of an expanded sample of Seyferts and

blazars at much higher S/N than with CGRO OSSE.

• Coordinated observations of Seyferts and blazars

with good sensitivity into the hard x-ray and

MeV bands.

Extragalactic Requirements:

• Hard X-ray telescope.

• Hard X-ray deep survey instrument

• Flux sensitivity at high energies at least a factor of

10 better than EGRET with comparable angular

resolution.

2.4 THE MYSTERY OF THE

UNIDENTIFIED HIGH-ENERGY

GAMMA-RAY SOURCES

The history of astronomy has been punctuated by breakthrough discoveries — galaxies,

quasars, pulsars, and X-ray binaries are

examples — where the sources were first seen as

“unidentified” objects by telescopes that fell short

of having the spatial, spectral, or timing resolution

needed to complete the identification. Such a population is now seen in the high-energy gamma-ray

sky. In addition to the known source classes of normal galaxies, pulsars, and blazars , the third EGRET

catalog contains 170 unidentified sources (out of a

total of 271). With this many gamma-ray-luminous

objects scattered across the sky, the potential for

exciting discoveries is high. 

The distribution of unidentified EGRET sources

suggests a largely Galactic population. Some possibilities are: (1) radio-quiet pulsars like Geminga.

Such pulsars would aid studies of neutron star

population and dynamics (see section 2.2.4). (2)

cosmic ray concentrations in supernova remnants.

Some EGRET sources appear to be associated with

SNR and may represent the sources of cosmic ray

protons. (3) X-ray binaries. Two tentative identifications suggest that winds or accretion processes

may result in gamma-ray emission from binary

systems. (4) gamma-ray flare stars. Some EGRET

sources appear to be transients not associated with

prominent radio or X-ray sources showing similar

variability. For all sources near the Galactic plane,

problems of source localization and source confusion present challenges to be solved with a future

gamma-ray telescope. 

Some of the high-latitude unidentified sources

are likely to be extragalactic, perhaps unrecognized

or radio-quiet blazars (many of the EGRET error

boxes do not contain bright, flat-spectrum radio

100

10

1

0.1

0.4 1 10 102 103 104 105 106 107

E (keV)

EIE (keV cm–2 s–1 sr–1)
16

sources), or some other type of active galaxy. The

tentative identification of Cen A in the EGRET catalog suggests that other AGN should produce highenergy gamma rays at some level. In all these

cases, the gamma rays provide critical information

toward understanding the physics of jets

(see section 2.3.2). 

The most important possibility is that the

unidentified sources contain some new type of

astronomical object, as many previous classes of

“unidentifieds” have revealed. This prospect is one

of the strongest incentives to make definitive

measurements of the gamma-ray source locations,

energy spectra, and time variability.

Figure 2.11. This optical image shows the relative sizes of the

EGRET and GLAST source localization capabilities. Improvements

such as these are critical for identifying counterparts to

unidentified sources.

Unidentified Objectives:

• Determine the locations of many of the unidentified sources with sufficient accuracy for identification with objects known at other wavelengths.

• Measure time variability, if any, on scales of one

day or less, and search for pulsations. Measure

the energy spectra of unidentified sources over a

broader energy range. 

Unidentified Requirements:

• Source location accuracy better than one arcmin.

The smallest possible error boxes are important.

• Increased sensitivity (factor of at least 10 greater

than EGRET) to detect more photons and monitor time variability of sources.

2.5 THE GAMMA-RAY BURST ENIGMA

Despite 25 years of effort, astronomers have

yet to explain cosmic gamma-ray bursts.

This phenomenon is currently the subject

of about one publication/day in the astronomical

literature, about the same as the observed event

rate. Bursts occur at random locations, and for

about 20 s can be the brightest objects in the

gamma-ray sky before fading into multiwavelength obscurity. Their isotropy and inhomogeneity have been well documented by BATSE.

Discoveries in the last two years have helped to

resolve the well known debate regarding the

gamma-ray burst distance scale but also heightened the mystery regarding what sort of model, or

group of models can provide the enormous energies implied by such a bright event occurring at

cosmological distances.

Beginning in 1997, the Italian X-ray mission

BeppoSAX was able to provide sufficiently accurate

prompt locations (about 5') to the ground-based

community that fading counterparts in other wavelengths, especially optical and radio, were discovered for a few bursts. To date at least 9 optical

counterparts have been detected and red shifts

obtained for a half dozen host galaxies (see Table 2.1).

It is important to note that, thus far, these bursts

have all been of the long complex variety since

BeppoSAX only triggers on bursts longer than 5 s.

Thus we still know very little about the characteristic

distance and sources of that discrete group of hard

short bursts with mean duration 0.3 s. However, it is

now clear that at least a large subset of the long

bursts seen by BATSE are at cosmological distance

with a mean redshift greater than one. This makes

them the most energetic explosions in the universe.

As this report goes to press, one of the most recent

bursts studied, GRB 990123, at red shift z = 1.61

has been determined to have an energy that, if

isotropic, would exceed the rest mass energy of the

sun turned into gamma-rays within no more than a

minute. It may well be that the burst sources are

beamed so that the characteristic energy is reduced

and the event rate much higher than one per day,

but in any case, the puzzles posed to the theorists

Lockman Hole

57.0

56.5

56.0

55.5

55.0

163.0

Right Ascension (J2000)

Declination (J2000)

162.5 162.0 161.5 161.0

EGRET

GLAST
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 17

are as exciting as they are difficult. How can one

release so much energy so rapidly and convert it

into gamma-rays with such high efficiency? How are

the “afterglows” at other wavelengths made? Is

there more than one model for gamma-ray bursts?

What do we learn about star formation, cosmology,

particle acceleration, and black hole physics?

GRB 990123 was also detected by the groundbased ROTSE telescope with an optical magnitude

of about 9 during the burst itself. While this was an

exceptionally bright burst, certainly in the upper

1% of the brightness distribution, the fact that

some bursts have bright counterparts must also

affect future mission strategy. Also of significance to

mission strategy was a burst that occurred on April

25, 1998 coincident with the onset of a bright

Type Ic supernova, SN 1998bw. Because of the

relatively large error box of BeppoSAX and the

possibility of source confusion, the congruence of

these two events remains controversial in the community. However if the burst and the supernova

(distance 38 Mpc) were the same event, the April

25 gamma-ray burst was about 100,000 timesTABLE 2.1. GRBS LOCALIZED BY BEPPOSAX AND BATSE/RXTE OR RXTE/ASM (COURTESY J. GREINER)

GRB GRB X-ray position Error Instrument IPNa XAa OTa RAa IAUC z

960720 17h30m37s +49 ̊ 05.8' 3' SAX/WFC n n 6467,

6569,

970111 15h

28m15s +19 ̊ 36.3' 3' SAX/WFC y y n 6533,

6569,

970228 05h01m57s +11 ̊ 46.4' 3' SAX/WFC y y y 6572

970402 14h50m16s –69 ̊ 19.9' 3' SAX/WFC y n 6610

970508 06h

53m28s +79 ̊ 17.4' 3' SAX/WFC y y Y 6649, 0.835

6654,

970616 01h

18m57s –05 ̊ 28.0' 40'*2' XTE/Uly y y n 6683,

6687,

970815 16h08m43s +81 ̊ 30.6' 6'*3' XTE ASM y n 6718

970828 18h

08m29s +59 ̊ 18.0' 2.5'*1' XTE ASM y y n 6726, 0.33?

6728,

971024 18h

24m51s +49 ̊ 28.9' 9.0'*1' XTE ASM y n priv.

comm.

971214 11h

56m30s +65 ̊ 12.0' 4' SAX/WFC y y y 6787, 3.14

6789,

971227 12h57m35s +59 ̊ 15.4' 8' SAX/WFC y? ? 6796

980109 00h25m56s –63 ̊ 01.4' 10' SAX/WFC n n 6805

980326 08h36m26s –18 ̊ 53.0' 8' SAX/WFC y ? y 6851

980329 07h02m41s +38 ̊ 50.7' 3' SAX/WFC y y y y 6853 ~5?

980425 19h34m54s –52 ̊ 49.9' 8' SAX/WFC y SN y 6884 0.0085

980515 21h18m04s –67 ̊ 14.9' 5' SAX/WFC ? 6909

980519 23h22m14s +77 ̊ 15.3' 3' SAX/WFC y y y y 6910

980613 10h17m46s +71 ̊ 29.9' 4' SAX/WFC y y 6938 1.096

980703 23h59m07s +08 ̊ 35.6' 4' RXTE/ASM y y y 6966 0.966

980706 10h48m00s +57 ̊ 20' 4 deg COMP/PCA y ? n –

981220 03h42m34s +17 ̊ 09’ 2'*4.5' ASM/Uly y y

981226 23h29m40s –23 ̊ 55' 6' SAX/WFC y ? 7074

990123 15h25m29s +44 ̊ 45' 2' SAX/WFC y y y y 7095 1.60

990217 3h02m52s –53 ̊ 06' 3' SAX/WFC n n 7110

(a) IPN = interplanetary network detection; XA = X-ray afterglow; OT = optical transient; RA = radio afterglow; z = redshift. (Complete through February 1999)
18

fainter than most of the other events for which

hosts have been determined. there may be a large

population of faint bursts beyond the capability of

BATSE to detect. Thus, much higher sensitivity

(increased by factors -5–20) and angular resolution

are needed to probe the logN-logS distribution and

possible SN contributions.

Another important relatively unexplored aspect

of gamma-ray burst science is their high energy

emission. Several bursts have been detected at very

high energies, up to 18 GeV, by the EGRET instrument on CGRO. In one case the burst continued at

high energy for almost two hours — much longer

than at the lower energies seen by BATSE. The

mission to accomplish this is

the Gamma-Ray Large Area

Space Telescope (GLAST).

While a complete theory of

gamma-ray bursts is lacking,

we expect that the panchromatic behavior of bursts, all

the way from the radio to 1

TeV, will be very important

in unraveling their nature.

Objectives:

• Obtain accurate prompt

locations, preferably of

order arc seconds, for

hundreds of bursts so as

to allow prompt follow up

studies from the ground

to determine distances. It

may be that gamma-ray

bursts are a diverse phenomenon and we need a large

sample to determine the

characteristics of the various

components.

• Obtain the distances and evidence for or against galactic

association for a significant

sample of short hard bursts 

• Study the very high energy

emission of gamma-ray bursts

in the energy band 100 MeV

to at least 300 GeV.

Coordinate the results with

behavior at other wav

lengths and obtain accurate locations for these

high energy bursts as well.

• Search for fainter bursts than seen by BATSE.

• Search for much fainter bursts. 

Requirements:

• Arcsecond source locations to allow prompt,

deep optical, X-ray and radio counterpart

searches.

• Rapid ground based followup

• Sensitivity to high energy photons 

• Increased hard x-ray sensitivity (~5–20 BATSE

_values) to search for GRBs at high redshift.

Figure 2.12. Over 2300 gamma-ray bursts have now been detected by CGRO/BATSE.

Figure 2.13. Small GRB error boxes have allowed sensitive optical searches for counterparts such as

this image associated with a burst detected by BATSE, COMPTEL, and BeppoSAX.
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 19

2.6 SOLAR GAMMA RAYS:

EXCEPTIONAL PHOTONS FROM AN

UNEXCEPTIONAL STAR

Gamma-ray lines from solar flares were first

observed in 1972 with the NaI scintillator

on OSO–7. It was not until 1980, however,

that routine observations of gamma-ray lines and

continuum became possible with the much more

sensitive Solar Maximum Mission (SMM). Most

recently, gamma-ray observations have been carried out with the CGRO instruments COMPTEL,

EGRET and OSSE, as well as with the PHEBUS

instrument on GRANAT. Two previously accepted

solar flare paradigms were drastically modified by

the gamma-ray work. Prior to the SMM observations, based on timing arguments, it was thought

that in the impulsive phase of flares only electrons

are accelerated; ion acceleration was believed to be

a delayed, second phase, phenomenon. This

paradigm has been overturned by the SMM data

which showed very prompt and impulsive gammaray emission, often in temporal coincidence with

the hard X-ray time profiles. Another accepted

paradigm of solar flare research has been that a

large fraction of the released flare energy resides in

nonthermal electrons of tens of keV, with the energy content in accelerated ions constituting only a

small fraction of this energy. This result depended

on the manner in which the ion spectrum was

extrapolated from around 10 MeV/nucleon, where

the bulk of the gamma-ray production takes place,

to lower energies. Recent work on abundances,

based on SMM data, and the requirement to

account for the very strong observed 20Ne gammaray line whose production threshold is near 1

MeV/nucleon, implies an extrapolation of the ion

spectrum as an unbroken power law down to that

energy. This yields an ion energy content comparable to the energy content in the low-energy electrons, placing ion and electron acceleration on an

equal footing: both components are impulsively

accelerated and contain approximately equal

amounts of energy.

Further gamma-ray work using SMM data has

provided important information on abundances of

both the ambient medium and the accelerated

particles. Concerning the ambient medium, the

analysis of gamma-ray lines has shown that in the

gamma ray production region the abundances of

elements with low first ionization potential (FIP) are

enhanced relative to those of elements of high FIP.

This FIP bias has been discovered previously using

accelerated particle data and atomic spectroscopy.

The nuclear spectroscopy is telling us that the bias

sets in quite deep in the solar atmosphere, probably already in the chromosphere where the bulk of

the gamma rays are produced. This result, which

was not known prior to the gamma-ray work, has

important implication on the dynamics of the solar

atmosphere.

There are also indications, based on gamma ray

lines from 7

Li and 7Be produced by alpha particle

interactions with ambient He, that there are

regions in the chromosphere with enhanced He

abundance. This also impacts solar atmospheric

dynamics.

Concerning the accelerated particles, the gammaray work has shown that these particles exhibit large

abundance enhancements for the heavy ions, particularly Fe, as well as enhancements of 3

He. The latter

is based on newly identified lines at 0.937, 1.04 and

1.08 MeV due to the interactions of accelerated 3He

nuclei with ambient O. The abundance of 3

He in the

solar atmosphere is only a few times 10–4 relative to 4

He. But based on particle observations we know

that it routinely becomes comparable to that of 4

He

in the accelerated particles from impulsive flares,

most likely due to gyroresonant interactions of the

particles with plasma waves. The gamma ray

observations complement the particle data and thus

have important implications for the particle acceleration mechanism.

The rising portion of solar cycle 22 (1988–1993)

was observed until 1989 with SMM. The maximum

of this cycle, however, was only studied with

non-dedicated instruments: Phebus/ GRANAT;

GAMMA–1; CGRO. Nevertheless, these instruments

found exceptionally interesting results, for example,

the observation of pion decay emission and nuclear

line emission lasting for hours. This indicates that in

post-flare conditions it is possible to either trap or

accelerate over extended time periods ions of energies as high as several GeV. While long lasting flare

emissions have been known previously, this was the

first instance that such time extended emission

could be associated with GeV ions. These observa-
20

tions were only possible because of the very good

sensitivities of the new instruments, in particular

CGRO.

The planned HESSI mission (launch in 2000)

with its Ge detectors will provide important new

data on many of the issues mentioned above,

including detailed abundance studies and for the

first time the imaging of the flare hard X-ray and

gamma-ray emissions. Solar flare gamma-ray observations with detectors of even higher sensitivity will

allow some entirely new types of investigations. For

example, it would be possible to observe in detail

the relatively long lived radioactivity (e.g., 7Be and

56Co) produced by accelerated particle interactions

in flares, thereby allowing the study of transport

and mixing in the aftermath of flares. Finally, the

flare observations will be used as a local laboratory

for the testing of the various proposed models of

the galactic sources of gamma-ray line emission. 

Objectives:

• Abundance measurements of both the ambient

medium and the accelerated particle spectrum

for a larger number of solar flares.

• High-energy (> 10 MeV) measurements on a larger number of flares to compare on a flare-by-flare

basis with hard X-ray and nuclear-line fluxes.

• Large duty cycle of solar observations since correlated data from ground-based observatories are

critical for proper interpretation. What distinguishes high-efficiency gamma-ray flares?

• Identification of gamma-ray production regions.

Requirements:

• Extended spectral coverage

(~ 100 keV – 100 MeV)

• Imaging at hard X-ray/gamma-ray energies

(~ few arcsecond)

• Good sensitivity with large dynamic range. 

• Good spectral resolution with “diagonal” energy

response.
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 21

CHAPTER

3

Current Program

The current set of missions related to the

gamma-ray astronomy program provide a

crucial resource. Valuable contributions will

continue to be made by this generation of instruments, setting the stage for the instruments in

development. This chapter provides an overview of

the experiments currently operating, or which will

be operating in the next three years.

3.1 COMPTON GAMMA RAY

OBSERVATORY

The Compton Gamma Ray Observatory

(CGRO) was launched in April 1991 with a

broad range of science objectives including

the understanding of gamma-ray bursts, studies of

black holes and neutron stars, the search for sites of

nucleosynthesis, probing the galaxy through the

interaction of cosmic rays with the interstellar medium, and studying the nature of active galaxies in

gamma rays. While significant insight has already

emerged in these areas, unanticipated discoveries

have challenged our understanding of the conditions

and energy-generating mechanisms for many astronomical sources as indicated in chapters 1 and 2.

Over 700 scientists from 24 countries have participated in the CGRO Guest Investigator program.

CGRO continues to operate flawlessly and none of

the instruments, except EGRET, has life-limiting

consumables. EGRET has a limited supply of spark

chamber gas which is being conserved through

carefully selected observations and operating

modes. Through this conservation program, EGRET

expects to support targets of opportunity over the

next five years. The science contributions which are

anticipated from future CGRO observations are:

• Understanding the GRB mystery. Counterparts

may continued to be identified using the near

real-time BATSE and COMPTEL notification system and the GRB Coordinates Network (GCN)

triggered ground observer network. 

• Continue all-sky monitoring for

transients, X-ray pulsars, and

sources of e+/e– annihilation

radiation.

• Nucleosynthesis in Type Ia

supernova. OSSE and COMPTEL

have the sensitivity to study

gamma-ray lines from such

supernovae to nearly 10 Mpc. 

• Continued galactic plane observations with OSSE and COMPTEL to improve the sensitivity to

diffuse line emissions such as

60Fe and 44Ti .

Figure 3.1. This image shows the diffuse glow of high-energy gamma-rays detected surrounding

the center of our galaxy by CGRO/EGRET. More sensitive high energy observations are needed to

quantify and understand this emission.
22

Figure 3.2. An image of the annihilation radiation detected by

CGRO/OSSE. The cloud of 511 keV emission coming from the

galactic center has surprised observers.

• High energy emission during the next solar

maximum. CGRO may well provide the only

opportunity to observe the Sun in gamma rays

during the entire next solar maximum. 

• Continued multiwavelength observations of

AGNs.

• Testing nucleosynthesis models for novae; 22Na

emission from novae within 1–2 kpc may be

detectable.

3.2 INTEGRAL 

The International Gamma-Ray Astrophysics

Laboratory (INTEGRAL) is an ESA mission

due for launch in 2001 that is dedicated to

fine spectroscopy (E/dE = 500) and imaging (12

arcmin FWHM) in the 15 keV to 10 MeV energy

range. The two main instruments on board are a

spectrometer (SPI) with high-spectral-resolution

germanium detectors and an imager (IBIS) that

employs high-spatial-resolution arrays of cadmium

telluride and cesium iodide detectors. Optical and

X-ray monitors complete the scientific payload. The

spectrometer has a field-of-view of 16 degrees (fully

coded), an angular resolution of 2 degrees FWHM

and a sensitivity to narrow spectral lines of ~5x10–6

ph cm–2 s–1 in a 106 s observation. The imager has a

field-of-view of 9 degrees fully coded, an area of

~2600 cm2 and a continuum sensitivity of 4x10–7

ph cm–2 s–1 keV–1 (~1 mCrab) at 100 keV. The key

scientific objectives of the INTEGRAL include (1) the

study of explosive nucleosynthesis in Type I supernovae out to ~15 Mpc through the detection and

measurement of 56Co lines; (2) a survey of galactic

supernovae from the past 300 years through detection and mapping of 44Ti line emission; (3) a determination of the sites of nucleosynthesis in the

galaxy over the past million years through the

mapping of the 26Al line emission; (4) a broad-band

(5 keV – 10 MeV) study of AGNs and the spectral

characteristics of different classes such as Sy 1, Sy

2, and blazars; (5) a study of the galactic center

region and galactic plane to determine the positions, spectra and nature of the compact objects;

(6) a sensitive, multi-year survey of the galactic

plane for study of galactic transient sources such as

X-ray novae and Be transient pulsars. The INTEGRAL is being developed primarily by ESA and the

European member countries, but will include Russia

(Proton rocket) and the United States (tracking,

instrumentation). A science data center is located

at the Geneva Observatory in Switzerland. The

observing program will consist of a Core Program

(~30% of the time) and General Program that will

have observations chosen from an open competition of proposals submitted by the members of the

community at large. The Core Program will be

largely devoted to galactic plane scans and a deep

exposure of the central radian of our galaxy.

Figure 3.3. These two images from the GRANAT/Sigma instrument

show the region around the galactic center at two different times.

Sources appear and disappear in this highly active region of our

Galaxy. Long-term monitoring of such sources is an important

aspect of gamma-ray science.

3.3 COMPLEMENTARY X-RAY MISSIONS 

T wo recently launched X-ray missions have

instruments observing in the hard X-ray

band which provide complementary observations to those of CGRO in addressing the key

science objectives identified in sections 1 and 2.

The Rossi X-ray Timing Explorer (RXTE) was

launched by NASA in December 1995, as a mission

to study the temporal and spectral variability of

X-ray emission from a broad range of astronomical

Galactic Longitude

20

10

0

–10

–20

–40 –20 0 20 40

Galactic Latitude
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 23

objects. A complement of three scientific instruments observing in the 2–250 keV energy band is

addressing important questions concerning the

structure and dynamics of compact X-ray sources

such as accreting neutron stars, white dwarfs and

black holes in our galaxy as well as the massive

black holes thought to be present in the nuclei of

distant active galaxies.

Important features of RXTE are the large

collecting area of its instruments which permit

sub-millisecond temporal resolution of bright

sources, the incorporation of the All-Sky Monitor

that views approximately 70% of the sky per orbit

for the detection of new transient sources, and its

ability to re-orient quickly (within 7–24 hours) for

detailed study of new transients. 

These characteristics have facilitated several

major accomplishments including: discovery of kilohertz quasi-periodic oscillations (QPO) in some 20

accreting neutron star systems, discovery of a millisecond accreting pulsar, discovery of 6 black hole

systems, some of which exhibit jets and evidence

for general relativistic frame dragging, and the

discovery that a soft gamma-ray repeater probably

has a superstrong magnetic (2x1014 G)

The RXTE spacecraft has an expected orbital lifetime extending at least through 2004 and, since it

is not constrained by consumables, may extend for

an additional five years

The primary objective of the Italian-Dutch X-ray

mission BeppoSAX is the broad band spectral characterization of galactic and extragalactic X-ray

sources. Launched in April 1996, it carries a complement of four co-aligned narrow field-of-view

instruments observing in the 0.1–300 keV energy

band and two wide-field cameras for the detection

of new transient sources in the 2–30 keV range.

The SAX detectors have relatively large area, good

energy resolution and approximately 1 arcmin

imaging at low energies. The wide-field cameras

provide milliCrab sensitivity for transient detection

and monitoring. 

This particular combination of large field of view

and good angular resolution instrumentation on

BeppoSAX has facilitated the key observations

proving the gamma-ray-burst distance scale to be

cosmological. This has been done by providing

accurate localization of ~ a dozen GRB X-ray counterparts thereby allowing identification of optical

afterglow counterparts.

Despite suffering the loss of two ACS gyros,

BeppoSAX has been able to maintain functionality

and is expected to have a mission life extending at

least through the year 2001. 

3.4 GAMMA-RAY BURST INSTRUMENTS 

BeppoSAX observes ~8 bursts/year in its Wide

Field Camera, with location accuracies

better than 10'. When the burst position is

observed with the Narrow Field Instruments by

slewing the spacecraft, fading X-ray counterparts

can often be identified and localized to < 1'. The

time required to acquire and analyze the data is

~hours. The mission has been approved through

2001.

The Global Coordinates Network (GCN) distributes ~300 GRB positions/year with delays of the

order of seconds, determined directly onboard the

CGRO spacecraft by BATSE. The error circle radii

are ~4o

. The “Locburst” procedure, based on rapid

ground- based processing of the more intense

BATSE bursts, results in ~100 bursts/year with error

circle radii as small as 1.6o

. The CGRO mission has

been approved by the Senior Review through

2002.

The 3rd Interplanetary Network (IPN) consists of

Ulysses and the Near Earth Asteroid Rendezvous in

deep space, as well as numerous near-earth missions such as CGRO, RXTE, WIND, and BeppoSAX.

Mars Surveyor Orbiter will join the network in

2001. The IPN observes and localizes ~70

GRBs/year to arcminute accuracies. The time to

process and distribute the positions is ~1 day. The

IPN will operate through 2001 and possibly until

2004, if the Ulysses mission is extended. 

The Rossi X-Ray Timing Explorer All Sky Monitor

(RXTE ASM) detects ~3 bursts/year and determines

their positions to arcminute accuracy in minutes.

In addition, the RXTE Proportional Counter Array is

used ~once/month to scan BATSE Locburst positions for fading X-ray sources. When successful,

the counterpart position can be determined to

~10' within hours. RXTE has been approved

through 2002.
24

Several automated camera systems are now

operational, which slew to the position of a burst

and image it within ~10 s of the burst onset. LOTIS

(Livermore Optical Transient Imaging Survey),

ROTSE (Robotic Optical Transient Search

Experiment), and TAROT (Rapid Action Telescope

for Transient Objects) are three examples. In particular, ROTSE has succeeded in detecting the optical

emission from a gamma-ray burst in progress,

990123, which reached m~9.

The High Energy Transient Explorer II (HETE-II)

combines a Wide Field X-ray Monitor and a Soft

X-Ray Camera to localize ~50 GRBs per year to

accuracies of 10" to 5', and transmit them to the

ground in near-real-time. HETE-II will be launched

into an equatorial orbit by a Pegasus from

Kwajalein Island around the end of 1999, with a

nominal 2 year mission.

The Cooperative Astrophysics and Technology

Satellite (CATSAT) will contain a soft X-ray spectrometer to measure the 0.5–20 keV spectra of

~12 GRBs/y with unprecedented energy resolution.

Although CATSAT has only modest localization

capability, these spectral measurements will constrain the hydrogen column density towards the

sources, and help to answer the question of the

locations of burst sources with respect to their host

galaxies. CATSAT is planned for launch in 2000,

with a nominal one year mission.

INTEGRAL, the International Gamma-Ray

Laboratory, can detect bursts with its Ge spectrometer array, with its IBIS imager, and in its anticoincidence shield. Locations for ~20 GRBs/yr can be

localized to arcminute accuracy and the positions

distributed to observers in

< 100 s. INTEGRAL will also

be part of the 3rd IPN when

it launches in April 2001.

The nominal mission lifetime is 2 years.

SWIFT, a dedicated

MIDEX mission, has been

approved for a 6 month

study phase. If selected for

flight, it will revolutionize

GRB studies by localizing

~300 GRBs/y to arcsecond

accuracies, measure their

optical spectra, and transmit all data to the ground

with delays of seconds to minutes. A nominal 3

year lifetime starting in 2003 is planned.

3.5 HESSI

The High Energy Solar Spectroscopic Imager

(HESSI) is a NASA Small Explorer scheduled

for launch in July 2000 to observe solar

flares through the peak of solar cycle 23. It will

explore the causes and characteristics of particle

acceleration in flares via the X-ray and gamma-ray

emission caused by high-energy electrons

(bremsstrahlung) and protons or nuclei (gammarays from nuclear excitation).

Figure 3.5. A view of the HESSI spacecraft. HESSI will provide high

resolution spectroscopy and imaging of Solar flares.

Figure 3.4. Rapid localization of GRB positions continue to allow searches for counterparts. This BeppoSax

observation of GRB 971214 is an example of the benefits of the current set of Burst detectors. 

BeppoSAX Observation of GRB 971214 Afterglow

t=t_burst+6.5h

11h57m42511h56m25511h55m08511h53m515

t=t_burst+12.5h

11h57m42511h56m25511h55m08511h53m515

t=t_burst+54h

11h57m42511h56m25511h55m08511h53m515

+65 ̊22'30"

+65 ̊15'00"

+65 ̊07'30"

+65 ̊00'00"
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 25

HESSI will achieve spectral resolution of 0.5 to a

few keV over more than three orders of magnitude

in energy (3 keV to 15 MeV) with a single set of 9

cooled coaxial germanium detectors. Each detector

will sit below a pair of grids with a unique spacing,

which will modulate the signal from a flare as the

whole spacecraft rotates, producing images with

2 arcsec resolution.

HESSI will resolve the shapes of every known

solar gamma-ray line except the very narrow 2.2

MeV neutron-capture line, for which its high resolution will provide high sensitivity. Since this line

traces proton acceleration, HESSI observations will

determine the partition of flare energy between

electrons and protons over a wide range of flare

fluences. The shapes of nuclear de-excitation lines

will probe the angular distribution of accelerated

ions, and the width of the 511 keV positron-annihilation line will reveal the temperature (and therefore location) of the annihilating medium. Ratios

between lines from different elements will constrain

compositions in the accelerated and ambient populations and also constrain the spectrum of the

accelerated protons (due to different thresholds for

different excitations). HESSI’s ability to image at

gamma-ray energies will provide the first look at

the location and size of the regions where flare

protons and secondary neutrons stop.

In addition to gamma-ray observations, HESSI

will produce rapid images and spectra of hard

X-rays due to bremsstrahlung from accelerated

electrons, providing information on their birthplace, motion, and energy loss. Since HESSI is

unshielded, it will also perform extra-solar observations, such as the lineshapes of the Galactic 1809

keV and 511 keV lines, spectroscopy of gamma-ray

bursts, and pulse-phased spectroscopy of pulsars.

It will image the Crab nebula when it enters the

solar field of view once a year.

3.6 AGILE

AGILE (Astro-rivelatore Gamma a Immagini

LEggero, Light Astro Gamma Imaging

Detector) is a high-energy gamma-ray

mission proposed to the Italian Space Agency (ASI)

Program for Small Scientific Missions. The Mission

completed a Phase A study in December 1998.

It has been selected for flight, although the final

funding decision has not been made at this writing.

Figure 3.6. Schematic view of the AGILE telescope. This pair

production telescope uses a silicon strip tracker with a plastic

scintillator anticoincidence detector and a thin CsI calorimeter.

AGILE is conceived as a bridge between EGRET

and GLAST, covering the energy range above 30

MeV. AGILE has about half the on-axis sensitivity of

EGRET but a field of view more than 3 times larger.

It will be a valuable tool for studies of bright AGN,

GRB, and other gamma-ray transients, where the

wide field of view will enable detection of flaring

sources that can be coordinated with observations

at other wavelengths. AGILE will help optimize the

science return from GLAST by identifying those

sources that require the much higher sensitivity

and broader energy range of GLAST

3.7 TEV TELESCOPES 

At energies above 100 GeV, observations

are conducted with ground-based

gamma-ray telescopes. By observing the

Cherenkov light from air showers produced by

gamma rays interacting in the upper atmosphere, it

is possible to detect discrete sources of gamma rays

with great sensitivity. The detectors are simple,

inexpensive and well-understood. There are now at

least five well-established TeV gamma-ray sources

(three of them pulsar/plerions and two of them

AGNs). The technique is also sensitive for timevariation (burst and pulsar) searches. In the past

decade ground-based gamma-ray astronomy has

become a viable discipline and an important

complement to observations from orbiting gammaray telescopes. The Atmospheric Cherenkov
26

Figure 3.7. This detection of the Vela supernova remnant by the

CANGAROO instrument, the third TeV detection of a supernova

remnant, shows the continued development of ground-based

gamma-ray astronomy. These high-energy instruments are an

important complement to their space-based counterparts.

Imaging Technique (ACIT) is the most effective

method of detecting sources of gamma rays with

energy > 200 GeV. It was developed at the

Smithsonian’s Whipple Observatory by a collaboration of U.S., Irish, and British institutions. The ACIT

has now been adopted by most of the groundbased gamma-ray observatories overseas. The

Whipple telescope is a 10m optical reflector with a

109 pixel camera. The flux sensitivity (5 sigma

level) at an energy threshold of 300 GeV is 8x10–12

photons cm–2 s–1 for an exposure time of 50 hours.

There are more than ten “second-generation”

ground-based gamma-ray observatories in operation or under construction. There is one collaboration active in the United States and there are major

groups in Germany, France, the U.K., the former

U.S.S.R., India, Japan, South Africa, and Australia. In

addition, the Milagro instrument is coming on-line.

It is an underwater Cherenkov telescope that has

the distinct feature of a large field-of-view in the

TeV range. Ground-based telescopes improve continuously in small increments; there is no technical

barrier to further increases in both flux sensitivity

and reduced energy threshold. In principle, a

telescope (e.g. MAGIC) can be built with an energy

threshold as low as 10 GeV.

3.8 ULTRA LONG DURATION BALLOON

PROGRAM FOR HIGH-ENERGY

ASTROPHYSICS

Since the 1960’s numerous U.S. and foreign

X-ray and gamma-ray astronomy groups have conducted balloon-borne astronomy experiments to

study celestial sources as well as to verify satellite

instrument concepts. The latter use of balloons has

been particularly important in the proof-of-concept

for the CGRO instruments. Balloon instruments

have also produced many scientific results. For

example, the pulsating X-ray binary source GX 1+4

and the 511 keV annihilation line, both in the

galactic center region, were discovered on balloon

flights conducted in the Southern Hemisphere.

Experiments such as these helped pave the way

for current satellite missions like BeppoSAX and

CGRO. With a reduced number of satellite opportunities and the emphasis on reducing the size and

cost of these missions, reliance on a strong balloon

program becomes even more important. For example, gamma-ray instruments tend to be relatively

heavy to attain a high efficiency to image weak

sources. Consequently, unless the satellite version

of the experiment can be conducted on a MIDEX

or smaller mission, some vital X-ray or gamma-ray

observations may never be conducted from satellites because of the mass constraint. There are

numerous astrophysics experiments that can be

inexpensively and quickly carried out on heavy lift

balloons. Experience with operations in the

Southern Hemisphere with long-duration balloon

flights of two- to four-week duration clearly

demonstrates the capabilities of the balloon platform for long flights. High-energy astrophysics

would benefit greatly from further development of

Southern Hemisphere long duration flight balloon

operations and extension of this capability to the

Northern Hemisphere.

The development of an Ultra-Long Duration

(100d) Balloon program has been initiated as the

Olympus Program. This would build on the long

duration (LDB) program (10-14d) and allow

gamma-ray science and missions with capabilities

comparable to MIDEX-class missions. For example,

a hard X-ray imaging sky survey at the ~1mCrab

level could be conducted in two such flights

(northern and southern) and enable relatively long

continuous exposures on individual sources at 

Vela 1993 – 1995

Right Ascension (J2000)

08h 40m 38m

46'00" 40' 20' 45'00" 10'

0 500 1000

counts dng–3

20'

36m 34m 32m 30m

Declination (J2000)
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 27

Figure 3.8. The promise of long-duration balloon flights is shown

in this plot of the sensitivity of the InFocus hard X-ray imaging

instrument. As with many other proposed experiments, balloons

provide an inexpensive method for instrument development and

scientific return.

relatively low and constant backgrounds. The

GRAPWG urges that NASA continue the development of both LDB and ULDB flight capabilities with

expanded support for payload development in both

the UNEX and SMEX programs. Alternatively, a separate LDB/ULDB mission line should be established

under the Explorer Program. International agreements for over-flight (particularly in the northern

hemisphere) must be pursued immediately.

Over-flight concerns would be made less difficult if

NASA also increased its implementation of balloon

flight control and flight termination/landing systems (e.g. parasails) which are now relatively developed (e.g. for DoD) but have not been incorporated in scientific ballooning. The balloon program,

and particularly the ULDB program, will enable:

• rapid, relatively low-cost, development of large

missions for cutting-edge science and space

qualification for possible future orbital missions

(e.g. ISS) 

• continuation of development of instruments, and

researchers (graduate students, postdocs, etc.) for

the future missions in HEA

3.9 THEORY

As the sensitivity and resolution of NASA’s

astronomy missions improve, so too must

the realism of the theory used to interpret

the results and give them meaning. Progress in

theory accompanies the progress in experiment.

One cannot lead the other for long. Theory is both

interpretive and predictive. On the one hand, for

data that are relatively well understood, theory

builds models to extract the greatest amount of

information possible from them. From these

models emerge new predictions that can be tested

and improved until the phenomenon can be satisfactorily understood. On the other hand, measurements may uncover surprises that generate great

controversy and, if properly understood, offer

potential for scientific advancement. It is such

phenomena that pose the greatest challenge to

theory and require it the most. Many examples of

each category could be given. We present just two:

nucleosynthesis gamma-ray lines and gamma-ray

bursts.

It is widely accepted that the elements heavier

than helium are made in stars with supernovae

playing a major role. It is also well documented by

measurement and understood in theory that the

short time scales and high temperatures of supernovae lead to the creation of short and intermediate lived radioactive isotopes. The species 26Al, 44Ti,

56Co, and 57Co have been detected and studied.

The role of theory is to obtain quantitative agreement between physical models and the line measurements in terms of flux, line shapes, and angular

distributions. This then leads to constraints on

models of stellar evolution and supernovae and a

better understanding of their nature, improved

accuracy in our models for galactic chemical evolution, and a better depiction of massive star formation in our galaxy. Based upon these improved

models, theory makes predictions that can be

confirmed by subsequent measurements, e.g.,

a detectable signal from 60Fe.

3 Sigma Continuum Sensitivity (ph/cm2/s/keV)

104

HEXTE

HEAD3 Survey

Balloon 8m

100 Crab

Balloon 8m

Reduced bkg

LDB 8m

Reduced bkg

1 Crab

Satellite 36m

103/5

LDB 8m

103/5

105

106

107

108

109

10

Energy (keV)

20 40 60 80 100

103/5
28

Gamma-ray bursts represent science of a different sort. Although the puzzle has made good headlines for a decade or two, ultimately the goal of

science is understanding. Here the theorist is much

less constrained, but also highly challenged by an

unexpected and poorly understood phenomenon.

Quick, qualitative speculations are useful for a time,

but ultimately progress requires that speculation be

backed up by detailed physical analysis and simulations. Frequently these simulations lead to the

death of the model, but that too is progress.

Eventually a model, or set of models, will be found

that explains what is observed and makes predictions that can be confirmed. Meanwhile theory

guides observations in defining the sensitivity of

future missions required to see bursts from the halo

from Andromeda, for example, or whether X-ray

absorption lines should be visible in the spectra of

cosmological gamma-ray bursts. Three dimensional

general relativistic calculations of neutron star

merger can show whether or not relativistic beams

can emerge. Calculations of planetesimal accretion

on neutron stars in the halo reveal that tidal disruption will likely prevent the intact arrival of an object

at the neutron star. Some models predict a large

number of hard X-ray bursts for every gamma-ray

burst; others predict that enduring hard GeV emission should be a common characteristic, etc. All

these predictions can and must be refined and

eventually tested. 

New observations will drive theory as they

always do. It is important however that NASA

continue to provide support to theory particularly

during these times of constrained budgets. For a

comparatively modest investment, NASA ensures

the existence of a cadre of trained specialists interested in making the most of the valuable data.

Without the activities of these people, the value of

the data is greatly diminished. 
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 29

CHAPTER

4

Future Missions

Aparticularly exciting set of new missions

in the time frame beyond the next three

years are planned in gamma-ray astronomy. In this chapter, we describe those missions

in the current Strategic Plan (GLAST, Constellation-X

HXT) as well as the follow-on missions which will

capitalize on their successes.

4.1 MISSIONS IN THE 1998 OSS

STRATEGIC PLAN

4.1.1 GLAST

E GRET has provided important discoveries of

gamma-ray emission from a diverse population of astrophysical sources. However, our

understanding of the gamma-ray emission mechanisms operating in these sources is limited by current instrumental capabilities. A future high-energy

gamma-ray mission should provide an imaging,

wide field-of-view telescope that covers the energy

range from approximately 20 MeV to more than

100 GeV. In this energy range, gamma rays are

identified by recording the characteristic track

signature of the electron-positron pair that results

from pair conversion in the presence of a nucleus.

The telescope consists of interleaved thin converters (metal foils) and position sensitive charged particle detectors followed by a calorimeter for energy

measurement. Finally, the telescope requires a very

efficient anticoincidence system for rejecting the

much higher flux of background particles and an

on-board trigger and data acquisition system.

Modern particle tracking detectors (silicon

microstrip or scintillating fiber detectors for example), sophisticated on-board processing, and higher

telemetry rates will allow the required major

advance in observational capability over EGRET

within the constraints of an intermediate class

astrophysics mission. The mission to accomplish

this is the Gamma-Ray Large Area Space Telescope

(GLAST — http://glast.gsfc.nasa.gov/).

For the baseline mission parameters given in

Table 4.1, a factor of 30 improvement in flux sensitivity and a factor of 10 improvement in point

source location capability will be obtained.

Determination of the spectra of the sources over a

broad energy range will also be possible. A wide

field-of-view telescope will allow the detection of

many more transient sources such as AGN flares

and high-energy gamma-ray bursts.

The principal scientific objectives for the mission

include:

• Active Galactic Nuclei. Determine the mechanisms

of AGN jet formation, particle acceleration, and

radiation by studying gamma-ray emission from

all known blazars (and possibly other AGN

classes) and correlating these observations with

those at other wavelengths. Rolloffs in AGN

spectra above 1 GeV may be caused by pair production against the intergalactic photon field and

may be used to study the extragalactic background light as a function of redshift.

• Unidentified Gamma-ray Sources. Determine the

type of object(s) and the mechanisms for

gamma-ray emission from the unidentified EGRET

gamma-ray sources by precisely measuring their

positions, spectra, and variability. For many of

these sources, direct periodicity searches for

gamma-ray pulsars will be possible.
30

• Isotropic Background Radiation. Determine if the

high-energy background is resolvable into point

sources or if there is a truly diffuse component,

by deeply surveying high-latitude fields.

• Gamma-Ray Bursts. Provide constraints on physical mechanisms for gamma-ray bursts by detecting high-energy radiation from about 200 bursts

per year and studying the GeV : keV–MeV emission ratio as a function of time; image burst positions to a few arcminutes or better, allowing deep

“real-time” multiwavelength observations.

Figure 4.1 This simulation of the GLAST view of the anticenter

shows the spectacular detail the improved GLAST sensitivity will

provide.

• Endpoints of Stellar Evolution (Supernovae, Neutron

Stars, and Black Holes). Provide direct evidence of

proton cosmic-ray acceleration in supernova remnants by gamma-ray mapping and energy spectral measurements; distinguish between models

for high-energy gamma-ray emission from pulsars

by measuring detailed phase-resolved spectra.

Determine whether or not galactic superluminal

jet sources emit significant high-energy gamma

radiation.

• Molecular Clouds, Normal Galaxies and Clusters.

Probe the cosmic-ray distributions in dense molecular clouds and in nearby galaxies (LMC, SMC,

M31) by gamma-ray mapping and measuring the

spectra of diffuse emission from these objects;

search for extended emission from possible cold

dark matter clouds in the galaxy and from galaxy

clusters as a signature of unusual concentrations

of unseen gas or cosmic rays.

TABLE 4.1. CHARACTERISTICS OF THE GLAST MISSION

QUANTITY GLAST REQUIREMENT

Energy Range 20 MeV–300 GeV

Energy Resolution1 10% (100 MeV–10 GeV)

Effective Area2 8000 cm2

Single Photon Angular <3.5 ̊ (@100 MeV)

Resolution – 68%3 <0.15 ̊ (E>10 GeV)

(on-axis)

Field of View4 2 sr

Source Location5,7 1-5 arcmin

Determination

Point Source 4 x 10-9 cm-2 s-1

Sensitivity6,7 (>100 MeV)

Mission Life 5 years, with no more

than 20% degradation

Telemetry Downlink — 300 kbps

Orbit Average 1 kbps near-realtime

Telemetry Uplink 4 kbps 

Pointing Accuracy 2 ̊ accuracy

30 arcsec knowledge

Observing Modes Rocking zenith pointing

Pointed mode

1 Equivalent Gaussian sigma, on-axis.

2 Peak effective area, including inefficiencies necessary to achieve

required background rejection.

3 Space angle for 68% and 95% containment.

4 Integral of effective area over solid angle divided by peak

effective area. Geometric factor is Field of View times Effective

Area.

5 Range: bright sources to sources of 10-8 ph cm-2 s-1 flux

at >100 MeV.

6 Sensitivity at high latitudes after a 2-year survey.

7 Derived quantities delimited by double-lined box

4.1.2 CONSTELLATION X

The Constellation X mission is a high

throughput, broad-band X-ray facility emphasizing

spectroscopic observations of a wide variety of

astrophysical sources, ranging from stars to galaxy

clusters. The primary science goals emphasize

spectroscopy in the soft X-ray band (0.1–7 keV),

which is rich in absorption and emission features

from nearly all charge states of all cosmically abundant metals. However, unambiguous interpretation

of the atomic features often requires measurements

at hard X-ray energies (10–40 keV) to characterize

any accompanying non-thermal continuum (i.e. in
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 31

supernova remnants, AGN and stellar flares). For

this reason, Constellation-X will include the focusing Hard X-ray Telescope (HXT), based on Wolter-I

or conical approximation optics, to extend sensitive

continuum observations above 10 keV (see

http://constellation.gsfc.nasa.gov). Independent

of the goals emphasized for the soft X-ray spectroscopy telescopes, compelling motivation for the

HXT is provided both by the existence of sources

whose energy output peaks in this range, and by

astrophysical processes which are uniquely observable there. HXT observations therefore uniquely

address a number of scientific investigations identified as priority objectives of the GRAPWG. Below

we illustrate a few of the many examples:

Scientific Objectives Unique to the HXT

• Heavily Absorbed AGN and the X-ray Background.

The energy density of the X-ray background

peaks at ~30 keV; less than 15% of this total

energy density can be accounted for by the

ROSAT AGN population. If AGN comprise the

XRB, then most must have huge absorbing

columns (NH ~1022–1025 cm-2). Surveys by XMM,

ABRIXAS, and AXAF, limited to energies less than

~10 keV, will only probe absorbing columns up to

a few times 1023 cm-2. With its much higher sensitivity, the Constellation-X HXT can survey IR-selected AGN, probing faint and high-redshift AGN

populations thus far inaccessible in the X-ray

band. The data will also provide a probe of the

geometry of the absorbing material.

• Non-Thermal Emission from Galaxy Clusters. The

HXT will enable us to measure the inverse

Compton scattering radiation predicted in

clusters of galaxies. Combined with radio measurements, the hard X-ray observations will yield

a lower limit on the intracluster magnetic fields

independent of equipartition or equal energy

hypotheses. Detection of the nonthermal emission in clusters has significant impact on the

underlying cosmological models through the

estimation of the cosmological parameter . It

also affects cooling flow arguments, since the

presence of the magnetic field can suppress the

conduction of the cooling gas. Many clusters are

believed to have formed through merger events

as predicted in hierarchical large-scale structure

models. As a result, due to the collision of subclumps and merging effects, shocks of very high

temperature can be produced in the ICM. The

hard X-ray observations will also shed light on the

nature of clusters that show evidence of extremely high temperatures.

Figure 4.2. This plot of the spectrum of NGC 4945 as compared to

the Constellation-X HXT sensitivity shows the value of extending the

spectral coverage of such objects to hard X-rays.

• Hard X-ray Source Populations in the Local Group.

Compact stellar remnants such as accreting X-ray

pulsars and LMXBs (with hard Comptonized tails)

have energy output peaking (in f) above 10

keV. These sources both inform us about the endpoints of stellar evolution in binaries and sample

stellar populations as a function of Hubble type of

the host galaxy. With high sensitivity and hard

bandpass, the HXT can probe these highly

absorbed and hard stellar populations throughout

the Local Group.

• Shock Acceleration in Young Supernova Remnants. In

addition to the soft thermal X-radiation produced

in the shocked ejecta, the spectra of young supernova remnants like Cas A and Tycho exhibit hard

X-ray ‘tails’ extending to ~50 keV. This high energy emission may be due to diffusive shock acceleration of particles at the shock front. The diffusive

acceleration theory provides an unambiguous

determination of the magnetic field distribution at

the SNR shock, from spatially-resolved radio and
32

hard X-ray maps. The broad bandpass and

superior sensitivity allow the HXT to map this

non-thermal emission.

4.2 MEDIUM RANGE NEW MISSIONS

4.2.1 HIGH-RESOLUTION SPECTROSCOPIC IMAGER (HSI)

As described in Section 2.1, gamma rays

emitted by the radioactive decay of material synthesized in SNe provide one of the

few direct observational tools available for probing

the physics of the explosion itself, the compact

object formation, as well as the production and

dispersion of heavy elements. While detection of

44Ti in two young SNR, and the measurement of

radioactive decay lightcurves of Cobalt in SN 1987A

have given hints of the potential diagnostic power

of such observations, detections to-date have been

limited to a small number of sources. In addition

with few exceptions, spectral resolution has been

insufficient to measure Doppler shifts and line

widths, and spatial resolution too coarse to map

the distribution of the emission in typical remnants.

For example, Cas A, one of the two remnants

detected in 44Ti to-date, is ~5arcmin in diameter,

and requires sub-arcminute imaging capability to

discern features at interesting levels.

Figure 4.3. Experiments extending the energy range of focusing

telescopes to as much as 170 keV, represented by this drawing of

the HSI mission, can address many nuclear astrophysics objectives.

The recent laboratory demonstration of hard

X-ray focusing optics, and the possibility of extending these techniques to higher energy, into a

region containing several key gamma-ray lines

(

44Ti (68,78 keV), 57Co (122 keV), 56Ni (158 keV)),

provides compelling motivation for planning a

dedicated focusing mission with sensitivity up to E

≥170 keV. With moderate extensions of existing

multilayer optics and Germanium detector technologies, a focusing telescope with collecting area

≥500 cm2 (1–170 keV), 10'' spatial resolution, and

700 km/s (68/78 keV) (450 km/s) (158 keV) spectral resolution is achievable on an intermediate

size mission with a new start compatible with the

2006–2008 timeframe. The signal-limited, high

spatial and spectral resolution afforded by this

mission could address, in addition to many other

diverse goals, several of the top nuclear astrophysics objectives identified in Section 2. We briefly

describe the scientific goals of this mission below:

NUCLEOSYNTHESIS AND DYNAMICS IN TYPE II SNE.

Detailed mapping of the radioactive decay from 44Ti in young SNRs provides a sensitive probe of

several important parameters in Type II and Type

1b SNe. Because 44Ti is synthesized near the mass

cut, its production and ejection are very sensitive to

the explosion mechanism and the ejecta dynamics.

The 44Ti spatial distribution can also be used to

reveal global asymmetries present in the initial SN

event. Besides revealing the global asymmetries,

the clumpiness and velocity profiles of the 44Ti

Figure 4.4. Simulated 100 ksec map of the Cas A SNR in 44Ti

emission (68/78 keV). The model assumes an axisymmetric,

clumpy explosion which is clearly spatially resolved. The velocity

distributions of individual knots (here assumed to be 3000 km/s

(blue), –5000 km/s (orange), and –2000 km/s (yellow)) can also

be clearly determined.
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 33

would provide valuable information on ejecta mixing. This information is unique to the radioactive

decay measurements: overall Ca and Ti elemental

abundances measured in the optical and infrared

are dominated by 40Ca and 44Ti production in

regions outside the mass cut. Figure 4.4 shows the

detailed mapping of the Cas A remnant that could

be carried out in a 100~ksec observation with a

focusing system.

EXPLOSION MECHANISM AND DYNAMICS IN TYPE IA SNE.

As described in Section 2, direct observation of

the 56Ni decay emission (the center of the nuclear

abundance peak of degenerate nuclear burning) is

a key diagnostic for understanding the SN Type Ia

mechanism. Outside of the degenerate conditions

of the WD, 56Ni is unstable and decays to 56Co with

a mean lifetime of 8.8 days, producing dominant

emission lines at 158 keV (100%) and 812 keV

(86%). The daughter nuclei then decay to 56Fe with

a mean lifetime of 111.3 days, producing the well

known emission lines at 847 keV (100%) and 1238

keV (68%). The 56Ni → 56Co → 56Fe decay chain

powers the SN light curve during the first few 

Figure 4.5. 56Ni 158 keV emission line flux evolution for several

Type I SNe models. Fluxes are presented for the first 100 days after

the explosion for a source at 16 Mpc (Virgo Cluster). The

instrumental sensitivity (5 sigma) is shown for 80, 300, and 1000

ksec observations (short dash lines). Fluxes are adapted from

Chan and Lingenfelter (1991).

hundred days of the event. As illustrated in Figure

4.5, the time-dependence of the 56Ni (158 keV)

emission is extremely sensitive to the explosion

model, as are the line centroids and widths. A

focusing experiment is capable of making detailed

lightcurves of these lines out to D > 16 Mpc, even

for the weakest case (assuming no mixing).

TABLE 4.2. CHARACTERISTICS OF THE

HSI MISSION

Effective Area 700 cm2 (68 keV)

400 cm2 (156 keV)

Angular resolution 10"

Energy range 2–170 keV

Spectral resolution

(E/DeltaE) 160 @ 158 keV

Line sensitivity (106 s) 44Ti 6.5108

ph/cm2/s (5 sigma) 156Ni 810-8

ph/cm2/s (5 sigma)

Continuum sensitivity (106 s) 310–8 ph/cm2

/s/keV

(@50 keV, E/DeltaE =

0.5, 5 sigma)

Optics Graded multilayer

Wolter

Detectors Ge pixel 

Focal length 6–8 m

# modules 14

TOTAL S/C mass 1000 kg

Power 200 Watts

Launch Delta II (7320)

AGN.

A high-energy focusing mission would provide

sensitive studies of AGN, obtaining for the first time

high-quality hard X-ray/soft gamma-ray spectra for

a significant number of quasars and Seyfert galaxies. Observations in the energy band from 20–170

keV provide a measure of the intrinsic luminosity of

the central source and address many basic questions, including the relationship among the various

classes of AGN, in particular Seyfert Is, IIs, and

quasars (QSOs).
34

ICM IN CLUSTERS.

As described in Section 3.3.2, hard X-ray observations can provide a direct measurement of the

magnetic field strength in galaxy clusters. Because

thermal emission will dominate in many cases at E

< 60–70 keV, extending sensitivity to energies higher than planned for Constellation-X is crucial. The

excellent flux sensitivity and good angular resolution make focusing telescopes ideal for this important measurement.

POPULATION STUDIES IN THE LOCAL GROUP.

Studying the compact stellar remnants, such as

weakly magnetic (B <

~ 109 G) neutron stars in lowmass binaries, and black hole candidates with both

high and low-mass companions outside of our own

Galaxy at hard X-ray energies is another goal possible only with a focusing instrument. Such studies

would both inform us about the endpoints of stellar

evolution in binaries and sample stellar populations

as a function of the Hubble type of the host galaxy.

4.2.2 ENERGETIC X-RAY IMAGING SURVEY

TELESCOPE (EXIST)

The hard X-ray sky, defined broadly as from

10 keV–600 keV, is both relatively poorly explored

and yet rich in promise. This is the energy range

where fundamental transitions from primarily

thermal to primarily non-thermal sources and

phenomena are expected, and where compact

objects are usually most variable. Thus over the

next decade, the scientific case is compelling for

one or more missions in this broad band. Here we

briefly outline the case for first a broad-band sky

survey mission followed by a focussing mission

(section 4.2.1) for detailed study of individual

sources over a more limited energy range. 

Only one truly all-sky survey has been conducted

in the hard X-ray band: the pioneering HEAO-A4

survey which yielded a catalogue of some 80

sources down to flux levels of typically 50 mCrab in

the 13–180 keV band. Meanwhile, the soft X-ray

sky has now been explored fully to flux levels a

factor of ~103 times fainter with ROSAT but only up

to energies of 2.5 keV. The German ABRIXAS mission (launched April 28, 1999) was planned to

extend the soft X-ray sky survey up to ~10keV with

~0.1mCrab sensitivity and ~3arcmin resolution but

with limited temporal coverage and essentially no

monitoring capability. Although a spacecraft power

failure aborted the mission after launch, it is likely

(and highly desirable!) that Germany will re-fly an

ABRIXAS2 by c. 2003. At energies above ~600 keV,

the sky has been imaged with COMPTEL and

EGRET. The need is therefore acute for a sensitive

hard X-ray (~10–600 keV) all sky imaging survey to

close the gap and achieve sensitivities comparable

to ABRIXAS (i.e. 0.1mCrab or below). 

Figure 4.6. The EXIST telescope assembly would mount to one of

the Payload Attach System points on the S3 segment of the truss.

An all-sky hard X-ray (HX) survey mission at high

sensitivity must be imaging to avoid the source

confusion problems that have plagued the collimated HX detectors on HEAO-1 and OSSE. Angular resolution of at least 15 arcmin is needed, particularly

in the galactic plane (where SIGMA discovered the

black hole candidate GRS1758-25 only 40arcmin

from the bright soft X-ray binary GX5-1) but also at

high latitudes (where SIGMA discovered the HX

source GRS1227+025, possibly identified with a

z = 0.57 QSO, and only 20arcmin from 3C273.)

The desired HX survey sensitivity (~0.05mCrab),

and current estimates of AGN number counts at

5–10 keV from BeppoSAX (section 2.3.1) suggest

a detection density of ~0.25/square degree (or

~10,000 all-sky). Thus a reasonable confusion
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 35

limit (resolution at ~1⁄40 expected source density)

suggests ~5arcmin as the needed deep HX survey

resolution, with source locations correspondingly

finer (e.g. <

~1arcmin for >~5sigma detections).

An imaging deep all-sky HX survey should also

provide unprecedented sensitivity and temporal

coverage for time variablility studies since the HX

sky is so inherently time variable. With only

~100mCrab sensitivity, BATSE has demonstrated

the power of broad temporal coverage for both

transients and pulsar studies. The HX survey should

therefore have all-sky coverage each orbit and

would then greatly extend (but not be replaced by)

X-ray (2–10 keV) all sky monitors (e.g. MAXI,

planned for the JEM on ISS) which will be less sensitive to outbursts of obscured AGN (e.g. SeyIIs)

and Blazars as well as heavily obscured galactic

transients.

The Energetic X-ray Imaging Survey Telescope

(EXIST) was selected as a New Mission Concept

(NMC) by NASA in 1994 and has been studied as

have several other HX survey telescope concepts

proposed for MIDEX missions. The deep HX survey

mission requirements for sensitivity (to match

ABRIXAS), energy range and temporal coverage

can be met with a very large area array of coded

aperture telescopes which would almost certainly

exceed the MIDEX envelope but could make very

effective use of the International Space Station

(ISS). A generalized EXIST mission concept for ISS

is shown in Figure 4.6 and would yield survey

sensitivities shown in Figure 4.7.

Figure 4.7. The hard X-ray sensitivity for the EXIST concept as an

instrument on the International Space Station.

The key Scientific Objectives for such a deep HX

survey mission would include:

• Obscured AGN and Diffuse Hard X-ray Background:

The recent exciting discoveries of obscured AGN

made by BeppoSAX and ASCA (cf. section 2.3

and fig. 2.9) point to the need for an unbiased

HX survey to map the entire sky. The narrow-field

instruments, and follow-on focusing HX telescopes, will observe limited samples but not

select the most extreme (obscured) ones which

require the widest field coverage at any limiting

survey sensitivity. A complete HX-selected sample

will not only allow their contribution to the cosmic diffuse background radiation (fig. 2.10) to be

measured, but will also constrain the accretion

history of the universe as measured by the complete sample of accretion luminosities it would

detect. This more complete census of BH luminosities may have far-reaching implications for

galaxy formation.

• Blazars and the Diffuse IR Background: Since the

HX Blazars have their synchrotron emission peaks

in the HX band, monitoring and measuring their

break energies in coincidence with the wide-field

capabilities of GLAST and followup studies with

ground-based TeV telescopes (e.g. VERITAS) can

constrain the intrinsic GeV/TeV spectral shape

and thus the cosmic diffuse IR background from

measured spectral breaks in their gamma-ray

inverse Compton components. 

• Studies of Black Holes on All Scales: A deep HX

survey will probe the emission mechanisms and

geometry closest to black holes: from stellar mass

BHs in the numerous transients it would discover,

through the large sample of massive BHs it would

discover in AGNs. High sensitivity and spectral

resolution out through 511 keV will allow the

contribution of pairs to hoth quiescent and (particularly) flare emission in both AGNs and galactic

black hole candidates to be measured and the

contribution of BH transients to the diffuse 511

keV emission in the Galaxy to be established. 

• Gamma-ray Bursts at the Limit: GRB sources thus

far identified (section 2.5) are at cosmological

distances and may probe star formation as well as

the most extreme explosions in the universe. The

faintest GRBs may thus probe both the extremes

of the GRB luminosity function (e.g. beaming

effects) and the SFR at largest redshifts. The
36

desired high sensitivity (~0.05mCrab in 107s) and

wide-field (~2sr) for the survey yield a GRB detection sensitivity some 20X better than BATSE. Thus

without further flattening of logN-logS below the

BATSE limits, GRBs might be detected at a rate of

~6/day, and the most luminous events (e.g.

GRB990123) could be detected at z ~ 10.

• Supernova Rate in Galaxy From 44Ti Galactic Survey:

High sensitivity maps from stacked survey images

of the galactic plane at high (~2%) energy resolution will also enable a much deeper (10X) search

for 44Ti (68,78 keV) emission from obscured SNR

than will be possible with the INTEGRAL galactic

plane survey. With an all-sky line sensitivity of ~2

x 10-6 photons/cm2-sec at ~70 keV, or ~30X

below the COMPTEL detection of Cas-A,

obscured SNR could be detected throughout the

Galaxy at similar ages (~300y) and the galactic

SN rate constrained. Candidates could then be

mapped at higher spatial resolution in followup

narrow-field HX focusing telescopes. 

Whereas the Burst Alert Telescope (BAT) on Swift

could conduct valuable survey and partial monitoring up to ~150 keV at ~1mCrab sensitivity, it is not

optimized for HX surveys (e.g. its fixed pointings

will engender flat fielding limits) or high time resolution imaging (e.g. pulsar timing would not be

possible for the onboard detector binning planned

for BAT) and a dedicated deep survey mission is

needed. INTEGRAL, with its relatively narrow field

(10-16deg) imagers, will conduct only limited

surveys without all-sky monitoring. Thus the

GRAPWG recommends a high sensitivity and widefield imaging HX survey mission be carried out

within the next decade with the following capabilities and approximate instrument parameters: 

Both the desired wide-field (monitoring) and

extended energy range (and thus shielding) of this

HX survey preclude its placement as the top detector of a large area Compton telescope (Table 4.4).

Such a deep survey mission would ideally be flying

prior to, or coincident with, the Constellation-X

mission and its pointed HXT. Blazar and GRB studies would be optimized if it were at least partially

simultaneous with GLAST. As precursors to this

mission, there could be lower sensitivity initial

surveys carried out by Swift/BAT (2003) and prototype survey instruments flown on both LDB and

ULDB balloon missions (2002–2005). 

TABLE 4.3: DEEP HX SURVEY MISSION (EXIST)

PARAMETERS

Energy range: –600 keV

Sensitivity 0.05 mCrab (5–100 keV);

(5 sigma; 107sec) 0.5mCrab (100–600 keV) 

Field of view 40deg160 deg 

Angular resolution 5arcmin

Source locations <1arcmin

Energy resolution 2% (60 keV); 1% (500

keV)

Temporal resolution 100% sky each orbit;

1microsec imaging 

Telescopes 8 coded aperture

telescopes (40deg

FWHM, each) at fixed

offsets (20deg, each)

Detectors 10,000 cm2 CdZnTe

(0.5cm thick), 400 

400 pixels, each telescope

Mass 2500 kg 

Power 1.5kW

Telemetry 1.2mbs; 32 kbs (compressed daily survey)

Mission implementation Zenith-pointing

(+/–80deg) telescopes in

LEO (ISS?); combined

160deg (FWHM) FOV

perpendicular to orbital

direction

Mission life 2 years+ (>2 sky surveys

at limiting sensitivity)

4.2.3 ADVANCED COMPTON TELESCOPE (ACT)

The realm of nuclear astrophysics and medium

energy gamma rays probes some of the most energetic phenomena in astronomy — the endpoints of

stellar evolution in supernovae, neutron stars, and

black holes. Measurements in this band can address

fundamental questions in astronomy such as star

formation, supernova physics, galactic structure,

and chemical evolution. The observational challenges of nuclear line astrophysics have been

addressed by several missions in the past, beginning with the High Energy Astrophysics

Observatory (HEAO) series in the 1970’s through
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 37

Figure 4.8. The characteristic structure of a Compton Scatter

telescopes point-spread function is apparent in this simulation.

The intersections of the event circles defines the location of a

point-source.

the current Compton Gamma Ray Observatory.

These missions, along with balloon flight experiments, have provided several notable achievements: 1) maps of galactic diffuse 26Al and 0.511

MeV emission with a few degree resolution, 2)

study of 56Co and 57Co lines from the Type II

supernova, SN1987A, and interesting limits on

56Co from Type Ia SN, and 3) detection of 44Ti

from Cas A. Some of these detections are of low

statistical significance; better sensitivity is needed to

provide diagnostics into the phenomena involved.

The planned INTEGRAL mission, which is to be

launched in 2001, is expected to provide a significant improvement over existing capabilities by having 3 degree imaging resolution, 2 keV spectral resolution, and a sensitivity to narrow lines of 5x10–6

photons cm–2 s–1 for a 106 s observation of a

narrow line. This is approximately a factor of 10

improvement over OSSE or COMPTEL. At this level,

one can begin to make detailed maps of 26Al

showing regions of recent star formation and

supernovae production. However, INTEGRAL’s

sensitivity to diffuse or broadened line emission is

degraded. Studying the physics of supernova

explosions will, lacking great serendipity, be

restricted to Type Ia, and Type II and Ib occuring in

the Local Group. A Type Ia in Virgo (20 Mpc)

would have a flux at peak of about 8x10–6 photons

cm–2 s–1. However the lines are broad, about 30–40

keV FWHM, so the sensitivity of INTEGRAL is reduced to just detecting the typical event in a 106 s

observation. Investigating the physics of SN Ia in

Virgo will require greater sensitivity. A meaningful

search for lines from 60Fe, estimated at an intensity

at least a factor of 10 below 26Al, is likely to be

beyond INTEGRAL’s reach and detailed study of

both the diffuse and central galactic pair annihilation lines will be somewhat limited by the large

0.511 MeV background feature.

Development of a nuclear astrophysics mission

that applies new technology to improve sensitivity

and to address the deficiencies in the current and

planned instruments is the priority of gamma-ray

astronomy. Such a mission would address key scientific priorities which have been identified by the

scientific community: 1) understanding of the

evolution of stars, in particular the SN Ia and corecollapse SN, 2) understanding the origin of the elements through quantitative understanding of the

physics of SN and their associated nucleosynthesis,

and 3) understanding of the behavior of matter

under extreme conditions — investigations of the

environment and processes associated with black

holes of all sizes (AGN and galactic microquasars).

The goal of the development would be an intermediate mission new start in the 2008–2010 time

frame. The characteristics of such a mission are

summarized in Table 4.4. A large field-of-view with

good imaging capability is required to provide

sensitivity to diffuse emission from the Milky Way as

well as to support a high-sensitivity sky survey. An

example of a mission concept with these capabilities

is an advanced Compton telescope (similar to

COMPTEL on CGRO) using the newly emerging

solid state detector technologies, such as

Germanium or CdZnTe strip or pixel detectors.

Liquid Argon or Xenon detectors also have interesting characteristics for MeV astronomy missions.

In the Compton telescope configurations, the good

position resolution of solid state strip or pixel

detectors and the excellent energy resolution of

Ge and possibly CdZnTe are critical in reducing the

background and thus providing the sensitivities

indicated in the table.

The priority objectives of this mission include: 

• Understand Type Ia Supernovae explosion mechanism and dynamics. The mission would measure

the structure of SN Ia via 56Co and 56Ni emissions

to probe the nature of progenitors and explosion
38

dynamics. Since SN Ia are used as meter sticks to

measure the size and shape of the universe, key

measurements of SN properties vs. galaxy type

could be made as well as cosmic star formation

rates vs. redshift. With typical expansion velocities

of 5000 km/s expected for the radioactive debris,

this implies that a broad line (dE/E ~ 3%) sensitivity of about 10–6 photons cm–2 s–1 in a few days is

needed to detect 56Co lines from SNe Ia to distances of 70 Mpc. Based on current discovery

rates in the optical, this translates into about

6–8 SNe/yr, each detectable over a period of

about 6 months.

Figure 4.9. The errors in scatter angle for a Compton scatter

telescope are directly related to source localization capability. As

shown in this figure, advanced materials such as Germanium which

provide superior spatial resolution can reduce the errors in

reconstructing the source direction.

• Map the Galaxy in nucleosynthetic radioactivity.

This mission should map the Galaxy with good

angular resolution in line emissions from 26Al,

60Fe, 44Ti, 12C, 16O, 56Fe and positron annihilation

and positronium continuum. These maps will

reflect the nucleosynthetic contributions of supernovae, novae and massive stars, and discover

sites of galactic supernovae. The 100-fold

improvement in sensitivity and angular resolution

over COMPTEL should permit localization of

young individual star clusters by the core collapse

supernova remnants they contain.

• Map Galactic Positron Emission. A large component of the observed galactic 0.511 MeV radiation is likely attributable to galactic Type Ia SN.

Thus Ia SNRs could be detected by their residual

0.511 MeV positron annihilation radiation associated with 56Co production. Diagnostics of the

physical properties can be provided by the line

profiles and positronium fraction. Positrons are

also excellent diagnostics of many high-energy/

compact environments. Electron-positron pairs

should be created in the intense magnetic fields

at the polar caps of pulsars and are expected

components in galactic jet sources (microquasars)

similar to AGN jets.

Science Objectives of secondary priority are:

• Use gamma-rays to understand the dynamics of

Galactic Novae. Detection of nuclear emission

lines from novae would provide critical tests of

the models of novae as thermonuclear runaways

on white dwarfs. Early -rays from 13N and 18F are

the most direct measure of the burning and

dynamics. CNO-rich novae should be detectable

in later -rays from 7

Be and 22Na to a distance of

1 kpc and positron annihilation radiation will be

detected from all novae within 3 kpc during the

first hours of the outburst.

Figure 4.10. Simulation of the structure in 26AI radiation which

might be seen from a galactic source such as the Vela supernova

remnant by a next generation Medium-energy instrument.

• Cosmic Ray Interactions with the Interstellar

Medium. Mapping of the galaxy will detect

nuclear gamma ray line emission from galactic

cosmic ray interactions with the ISM and molecular clouds. These measurements would provide

perhaps the only way to study the low-energy

(10s of MeV/nucleon) Galactic cosmic rays and
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 39

their substantial contribution to the ISM total

energy budget. Fluctuations would be expected

near particle acceleration regions such as in supernovae/cloud interactions. Continuum measurements in the MeV band will map the galactic cosmic ray electron distribution and energy extent.

• Active Galactic Nuclei. The high energy emission of

many blazar AGN have their peak energy content

in the 1–20 MeV range. These energies also provide some of the most compelling tests for relativistic beaming from – transparency arguments

and the Elliot-Shapiro relation. The detection of

e+/e– annihilation radiation would provide dramatic measurements of the conditions in the innermost regions near the central engine. These pairs

might be produced through – pair production in

Seyfert AGNs or indicate the presence of antimatter in the jet plasma in the case of blazars.

• Gamma Ray Pulsars. These powerful accelerators

produce e+

/e– in copious numbers from photonmagnetic field, photon-photon and interactions

with the stellar surface. The detection, redshift

and shape of an annihilation feature would probe

the environment of the production and annihilation sites.

The principle behind the Advanced Compton

Telescope is to measure the energy and position of

two gamma ray interactions, from which the arrival

direction of the incoming photon is constrained.

Conceptually this is done using two detectors,

shown in Figure 5.2. The direction of the scattered

gamma ray is determined from the positions of the

two interactions, and the energy loss in the two

interactions uniquely determines the scatter angle

( in the Compton formula).

Thus, the direction of the incoming gamma ray is

restricted to a cone centered on the direction of

the scattered gamma ray, with an opening angle

equal to the scattering angle. Figure 5.3 shows a

simple cartoon of the telescope, gamma ray interactions, and the reconstructed cone. 

Reconstruction of the gamma ray sky is accomplished by the superposition of many such events.

Figure 4.8 shows a simple “ring-sum” image reconstruction of a point source in a laboratory demonstration. This experiment was performed using two

imaging germanium detectors. The D2 detector

was moved to four different locations to simulate a

larger array of detectors, hence the four bands of

rings in the image. The limiting sensitivity of the

Compton instrument can be understood from the

ring-sum, determined essentially by the number of

rings that pass through the location of the source.

The reconstruction algorithms used by COMPTEL

on the Compton Gamma Ray Observatory are

much more sophisticated and deconvolve the sidelobes or “point spread function” from the image.

The same reconstruction techniques also apply for

data from the ACT.

The sensitivity and reconstruction ability are

directly related to the sharpness of the ring that

describes the possible photon arrival directions. Two

factors contribute to the sharpness: 1) ability to

measure the direction of the scattered photon, and

2) the ability to measure the Compton scatter angle

. Good position resolution in the detectors relative

to the spacing between the detectors determine the

uncertainty, d , in the scatter direction. The detector energy resolution determines the d contribution to the measurement of the cone angle, given

by the formula shown below, where E1 and E2 are

the energies in the two detectors, and E0=E1+E2 is

the incident energy. It is clear that good energy

resolution is needed in both detectors to minimize

 , and thus improve the ring sharpness.

The importance of energy resolution is apparent

in Figure 4.9 that compares a Compton telescope

using germanium detectors with a telescope similar

to Comptel that uses NaI scintillators. The comparison is made for 1 MeV photons. The most useful

events scatter by ~45 degrees or less, which corresponds to greater than 635 keV on this plot. The

uncertainty in the scatter angle is more than an

order of magnitude smaller in the germanium

instrument for most of the scattered photons. The

number of background events that are consistent

with a source location are fewer if the rings are

sharp, thus background events are easily identified

and rejected. Efficient background rejection is critical to achieving high point source sensitivity.
40

TABLE 4.4. CHARACTERISTICS OF THE ACT MISSION

USING A HIGH RESOLUTION COMPTON

TELESCOPE

Energy Range 300 keV–20 MeV

(Compton mode)

25–500 keV

(Coded aperture mode) 

Energy Resolution <3 keV @ 2 MeV 

Detector Area ~10,000 cm2

Field-of-View ~60 degrees (Compton)

~10 degrees

(Coded aperture)

Point Source ~5 arcmin

Localization

Line Sensitivity ~210–7 cm–2 s–1

(1 MeV, Narrow Lines)

~110–6 (SN Ia lines,

broadened)

Continuum ~110–5 cm–2 s–1 MeV–1

Sensitivity (0.5 MeV)

Mass 3500 kg (320 kg active

mass)

Power 2500 W 

Telemetry 3 Mbps 

Mission Life >2 years 

Orbit Low inclination LEO 

Spacecraft Pointing 30 arcsec knowledge 

Operating Modes Pointed observation mode

any direction, any time 

The superb performance of the ACT is shown in

the simulated image shown in Figure 4.10. This

simple ring-sum image is of the Vela SNR with a

hypothetical distribution of 26Al. The total amount

of 26Al in the simulation is derived from the COMPTEL measurements. Each of our four knots receive

3% of the total flux with the remaining 88% a

broad gaussian distribution in the center. The simulation represents the image that would be produced with a 2 week observation using the ACT

telescope described in Table 4.4.

The energy domain of this mission could be

extended to lower energies by the inclusion of a

coded-aperture telescope above the Compton

telescope. The top detector array of the Compton

instrument provides a large array of position-sensitive detectors for hard X-ray measurements; in this

energy band photo-electric absorption dominates

Compton scattering in many detector materials.

Simply by placing a relatively thin, low-mass coded

aperture above this detector plane to create a

coded aperture instrument in the energy range 20

keV to ~300 keV. Such an instrument, with approximately 10 degree field of view, would have excellent continuum sensitivities. Careful simulations of

this dual instrument configuration would be needed to verify that the coded aperture mask did not

adversely affect the performance of the Compton

Telescope. Preliminary studies of this configuration

are encouraging in that regard.

The recent developments in gamma-ray detector

technology show the way to advanced Compton

telescope concepts which provide the significant

advance in sensitivities needed to address the

broad range of exciting scientific investigations

listed above. These sensitivities for nuclear line

detections are ~2 	10–7 cm–2 s–1 for narrow lines

and ~1 	10–6 cm–2 s–1 for broadened lines as in SN

Ia. The development of such a mission requires key

investments in the detector technologies but also in

the simulations of the instrument designs and their

background radiations.

4.3 LONG RANGE NEW MISSIONS

4.3.1 NEXT GENERATION GRB (NGGRB)

In the coming years, missions such as CGRO,

BeppoSAX, HETE-II, CATSAT, and the Interplanetary

Network will be the workhorses which assure that the

momentum we have gained in GRB studies is not lost.

After that, the next big step in gamma-ray burst

studies will be to accelerate the identification of multiwavelength counterparts. Today, we rely on groundbased photometric observations of ~arcminute burst

positions to produce optical positions which are precise enough to position optical spectrometers (arcseconds), and delays of hours or more are routine. Swift,

a gamma-ray burst MIDEX currently in Phase A study,

will eliminate these delays with its onboard multiwavelength instrumentation. A wide-field gamma-ray

camera gives arcminute positions for ~300 bursts per

year to a sensitivity better than that of BATSE. The

spacecraft then slews in 20–70 seconds to point sensitive narrow-field X-ray and optical telescopes at the

burst location. These instruments study the burst
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 41

afterglow and provide rapid arcsecond positions for

further ground-base follow-up.

At high energies, GLAST has the valuable capability to study the very high energy gamma-ray

emission associated with bursts, first discovered by

EGRET. To do this, the mission must have a modest

gamma-ray burst experiment, which is currently

part of the instrument complement.

Figure 4.11. A view of the Swift satellite along with the three

science instruments.

The recent ROTSE results on prompt optical

emission from one burst, GRB 990123, dramatically

demonstrate that ground-based, rapidly slewing

telescopes can greatly enhance our understanding

of the GRB emission mechanisms. Indeed, a network of such telescopes can support not only Swift,

but also GLAST observations of AGNs.

We have much to learn from missions like Swift,

and that knowledge will undoubtedly provoke

more questions. Among them are:

• is there a population of bursts an order of

magnitude or more weaker than the ones we are

observing? If so, what are their distances?

• do bursts have low energy X-ray iron line emission, as suggested by recent observations?

• can bursts be studied by their emission outside the

electromagnetic radiation band (e.g., neutrinos)?

The GRAPWG encourages further development

and timely flight opportunities for the next

generation of gamma-ray burst experiments. 

4.3.2 NEXT GENERATION HIGH-ENERGY GAMMA RAY

(NGHEG)

We expect a significant revolution in our understanding of the high-energy universe through the

GLAST mission. One thing that is already understood is that the spectral energy distribution peaks

in high-energy gamma rays for many sources

including young pulsars, blazars and at least a large

fraction of the unidentified sources. For blazar

AGN, source variability demands that a multiwavelength approach should take advantage of simultaneous observations. It would be highly beneficial to

maintain a high-energy capability beyond GLAST to

provide an essential component for studying such

varying sources in concert with future, sensitive

telescopes at other wavelengths.

Furthermore, it is likely that other fundamental

physics will need to be tested beyond the capabilities of GLAST. For instance:

• Using blazars and the high energy components of

GRB's as cosmological probes.

• Mapping our Galaxy, clouds, supernova remnants, and nearby galaxies for cosmic ray sources

at arcmin resolution.

• Identifying the counterparts of steep-spectrum

and confused high-energy gamma-ray sources

that require sub-arcmin positioning. It is already

clear from EGRET that not all the sources are

blazars and pulsars.

• Pursue aspects of astroparticle physics (high-energy lines from exotic particles, neutrino correlation, etc) beyond the capabilities of GLAST.

4.4 SUBORBITAL PROGRAM 

The future of gamma-ray astronomy will

continue to depend on the balloon program for the development of new instruments and techniques for major missions. The

advent of UNEX opportunities for Long Duration

Balloons (LDB) and eventually Ultra-LDB should

provide significant alternatives to space for investigations requiring large or heavy instruments. In

addition, ballooning provides a critical function in

the education and training of students in all aspects

of mission design, planning and implementation

and is thus essential for the future vitality of NASA.

Ballooning provides opportunities for critical
42

“hands-on” experience that is simply not practical

in today’s environment of highly schedule and cost

constrained missions.

Historically, the role of the sub-orbital program

has been particularly important for gamma-ray

missions, with all the instruments on CGRO (for

example) having heritage in the balloon program.

The role of the balloon program in gamma-ray

science has not been as widely appreciated.

Important scientific advances include the discovery

of galactic 511 keV line emission, 56Co line emission

from SN 1987A, the hard X-ray imaging study and

identification of several galactic bulge sources, and

the mapping and study of both the diffuse 511 keV

and 26Al 1.8 MeV emission.

With the newly revived and promising development of superpressure balloon technology, the

long-sought goal of ultra-long duration balloon

flights (~100 days) of large payloads (~3000 lb)

appears to be finally within reach. The GRAPWG

strongly endorses the current push to develop this

capability as a viable alternative for implementation

of UNEX class missions. To ensure the timely success of this effort, progress is needed in several

areas including mechanical, power, communication

and recovery systems and the maintainance of

overflight agreements. The GRAPWG encourages

NASA to pursue innovative approaches to these

problems through support of the balloon program

and by extending payload development opportunities to the user community.

4.5 SPACE STATION

Gamma-ray astronomy can potentially

benefit from the opportunity for locating

instruments, which are necessarily massive

and often have substantial power and telemetry

requirements, on the Space Station. Its highinclination orbit will have large particle backgrounds, but might not preclude all instrument

concepts. Narrow field, pointed telescopes would

be problematic, unless detached from the station,

but broad-field, survey or monitor instruments are

possibilities. For example, a very large area, hard

X-ray survey instrument might have otherwise

unattainable capabilities. Also, a low-density gas

time projection chamber could operate as a

Compton telescope with electron tracking in the

nuclear line regime. To achieve the sensitivity discussed herein, its size might require construction

in space, with the Space Station the obvious location. Frequent launch opportunities might also

make tests of prototypes for future major missions

feasible. We recommend that NASA commit

resources to study the suitability of instrument and

mission concepts discussed in this report for the

Space Station, and make the available opportunities

clear to potential investigators. 
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 43

CHAPTER

5

Technologies

The breadth of gamma rays in the electromagnetic spectrum — a range of more

than 109 in photon energy — demands a

wide variety of technologies. Advances in relevant

technologies span the full range of opportunities

for gamma-ray instruments, giving a high potential

for major improvements in many aspects of the

field. Section 5.1 describes the imaging techniques

needed in the different parts of the gamma-ray

spectrum. Section 5.2 illustrates how technologies

are currently being developed to take advantage of

the full range of gamma-ray possibilities. 

5.1 IMAGING TECHNIQUES 

The physics of gamma-ray detection is the

ultimate driver for all gamma-ray telescopes. In the keV energy range, gamma

rays interact primarily through the photoelectric

effect; in the MeV range, primarily through

Compton scattering; and at energies above a few

tens of MeV, almost exclusively by electron-positron

pair production. Only at the lowest gamma-ray

energies is any form of reflection possible. Most

gamma-ray telescopes require substantial detector

areas.

5.1.1 MULTILAYER MIRRORS

The familiar technical challenge to extending

traditional grazing incidence optics into the hard

X-ray band (E /10keV) is the decrease with energy

in incident angle (referred to as graze angle) for

which significant reflectivity can be achieved. For a

Wolter or conical approximation mirror geometry,

the graze angle, g, on a given mirror shell is related

to the focal ratio by g = 1/4 x (r/f), where r is the

shell radius and f is the focal length. Coating the

reflective surfaces with multilayer structures, which

operate on the principal of Bragg reflection, can

substantially increase the maximum graze angle for

which significant reflectivity is achieved over a relatively broad energy range, while maintaining realistic focal ratios. Other concentrating techniques and

mirror geometries such as polycapillary optics and

Kirkpatrick-Baez telescopes can also be extended

into the hard X-ray band; however, given the current state of technology and the desire for good

imaging performance, systems based on Wolter-I or

conical optics are the most attractive. 

The requirements for the multilayer materials are

that the K-shell absorption edges not lie in the

energy range of interest, and that the two materials

employed be chemically compatible for forming

TABLE 5.1 COMPARISON OF GAMMA-RAY IMAGING TECHNIQUES

Imaging Technique Energy Range Characteristics

Multi-layer mirrors below 100 keV high resolution, narrow field-of-view

Coded-Aperture mask below 10 MeV good resolution, wide field-of-view

Compton telescope ~1 MeV– ~100 MeV good resolution, wide field-of-view

Pair telescope above 10 MeV good resolution, wide field-of-view

Atmospheric Cerenkov above 100 GeV good resolution, narrow field-of-view
44

stable thin films. In the hard X-ray band, this is satisfied by several material combinations that have

been used to fabricate X-ray multilayers with the

appropriate dimensions. The most promising combinations for operation above ~5 keV include W/Si

(W K-edge at 69.5 keV), Ni/C (both edges below

10 keV, and therefore effective up to ~100 keV),

and Pt/C (Pt K-edge at 78.4 keV). Technical limits

restrict the operational energy band to below ~

120 keV. The graze angles for multilayer hard X-ray

telescopes are still smaller than those typically

employed at low energies. Therefore thin, lightweight, highly-nested mirror substrates are

required. Development of such optics is also critical

for future missions operating in the soft X-ray band

(the spectroscopy telescopes on HTXS, for example), and these efforts are directly applicable to

future hard X-ray focusing missions. 

5.1.2 CODED APERTURE IMAGING

Although multilayer coatings and small graze

angles allow imaging up to perhaps 100 keV (see

above), this is restricted to narrow fields-of-view

(typically less than 10 arcmin). Although Bragg

reflection can be incorporated into Laue lenses at

still higher energies (e.g., up to several MeV), these

focusing techniques are restricted to narrow energy

bands (typically less than 1–2% of the incident

energy). Therefore alternative concepts must be

used to achieve the important advantages that

imaging, with moderate to wide fields-of-view, can

provide: simultaneous measurements of source(s)

and background without the need to chop on and

off source; measurements of source locations with

resolution typically much higher than in nonimaging (e.g., collimated) detectors; and resolving

source structure for true imaging of extended

sources. All of these can be achieved by using

coded aperture imaging, whereby images are constructed from shadows of a coded aperture mask

cast on a position-sensitive detector located at focal

length, below the mask. Coded aperture imaging

is particularly well suited for the hard X-ray/soft

gamma-ray band (10 keV – 1 MeV) since it depends

on source photons being either absorbed (photoelectric) or scattered (Compton) if they strike a

closed cell of the coded mask. However the images

become increasingly blurred, with consequent loss

of sensitivity, as Compton scattering dominates and

Figure 5.1. This slide shows part of the coded aperture mask for

the INTEGRAL IBIS instrument.

the coded mask becomes (eventually) optically

thin; thus it is not optimum for energies above ~1

MeV. For a coded mask of open and closed holes

with usual open fraction 0.5, images are derived

simply by correlating the detected pattern of

source counts on the detector with the (known)

pattern of the mask. This may be understood simply as measuring the x- and y- shift of the detected

shadow on the detector and thus the angular position (in the orthogonal angles giving rise to x- and

y- offsets) of the source relative to the optical axis

of the telescope. The technique has now been well

developed and a variety of successful imaging telescopes have been flown from balloons and in

space. The premier space mission to date has been

the French/ Russian SIGMA telescope which

imaged selected regions of the sky (primarily the

galactic center region) down to sensitivities of

(typically) 30–50 mCrab in the 35–150 keV band.

Future missions are now planned (INTEGRAL) or

proposed (e.g., BAT on Swift) or mission concepts

studied (e.g. EXIST) which will be based on coded

aperture imaging. The proposed missions are all

survey missions and thus require very large fieldsof-view for maximum exposure time, temporal coverage and sensitivity. These requirements effectively

point to coded aperture imaging as the imaging

technique of choice. The technique requires position sensitive detectors of large area and high

spatial resolution. New CZT detectors (cf. section

5.2.1) are particularly promising since they provide

fixed pixels (which can be very small) and yet high

energy resolution.
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 45

5.1.3 COMPTON TELESCOPES

Compton telescopes have been used since the

early 1970’s for making astronomical observations

and measurements above 1 MeV. The first orbiting

Compton telescope, COMPTEL, on the CGRO has

performed the first all-sky survey at MeV energies

with a resolution of about one degree. The principle behind the instrument is that photons first scatter in a low-Z material in a forward detector. The

scattered photon is then detected by a second, or

rearward detector, typically made of a high-Z material to fully absorb the remaining energy. By requiring that the photon scatter twice, three advantages

become apparent. The first is that the telescope has

a natural directionality. The second is that if the

time-of-flight is measured between the triggered

detectors, then only photons traveling in the proper direction (forward or backward) need be accepted for further analysis. The third advantage is that

although the efficiency of the instrument is low

(two scatters are required and one pays the price in

effective area), it greatly reduces internal or local

background. However, when the direction of the

recoil photon is not measured, but only the energy

deposits, as with COMPTEL, background still comes

from the local sources and the true cosmic photons

from nearby points on the sky.

Figure 5.2. A representation of the operating mechanism of a

standard Compton Scatter telescope. Measuring the momentum

of the scattered electron in the upper detector can collapse the

annulur shape of the point spread function to a more standard

(and smaller) circle.

By measuring the energy deposits in the two

detectors one not only has a measure of the total

incident photon energy but also a measure of the

Compton scatter angle that can later be used to

construct an “event circle” and create an image.

Present day Compton telescopes only employ the

technologies that allow them to measure the location of the photon interactions, the energies of the

interactions and the time-of-flight. These data are

not sufficient to assign a unique direction to the

incident photon. One also needs the direction of

the scattered electron in the forward detector. This

is not easily accomplished. MeV electrons scatter

efficiently and unless the material in the forward

detector is tenuous and of low-Z composition, the

direction information of the electron is quickly lost.

The MeV region is home to strong sources of background that competes with the declining spectrum

of cosmic sources. In the design of a Compton telescope the lack of knowledge of the recoil electron

momentum vector allows real photons from other

points on the sky, i.e., on the event circle, to have

the same data signature as a photon from the true

source. The key to future Compton telescope

advances is to increase the instrument’s effective

area but, more importantly, to reduce background

rates, both internal and external.

Although low, significant background exists in

present day Compton telescope data, primarily

from multiple-photon cascades produced by energetic neutrons. These cascades when they take

place near the forward detector can produce signals in both detectors with the proper time-offlight. These prompt cascades coupled with multiple-photon radioactive decays are the dominant

backgrounds in COMPTEL. These background

events fog any good image of the sky, or worse, if

not controlled, produce artifacts. Three measures

individually or together can ameliorate this background. The first is that the track of the recoil

electron in the forward detector can be measured,

thereby collapsing the “event circle” to an arc

segment or a point. This requires tracking technology similar to that being proposed for GLAST or

with liquid-xenon “drift chambers.” This technique

is effective above a few MeV, above the range

where electron scattering is strong. This is generally

above the range of most nuclear lines. At these

energies different production processes in space are

responsible for the emission, so that the objects






ring

Projected Sky

E1

E2

Detector 1

Detector 2




46

that are bright at a few MeV (the continuum) may

not be bright at many tens of MeV, and are likely

different from those emitting nuclear lines. 

The second method of reducing background is

to reduce the width of the “event circle.” Superior

energy resolution is necessary for this technique.

When the full recoil photon energy is absorbed in

the rearward detector, the event circle passes

directly through the incident direction vector and

its small width brings in minimal background. Solid

state detectors, e.g., germanium, are ideal for this.

However, it is important to fully absorb and measure the scattered photon so that the narrow event

circle contributes only to the source and not the

background of other points on the sky and that

event circles from other (sometimes far removed)

points on the sky, likewise, do not interfere with

the source measurement. The fine energy resolution of Ge also would greatly improve the signal-tonoise of diffuse narrow line sources. (The benefit to

point source narrow lines has already been realized

in the narrow event circle and cannot be realized a

second time for point sources.) These first two

methods of reducing background are also accompanied by finer angular resolution performance,

thereby improving the positioning and resolution

of sources while also improving the signal-to-noise

ratio. Angular resolutions of a several arc minutes is

attainable with technology currently under development producing a corresponding improvement

in signal-to-noise ratio. 

Lastly, in any Compton telescope design the

passive material around the primary scatterer must

be minimized, because it is the source of the local

background.

The ideal Compton telescope design would be

in the form of an electron-tracking forward detector with germanium resolution and with the high

density electronics necessary to support the large

number of data channels. Such an ideal configuration may not fall within the constraints of a new

mission, but it is important to note that (1) an electron-tracking Compton telescope will undoubtedly

provide significantly better performance than existing Compton telescopes in the nuclear line region

where electron tracking is not efficient and similarly

(2), a non-electron-tracking high-resolution

Compton telescope will undoubtedly provide significantly better performance in the continuum

regime above the nuclear lines, where electron

tracking is efficient. In this case the choice must be

made based upon the available technology and its

ability to adapt to the wide range of energies,

backgrounds and detection processes that are present in the MeV range. In other words, the different scientific goals of measuring the nuclear lines

and the continuum emission may be achieved by

either an electron tracking telescope or one that

does not perform electron tracking, albeit with different levels of success. This is especially true given

that there are likely few opportunities for measuring intrinsically narrow nuclear lines. 

Figure 5.3. Cross-section of a pair-conversion telescope. The

four basic elements — anticoincidence, pair converter, pair

trackers, and a calorimeter — have not changed in decades.

What has changed is the emergence of new detector

technologies allowing vastly improved sensitivity.

5.1.4 PAIR PRODUCTION TELESCOPES

The observation of high-energy gamma rays is

done indirectly, by detecting the electron and

positron produced when the gamma ray undergoes

pair production in the presence of a nucleus. A

high-energy gamma-ray telescope consists of highZ pair-production material (metal foils) interleaved

with position sensitive charged particle detectors.

The direction and energy of the incident gamma

ray is reconstructed from the direction and energy

of the electron and positron. Thus, a gamma-ray

telescope is, in effect, a track imaging charged particle detector and calorimeter. An anticoincidence is

needed to screen the track imaging detector from

the charged particle cosmic rays which outnumber

the gamma rays by a factor of approximately 104.

Since the electron and positron emanate from a

charged-particle

anticoincidence

shield

pairconversion

foils

particletracking

detectors

calorimeter c+ c–


Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 47

common vertex, giving the inverted V signature of

the gamma-ray conversion to an electron/positron

pair, the two-track resolution of the track imaging

detector is important for both event recognition

and subsequent direction determination. The

instruments which define high energy gamma-ray

astronomy — SAS-2, COS-B, and EGRET — all used

this same basic design. The great advance possible

in this area comes from the application of recent

developments in particle tracking detectors (see

below), along with improvements in on-board processing and telemetry bandwidth. 

5.1.5 ATMOSPHERIC CERENKOV TELESCOPES

At sufficiently high gamma-ray energies, typically

above 100 GeV, the Earth’s atmosphere itself can

be used as part of a gamma-ray telescope. As these

high-energy photons collide with the upper atmosphere, they convert to electron-positron pairs just

as 100 MeV photons do, but these particles are sufficiently energetic to produce a cascade of secondary particles traveling fast enough through the

air to produce a flash of Cerenkov radiation. This

radiation is then detected by large-area optical collectors on the ground. The Whipple Observatory,

CAT, HEGRA, and CANGAROO telescopes are all

active now, and new telescopes (e.g., CELESTE,

STACEE, MILAGRO) begin operation in 1999.

Within a few years it is expected that these will

be joined by VERITAS, HESS, MAGIC and

CANGAROO IV.

5.2 DETECTOR TECHNOLOGIES 

Anumber of detector technologies are used

for gamma-ray imaging and spectroscopy.

Many aspects of such detectors are well

established, such as scintillators and photomultiplier

tubes. Dramatic progress is now obtainable in this

field because of new and developing technologies

influencing all types of gamma-ray imaging. Several

general areas stand out as key new technologies.

5.2.1 STRIP AND PIXEL DETECTORS

5.2.1.1 CDZNTE

CdZnTe detectors are at the threshold of becoming a widely used tool in gamma-ray astronomy.

The basic properties that make them interesting

are: 1) large enough band gap energy (1.6 eV) to

Figure 5.4. This artist’s conception of the proposed VERITAS

instrument is one example of the future direction of groundbased gamma-ray astronomy.

permit room temperature operation; 2) high density (~6 g cm–2) for good stopping power; 3) high

atomic numbers (48 for Cd, 52 for Te) for photoelectric absorption up to high energies (For example, CdZnTe has a photoelectric attenuation coefficient that is more than 10 times the Compton scattering coefficient up to 110 keV compared to 60

keV for Ge and 25 keV for Si.); 4) low bias voltages

of typically 200 volts compared with thousands of

volts for Ge; 5) ease of electrode segmentation for

fine imaging; 6) low susceptibility to contamination problems so that the detectors can be easily

fabricated and handled; 7) availability of suitable

crystals so that multi detector arrays can be fabricated at low cost; and 8) increased resistivity with

introduction of Zn to improve performance over

CdTe (though PIN CdTe and PIN CdZnTe have best

performance so far). The high density and particularly high atomic number of CdZnTe combine to

give several important characteristics. The domination of photoelectric attenuation means that

CdZnTe has single-site absorptions (good for imaging) throughout the low-energy gamma-ray band.

Also, the large attenuation coefficient for photoelectric absorption means that CdZnTe detectors

can be very thin and still efficient at stopping

gamma rays. The typical size of a CdZnTe detector
48

is 1 cm2 in area by 2 mm thickness, with the thickness limited by hole trapping effects. Recently,

however, it has been shown that detectors with

segmented electrodes can achieve reasonable spectroscopy using only the electron signal due to the

“near-field” effect, thus enabling thicker detectors.

The small sizes of CdZnTe detectors means that

large-area detection planes will require arrays with

hundreds or thousands of individual detectors. In

order to keep the electronics power level within

reasonable levels for spaceflight applications, VLSI

front-end amplifiers must be used. Power levels of a

few mWatts per detector are then achievable. The

ability to finely segment the contacts of CdZnTe

detectors, combined with their high photoelectric

attenuation coefficient, means that they are ideal

for high-resolution imagers. Detectors with strip

contacts and with pixel contacts have been fabricated with pitches of 100 micro-m or less.

Applications of such finely segmented detectors

include wide-field coded mask instruments with

better than arcminute (in some cases approaching

arcsecond) angular resolutions and high-sensitivity

focusing hard X-ray telescopes with arcminute resolutions. Examples of instruments that incorporate

CdZnTe or CdTe detectors are: the INTEGRAL

imager; the Swift and EXIST mission concepts; and

several recently-proposed balloon instruments such

as In-Focus.

5.2.1.2 GERMANIUM

Germanium remains the only solid state detector

capable of high-resolution spectroscopy in the

nuclear energy band. It is also the only semiconducting detector that can be made in large volumes, ~10 cm diameter. These are the reasons it

has been used in satellite (HEAO C–1 and INTEGRAL) and balloon missions (Bell/Sandia,

Lockheed/MSFC, GRIS, Hexagone) requiring both

the best possible energy resolution and large collecting areas. However, the next generation of

instrumentation is also going to need fine imaging

capabilities that standard large volume co-axial

detectors do not provide. These instruments must

combine fine angular resolution with high sensitivity well beyond that of INTEGRAL. Planar configurations of germanium detectors with strip or pixel

electrodes are being investigated. Both of these

concepts achieve good position and energy resolution for applications in coded-aperture or Compton

telescope imaging systems. Germanium detectors

with 2-mm spatial resolution have been demonstrated in the laboratory in a device measuring

5x5x1 cm in volume. Even larger devices with finer

spatial resolution are possible. The principle challenge will be to make large arrays of germanium

detectors with high reliability and a reasonable

cost. Production of the detector quality germanium

material is already a routine manufacturing process.

Cost savings will be realized by improvements in

the contact technology and device packaging or

handling procedures. A germanium Compton telescope using such pixelated detectors could achieve

sensitivities 10 – 50 times better than INTEGRAL

and have good sensitivity to both diffuse and

broadened line emissions. This is because the good

energy resolution of germanium also reduces the

background by improving the angular resolution in

Compton telescopes so that the sensitivity

improves proportionally with resolution for point

sources, not simply as the square root of resolution

in an INTEGRAL-like spectrometer.

5.2.1.3 SILICON STRIPS

Silicon microstrip detectors have been developed

at accelerators and are now readily available from

several commercial manufacturers. These devices

are, in effect, big integrated circuits fabricated on

Si. The spatial resolution is determined by the

width of the semiconductor strips fabricated on the

Si. Resolution of 50 m m is easily attainable by

modern photolithography. Because the Si

microstrips are fabricated on thin layers of silicon,

they have very good two track resolution, typically

3 times the strip pitch. Si microstrip detectors are

available in sizes up to ~9 cm x 9 cm, although 6

cm x 6 cm is more common. Si strip detectors are

applicable to both Compton and pair telescopes. 

5.2.1.4 LIQUID XENON

Liquid xenon, with its high density (3 g cm–3)

and high atomic number (54) is a very good detector material for gamma rays. Liquid Xe detectors

can be realized in a variety of configurations, using

either its good ionization (64,000 electrons/MeV)

or scintillation (similar yield as NaI, < 5 ns time

response) properties, or both. In a liquid Xe Time

Projection Chamber (LXeTPC), both the ionization

electrons and the scintillation light signals are

detected to infer the energy deposit (resolution of
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 49

Figure 5.5. A prototype of a Compton scatter telescope with a

liquid Xenon Time Projection Chamber as the upper and lower

layer.

~6 at 1 MeV) and the three-dimensional spatial

coordinates (resolution < 1 mm) of each gammaray interaction. Events with multiple-site interactions are unambiguously recognized and the original gamma-ray direction reconstructed from kinematics, resulting in high detection efficiency for

true source events. The characteristics of a LXeTPC

for gamma-ray imaging and spectroscopy have

been demonstrated with a prototype of 400 cm2

active area and 8 cm thickness. A double-scatter

Compton telescope based on a coincidence of two

liquid TPCs is being studied. With a Liquid Ar TPC

as the low Z converter and Compton electron

tracker, at a fixed distance from a LXeTPC as

calorimeter and imager, a substantial improvement

in sensitivity and angular resolution over COMPTEL

in the 1–10 MeV range could be achieved.

Moreover, such a configuration, with TPCs of ten

times the area of the existing prototype, appears

feasible within a MIDEX mission cost and weight

constraints.

5.2.2 VLSI/ASIC

A characteristic common to many of the current

and future technologies for gamma-ray detector

systems is the large number of channels with relatively small signal outputs. Most such applications

will, therefore, rely on Application Specific

Integrated Circuits (ASICs) and Very Large Scale

Integration (VLSI) techniques in order to keep the

power consumption to a modest level for spaceflight. Although the specific circuits are tailored for

individual applications, general approaches to

designing and building such electronics can be

improved. Any technology developments that

allow faster or cheaper production of these electronics will directly benefit a broad range of

gamma-ray telescope designs.

5.3 COMPUTATIONAL CAPABILITIES

The information explosion that will come

with the next generation of satellite

gamma-ray experiments will put severe

demands on the current computing capabilities

both on-board and ground-based. Increased data

rates, even without better resolution, will demand

computational upgrades. Increased angular resolution will force better image analysis, better time

resolution will drive deeper pulsar and quasi-periodic searches and more sophisticated spectral techniques will require more complicated analysis routines. Fortunately, this is a field which can be

expected to rapidly improve with time so that it is

only necessary to ensure that state-of-the-art computer facilities are available for gamma-ray data

analysis.
50
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 51

CHAPTER

6

Supporting Programs

The scientific goals of the gamma-ray astronomy program cannot be achieved without

support beyond mission opportunities.

Supporting programs in theory, data analysis, and

other complementary disciplines are vital to achieving the breakthroughs possible in the next decade.

6.1 DATA ANALYSIS

The recent exciting discoveries in gamma-ray

astrophysics are the results of detailed data

analysis and correlative studies of past and

currently operating U.S. and foreign missions, as

well as theoretical studies. However, MO & DA

funding is presently inadequate to cover, for example, the CGRO mission, and much excellent science

will simply not be done with this great observatory

in the coming years. The GRAPWG recomments

augmenting the MO & DA funding, and also

strongly supports both the Astrophysics Data

Program, and the Long-Term Space Astrophysics

Program, which are key elements in many correlative, multi-wavelength, and single mission data

analysis efforts. To put these results into context,

theoretical investigations into the nature of exotic

high-energy sources is also essential. 

6.2 THEORY

Gamma-ray observations probe exotic

physics from remarkable, energetic

sources. However, the nonthermal nature

of the emission, the modest photon statistics and

the need to connect the high-energy radiations

with lower energy observations makes progress in

the field particularly dependent on adequate

theoretical support. In turn the puzzles posed by

high-energy observations have spurred a ferment of

theoretical activity, as exemplified by the continuing stream of papers on gamma-ray burst models.

As discussed earlier, many CGRO observations

remain unexplained. Late in the CGRO era, support

for theoretical work on high-energy problems is

becoming very limited. The NASA theory program

plays an important role, but experiences extreme

pressure from other disciplines. Because new

understanding spurred by CGRO and other recent

missions offers hope of important advances in our

understanding of compact objects and other highenergy sources, expanded support of theoretical

work in this area can provide important progress in

the post-CGRO era. It will also be important to

continue to refine theoretical predictions, looking

forward to the sensitive observational tests of

future missions.

6.3 GROUND BASED 

Many ground-based “third-generation”

atmospheric Cerenkov systems are now

under consideration which will have

sensitivity in the 10–100 GeV energy range. The

energy threshold varies inversely as the product of

the square root of the total mirror area, the light

collection efficiency and the quantum efficiency of

the detectors. Hence if a threshold of 200 GeV can

be achieved with a 10m aperture reflector, then a

threshold of 20 GeV is feasible with a reflector with

an effective aperture of 100m. In practice an array

of detectors offers better background rejection and

more economical construction than a single reflector. One such approach is VERITAS (Very Energetic
52

Radiation Imaging Telescope Array System); this is a

logical development of the imaging atmospheric

Cerenkov concept and consists of an array of seven

telescopes of 10m aperture each closely based on

the proven design of the Whipple 10m optical

reflector. This array would easily reach a threshold

of 50 GeV with conventional photomultipliers; with

advanced technology detectors it could be as low

as 30 GeV. Its flux sensitivity for discrete sources

would be very competitive with planned high-energy gamma-ray space missions i.e., 2x10–12 photons

cm–2 s–1 at 100 GeV. It would also have excellent

spectral resolution. VERITAS will be built in Southern

Arizona; similar arrays will be built in the southern

hemisphere: the German-French HESS in Namibia

and Japanese-Australian Super-CANGAROO in

Australia.

The VERITAS approach is

not unique among atmospheric Cerenkov observatories proposed as a next generation system although it is

probably the most conservative and predictable. Other

approaches include a large,

steerable, single dish (17 m

aperture) with a high resolution camera (MAGIC), and

the Solar Array approach

whereby existing arrays of

heliostats (built as solar

energy collectors) are utilized as large area light collectors with a central detector (CELESTE, STACEE and

Solar-Two). Mention should

also be made of large water

Cerenkov systems e.g.,

MILAGRO which are most

useful as burst monitors and for all-sky surveys.

Developments in space-and ground-based detector

technology have an obvious impact on one another; ground-based detectors will rely on space missions for selection of suitable sources and for all-sky

monitoring of source activity. In return, groundbased observation can supply improved spatial

localization, high-energy spectrum measurements

and high count statistics to probe short-time variability. It is clearly advantageous in the planning of

future missions/telescopes that the development of

the overlapping techniques proceed in parallel.

Ground-based observatories will continue to operate after the demise of EGRET and will thus provide

continuity in the field; they will be continually

upgraded to achieve maximum sensitivity before

the launch of an EGRET successor.

Figure 6.1. A view of the HEGRA gamma-ray telescope. Ground based facilities continue to multiply

and improve.
Recommended Priorities for NASA’s GammaRay Astronomy Program 53

CHAPTER

7

Education and Public Outreach

Since its inception, one of NASA’s mandates is

to disseminate the results of its programs to

the general public. Because of people’s

inherent interest in astronomy and spaceflight,

NASA has been particularly successful in this outreach endeavor. Programs in gamma-ray astronomy

are making a significant contribution to NASA’s

goals of education and to raising the level of public

understanding and appreciation of science and

technology. As our knowledge of the high-energy

sky increases, so does our ability to communicate

the promise and excitement of gamma-ray astronomy. The mystery of gamma-ray bursts, direct measurement of ongoing galactic nucleosynthesis, and

observations of exotic objects such as black holes

are examples of fields to which gamma-ray astronomy contributes.

Public information flourishes as a result of the

many discoveries and new mysteries that have

accompanied the growth of gamma-ray astrophysics in the last five years. This includes public

talks by leading scientists at museums and planetariums around the country and front page articles

in newspapers and magazines. More concerted

efforts at public outreach activities in the CGRO era

alone include posters and brochures put together

by the CGRO Science Support Center and other

organizations as well as numerous World Wide Web

pages created by groups and individuals on specialized topics. A well-received exhibition at the

Smithsonian Air and Space Museum is another

example. Continuing this legacy of public outreach

should be an important component of the gammaray astronomy programs in the future. Gamma-ray

astronomy has touched education in a number of

ways. Special grants to use the success of CGRO for

educational purposes under the IDEAS program,

from elementary to high schools, to college undergraduate and graduate education have also been

highly successful. In addition, efforts such as the

HEASARC’s Learning Center page are beginning to

present gamma-ray astronomy resources to

younger students using the Internet. We encourage

the use of add-on grants for educational purposes

as an effective way to help active researchers contribute to science education.

Figure 7.1. Outreach efforts such as this CD version of a NASA web

site are increasingly teaching students and educators about the

science and opportunity of gamma-ray astronomy.
54
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 55

CHAPTER

8

Recommended Program

The preceeding chapters have described the

scope and status of gamma-ray astronomy.

This field is, of course, part of a broader

enterprise — that of Space Sciences as a whole.

While gamma-ray astronomy has its own history

and logic, it must also be assessed as part of this

wider effort. To that end, the goals and priorities

presented in the Executive Summary are repeated

here, with an eye to their relevance to the overall

goals of NASA’s Space Sciences Program.

Our reference point is the 1997 Space Science

Strategic Plan (http://spacescience.nasa.gov/strategy/1997/sseplan.htm). This represents a coherent

summary of the goals of NASA’s science themes. In

particular, the list of Science Goals in this plan

lends itself to a direct comparison with our priorities. The NASA Science Goals are:

1. Understand how structure in our Universe.(e.g.,

clusters of galaxies) emerged from the Big

Bang.

2. Test physical theories and reveal new phenomena throughout the Universe, especially

through the investigation of extreme

environments.

3. Understand how both dark and luminous matter determine the geometry and fate of the

Universe.

4. Understand the dynamical and chemical evolution of galaxies and stars and the exchange of

matter and energy among stars and the interstellar medium.

5. Understand how stars and planetary systems

form together.

6. Understand the nature and history of our Solar

System, and what makes Earth similar to and

different from its planetary neighbors.

7. Understand mechanisms of long- and shortterm solar variability, and the specific processes

by which Earth and other planets respond.

8. Understand the origin and evolution of life on

Earth.

9. Understand the external forces, including

comet and asteroid impacts, that affect life and

the habitability of Earth.

10. Identify locales and resources for future human

habitation within the solar system.

11. Understand how life may originate and persist

beyond Earth.

As reflected in the Executive Summary, the

recommendations of the GRAPWG are divided into

three categories: science priorities, mission priorities, and other recommendations. Below, we

further describe these priorities and indicate which

of NASA’s Science Goals are addressed by each.

8.1 1997 GRAPWG MISSIONS

In the previous GRAPWG report, the top-priority

was given to the GLAST high energy gamma-ray

mission to follow on the discoveries of the EGRET

instrument on CGRO. Other high priority missions

were a focusing hard X-ray telescope and a nextgeneration nuclear line and MeV continuum mission. For Explorer-class missions the top scientific

opportunities were found to be for gamma-ray

burst observations and hard X-ray surveys. Some of
56

these missions have now been started by NASA:

GLAST is in the OSS strategic plan for new start in

2002; the Swift gamma-ray burst MIDEX has been

selected for Phase A study with final selection of

two missions to fly out of five studies to be made in

September 1999. Also, the OSS strategic plan

contains the top mission of the X-ray community,

Constellation X, which has a focusing hard X-ray

telescope (HXT) onboard that achieves some of the

objectives identified by the GRAPWG for a focusing

hard X-ray mission.

The GRAPWG finds that the scientific case for

GLAST, Constellation-X HXT and Swift has grown

since 1997. The GRAPWG continues to give its

STRONGEST ENDORSEMENT to these mission,

which are the backbone of NASA’s future program

in hard X-ray and gamma-ray astronomy.

8.2 TOP PRIORITY SCIENCE TOPICS

The GRAPWG identifies the following PRIORITIZED list to be the most compelling science topics

that future hard X-ray and gamma-ray missions

can address beyond those covered by GLAST,

Constellation-X HXT and Swift. These are areas in

which hard X-ray and gamma-ray astronomy offers

unique capabilities for advancing our understanding

of the universe. Each science topic is followed by a

list of areas in which key contributions are expected.

The HIGHEST PRIORITY science topic is:

1. NUCLEAR ASTROPHYSICS: SITES OF GAMMA RAY

LINE EMISSION

Gamma-ray astronomy holds the promise of revolutionizing studies of nucleosynthesis in our galaxy

and beyond. Through the detection of nuclear lines,

sites of nucleosynthesis can be studied and elemental abundances can be measured. In addition, the

configuration and dynamics of the emitting gas can

be determined. Topics for future missions include:

• Abundance yields of explosive nucleosynthesis

• Mass cut between SN ejecta and core

• Supernova and nova explosion physics and

dynamics

• Sites of nucleosynthesis in the Galaxy and universe

• Cosmic nucleosynthesis rate from redshifted

SN Ia lines

• Supernova rate in the Galaxy

• Better understanding of SN Ia cosmological

distance scale calibration

• Cosmic ray interactions with interstellar gas

• Positron diagnostics of compact objects

Other high priority topics are:

2. GAMMA RAY BURSTS

Appropriate to their nature, gamma-ray burst

studies continue to change quickly and dramatically. The increasing number of counterparts at

lower energies when coupled with the impressive

BATSE database are leading to a new era in GRB

studies. Aside from the intrinsic astrophysics of

GRB’s, bursts will become an important probe of

the early universe. NASA Science Goals (1), (2) and

(4) are related to GRB studies. Topics for future

missions include:

• Links to star formation

• Evolution and populations of massive stars

• Possible sites of black hole formation

• New GRB populations and mechanisms

• Probes of dusty matter in distant galaxies

• Probes of the intergalactic medium out to high

redshift

3. HARD X-RAY EMISSION FROM ACCRETING BLACK HOLES

AND NEUTRON STARS

Hard X-ray and gamma-ray studies of accreting

sources are becoming increasingly critical for full

understanding of these objects. Detections of

galactic and extragalactic black hole systems at

high energies provide a laboratory for studying

black holes across a wide range of masses. Topics

for future missions include:

• First population study of absorbed Seyfert 2’s

• Constraints on blazar spectra and diffuse IR background

• Non-thermal components in galactic transients

• Jets associated with galactic BH’s and AGN

• Black hole parameters (spin, mass)

• Accretion physics

4. MEDIUM ENERGY (500 KEV–30 MEV) EMISSIONS:

Distinct from nuclear lines, the continuum emission in the medium energy range has been shown
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 57

to be important for understanding nonthermal

emission from objects such as pulsars and AGN and

sites of cosmic ray interaction with gas. This relatively unexplored band ties together studies at MeV

and GeV energies. NASA Science Goals (2) and (4)

are addressed by this science. Topics for future missions include:

• Search for MeV blazars and spectral studies to

understand emission

• Pulsar physics through broad-band spectral

studies

• Components of diffuse galactic emission

• Extragalactic diffuse emission in poorly measured

MeV band

• Nonthermal components from accretion-driven

sources

• Cosmic ray interactions with the ISM

8.3 TOP PRIORITY MISSION

RECOMMENDATIONS

Figure 1 shows the mission roadmap that the

GRAPWG has developed for hard X-ray and

gamma-ray astronomy. In addition to the three

missions mentioned above, the GRAPWG finds

three near-term missions and two long-term concepts to be the most exciting for addressing our

top-priority science topics. These are as follows.

ADVANCED COMPTON TELESCOPE (ACT)

The HIGHEST PRIORITY major mission recommended by the GRAPWG is ACT, a high-technology

MeV line and continuum Compton telescope mission operating in the 500 keV to 30 MeV range.

With a factor of 30 improvement in sensitivity compared to CGRO and INTEGRAL, it promises detailed

studies of sites of nucleosynthesis in the universe

and a deep survey of continuum sources. The

optimum configuration of large imaging detector

arrays based on either semiconductor or high density rare gases is being studied to enable a mission in

this challenging energy band. The mission addresses

science areas (1), (2) and (4) in the above list.

Two other high-priority missions are of particular

interest in the coming decade for accomplishing

our science goals.

HIGH-RESOLUTION SPECTROSCOPIC IMAGER (HSI)

A high priority intermediate or enhanced MIDEX

class mission recommended by the GRAPWG is HSI,

a focusing optics telescope operating in the 10 to

170 keV range. With a factor of 100 improvement

in sensitivity compared to RXTE, this mission will

answer key questions on the nature of accretion

onto neutron stars and black holes and will allow

detailed studies of sites of nucleosynthesis in the

universe. New multilayer mirror technology will

enable the upper energy bound of the mirrors to

be as high as 200 keV. The mission addresses science areas (1) and (3).

ENERGETIC X-RAY IMAGING SURVEY TELESCOPE (EXIST)

A high priority intermediate or enhanced MIDEX

class mission recommended by the GRAPWG is

EXIST. A factor of 100 improvement in sensitivity

compared to the only previous all-sky hard X-ray

survey (HEAO-1) will allow discovery of the predicted, but so-far unobserved, class of absorbed

Seyfert 2’s that are thought to make up half of the

total inventory of AGN’s. A large area array of newtechnology solid state detectors, used in conjunction with a wide field-of-view coded aperture, will

cover the 10-500 keV region and address science

areas (2) and (3). The International Space Station is

a possible platform for such an instrument.

Complementing these missions will be projects

which will extend and improve upon those already

in the strategic plan.

NEXT GENERATION GAMMA-RAY BURST MISSION

(NGGRB)

The GRAPWG believes that gamma-ray bursts

will continue to be one of the most important and

fascinating areas of astronomical research for tens

of years to come. A mission will be needed in the

post HETE-II and Swift era to further this field.

Emphasis in that time frame may involve observations of nonelectromagnetic radiation such as

gravitational waves and neutrinos and will certainly

involve multiwavelength electromagnetic instrumentation. To correlate these data with known

properties of bursts and to monitor the sky for

infrequent special events, it will be essential to have

a continuous gamma-ray burst monitor in space.

The GRAPWG recommends that such a mission,

NGGRB, be identified in NASA’s program.
58

NEXT GENERATION HIGH-ENERGY GAMMA-RAY MISSION

(NGHEG)

The discoveries of GLAST will produce strong

interest in the astronomical community in high

energy gamma-ray phenomena and will undoubtedly raise new fundamental questions. The band

width of the high energy range is huge, from 30

MeV to 300 GeV, and overlaps with the growing

number of very high energy (TeV and PeV) groundbased observatories. The GRAPWG recommends

that a mission called NGHEG be identified in

NASA’s program to follow on GLAST. GLAST will

address NASA Science Goals (1), (2), (3), (4)

and (7).

8.4 GAMMA-RAY BURSTS

The GRAPWG is particularly intrigued by the

gamma-ray burst problem and the promise that

bursts offer for fundamental studies in astrophysics.

There are multifaceted implications that bursts have

on many future missions. Below are listed some

topics and recommendations on gamma-ray burst

astronomy. What follows benefits NASA Science

Goals (1) and (2):

• HETE-II should be flown on schedule.

• The Swift GRB MIDEX mission, now in Phase A,

should have high priority for flight.

• Support should continue for the Interplanetary

Network as an effective means for deriving

arcminute GRB locations.

• Support should continue for BATSE and the

Gamma-ray burst Coordinate Network (GCN).

• The GRB monitor currently planned for GLAST

will greatly enhance its GRB capabilities.

• Synergism between space-borne GeV GRB observations and ground-base TeV observations should

be recognized and exploited.

• A global network of small, dedicated GRB robotic

telescopes should be developed.

• Support for a network of co-ordinated 1–3 meter

telescopes to establish lightcurves over the first

few days.

• Time on major ground- and space-based observatories should continue to be provided for GRB

follow-up observations.

8.5 OTHER RECOMMENDATIONS

Gamma-ray astrophysics is a broad enterprise

covering many efforts. The GRAPWG recommends

that the following items receive special consideration:

• Technology Development: Many exciting new

technologies are arising in gamma-ray astronomy,

including multilayer mirrors, Laue lenses (Bragg

concentrators), complex coded masks, solid-state

pixel and strip detectors, rare gas and liquid

detectors, and VLSI electronics. These form the

backbone and future of our field. The GRAPWG

strongly recommends a vigorous program of

technology development for hard X-ray and

gamma-ray astronomy.

• TeV Astronomy: Aside from their independent

successes, ground-based observatories will provide an important complement to future high

energy gamma-ray missions such as GLAST. The

GRAPWG endorses the continued development of

TeV telescopes with low energy thresholds.

• Balloon Program: The ultra-long duration balloon

(ULDB) program offers great potential for both

instrument development and significant science

in gamma-ray astronomy. The GRAPWG recommends strong NASA support for LDB’s and ULDB’s.

• International Space Station: The GRAPWG

views the ISS as an opportunity for gamma-ray

research. It is particularly well suited for wide-field

instruments and long-term monitors.

• Optical Telescope Support: Many areas of

gamma-ray astronomy research, particularly

gamma-ray bursts and AGN studies, benefit from

a multiwavelength approach. In particular, optical

telescopes can provide important monitoring

capabilities which are difficult to achieve at other

wavelengths. The development of a network of

optical telescopes capable of near-continuous

observation of gamma-ray transients is supported

by the GRAPWG.

• Data Analysis and Theory: Making the most of

the rich databases expected from future missions

is an important concern of the GRAPWG.

Adequate support for data analysis and theory is

a cost effective way of maximizing the return

from current and future experiments. 
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program: 59

IMAGE CREDITS

Figure 1.1: BeppoSAX instrument team

Figure 1.2: M. Catanese, Iowa State University (for

the Whipple Observatory Collaboration).

Figure 1.3: EGRET Team (Goddard Space Flight

Center)

Figure 1.4: D. Macomb (USRA/Goddard Space

Flight Center)

Figure 2.1: A. Iyudin & V. Schonfelder (MPE)

Figure 2.2: U. Oberlack (MPE for the CompTel

Collaboration)

Figure 2.3: Goddard Space Flight Center, NASA

Figure 2.4: Dr. Robert Mallozzi (University of

Alabama in Huntsville, and Marshall Space Flight

Center.)

Figure 2.5: C. Kouveliotou (USRA/Marshall Space

Flight Center)

Figure 2.6: M. McConnell (University of New

Hampshire for the CompTel Team)

Figure 2.7: D. Thompson (Goddard Space Flight

Center)

Figure 2.8: R. Romani (Stanford University)

Figure 2.9: G. Madejski (UMD/GSFC)

Figure 2.10: Zdziarski (N. Copernicus

Astronomical Center)

Figure 2.11: S. Digel (RITSS/GSFC), image from

Condon, J.J., et al. 1998, AJ, 115, 1693

Figure 2.12: M. Briggs (Marshall Space Flight

Center)

Figure 2.13: Steve Odewahn, Josh Bloom and

the Caltech-CARA-NRAO GRB Collaboration

Figure 3.1: D. Dixon (University of California,

Riverside)

Figure 3.2: D. Dixon (University of California,

Riverside) & W.R. Purcell (Northwestern

University)

Figure 3.3: SIGMA Team (SAp, Saclay; CESR,

Toulouse; IKI, Moscow)

Figure 3.4: BeppoSAX instrument team

Figure 3.5: HESSI Instrument team

Figure 3.6: M. Tavani (Istituto de Fisica Cosmica,

CNR, Milano)

Figure 3.7: T. Yoshikoshi (for the CANGAROO collaboration)

Figure 3.8: J. Tueller (Goddard Space Flight

Center)

Figure 4.1: S. Digel (RSTX/Goddard Space Flight

Center)

Figure 4.2: F. Harrison (California Institute of

Technology)

Figure 4.3: F. Harrison (California Institute of

Technology)

Figure 4.4: F. Harrison (California Institute of

Technology)

Figure 4.5: F. Harrison (California Institute of

Technology)

Figure 4.6: J. Grindlay (Harvard)

Figure 4.7: J. Grindlay (Harvard)

Figure 4.8: W. N. Johnson (Naval Research

Laboratory)

Figure 4.9: W. N. Johnson (Naval Research

Laboratory)

Figure 4.10: W. N. Johnson (Naval Research

Laboratory)

Figure 4.11: N. Gehrels (Goddard Space Flight

Center)

Figure 5.1: INTEGRAL/IBIS instrument team

Figure 5.2: W.N. Johnson (Naval Research

Laboratory)

Figure 5.3: P. Michelson (Stanford)?

Figure 5.4: Whipple Collaboration

Figure 5.5: E. Aprile (Columbia University)

Figure 6.1: Courtesy of the HEGRA collaboration

Figure 7.1: K. Smale (RSTX/ Goddard Space Flight

Center)
60

ACKNOWLEDGEMENTS

This document was written by the GRAPWG committee members in consultation with many colleagues in

the astronomy and physics communities. While we cannot name them all here, their contributions to this

publication and the GRAPWG discussions were invaluable. In particular, we wish to thank Paul Hertz,

Lou Kaluzienski and Don Kniffen of NASA Headquarters. The fine graphics work by Jeff Walker (Sutter Design)

is greatly appreciated, as was the hard work and advice of Carol Ladd (GSFC).
Recommended Priorities for NASA’s Gamma-Ray Astronomy Program 61

INDEX

3C 279 . . . . . . . . . . . . . . . . . . . . . . . 14

4U 0614+09 . . . . . . . . . . . . . . . . . . . . 9

511 keV line . . . . . . . . 7-8, 13, 25-26, 42

ABRIXAS . . . . . . . . . . . . . . . . . .31, 34-35

AGILE . . . . . . . . . . . . . . . . . . . . . . . . .25

ASCA . . . . . . . . . . . . . . . . . . . . . .13, 35

AXAF . . . . . . . . . . . . . . . . . . . . . . . . .31

accretion . . . 0,3,8-10,13,15,28,35,56-57

Active Galactic Nuclei (also AGN)

 . . . . 2, 12-16, 25, 29, 31, 33-35,37-39,

41, 56-58

Advanced Compton Telescope

(also ACT) . . . . . . . . . . . .36-37, 39-457

Andromeda . . . . . . . . . . . . . . . . . . . . 28

annihilation radiation . . . 7, 21-22, 38-39

Apollo 16/17 . . . . . . . . . . . . . . . . . . . .15

Atmospheric Cherenkov Imaging

Technique (also ACIT) . . . . . . . . . . . .26

BATSE . . . . . . . . . . 1-3, 9, 16-18, 21, 23,

35-36, 456, 58

Balloon Program . . . . . . 26-27, 41-42, 58

BeppoSAX (also SAX) . . . . . . 1, 9, 13-14,

16-18, 23-24, 26, 34-35, 459

black hole (also BH)

 . . . . . . . 2-3, 6-9, 12, 17, 23, 34-35, 56

blazar . . . . . . . . . . . 4, 12, 36, 39, 41, 56

CANGAROO . . . . . . . . . . . . . . 26, 47, 59

CAS A . . . . . . . . . . . . . . . . . 6, 31-33, 37

CAT . . . . . . . . . . . . . . . . . . . . . . . . . . 47

CATSAT . . . . . . . . . . . . . . . . . . . . 24, 40

CELESTE . . . . . . . . . . . . . . . . . . . . 47, 52

CGRO (or Compton Gamma-Ray

Observatory . . . . 1-4, 6-7, 9, 12-13, 15

18-23, 26, 37, 442, 45, 51, 53, 55, 57

COMPTEL . . . . . . . . . 3, 6-8, 115, 18-19,

21, 34, 36-445, 49, 59

COS-B . . . . . . . . . . . . . . . . . . . . . . 2, 47

CZT . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Cen A . . . . . . . . . . . . . . . . . . . . . . . . 16

cluster . . . . . . . . . . . . . . . . . . . 5, 14, 33

coma cluster . . . . . . . . . . . . . . . . . . . .14

compton scatter . . . . . . . . 37-39, 45, 49

cosmic ray . . . . . 8, 15, 38-39, 41, 56-57

Constellation X (also Con-X) . . . . . . .356

Crab . . . . . . . . . . . . . . . . . . . . 10-11, 25

cyclotron line (also cyclotron feature) . . . 9

data analysis . . . . . . . . . . . . 8, 49, 51, 58

diffuse galactic emission . . . . . . 8, 11, 57

EGRET . . . . . . . 2-3, 11-12, 14-16, 18-19,

21, 25, 29, 34, 41, 47, 52, 55, 59

ESA . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Energetic X-Ray Imaging Survey Telescope

(EXIST) . . . . . . . . . . . 34-35, 44, 48, 57

extragalactic diffuse emission (also diffuse

gamma ray background) . . . . . . . 5, 57

GLAST . . . . . . . . . 16, 18, 25, 29-335-36,

41, 45, 55-56, 58

GRANAT . . . . . . . . . . . . . . . . . . 3, 19, 22

GRAPWG . . . . . . . .27, 31, 36, 41-42, 51,

55-58, 60

GRB 970228 . . . . . . . . . . . . . . . . . . . . 1

GRB 970228 . . . . . . . . . . . . . . . . . . . . 1

GRB 971214 . . . . . . . . . . . . . . . . . . . 24

GRB 990123 . . . . . . . . . . . . . . .16-17, 41

GRB 990123 . . . . . . . . . . . . . . .16-17, 41

GRB 960720 . . . . . . . . . . . . . . . . . . . .17

GRB 970111 . . . . . . . . . . . . . . . . . . . .17

GRB 970402 . . . . . . . . . . . . . . . . . . . .17

GRB 970508 . . . . . . . . . . . . . . . . . . . .17

GRB 970616 . . . . . . . . . . . . . . . . . . . .17

GRB 970815 . . . . . . . . . . . . . . . . . . . .17

GRB 970828 . . . . . . . . . . . . . . . . . . . .17

GRB 971024 . . . . . . . . . . . . . . . . . . . .17

GRB 971227 . . . . . . . . . . . . . . . . . . . .17

GRB 980109 . . . . . . . . . . . . . . . . . . . .17

GRB 980326 . . . . . . . . . . . . . . . . . . . .17

GRB 980329 . . . . . . . . . . . . . . . . . . . .17

GRB 980425 . . . . . . . . . . . . . . . . . . . .17

GRB 980515 . . . . . . . . . . . . . . . . . . . .17

GRB 980519 . . . . . . . . . . . . . . . . . . . .17

GRB 980613 . . . . . . . . . . . . . . . . . . . .17

GRB 980703 . . . . . . . . . . . . . . . . . . . .17

GRB 980706 . . . . . . . . . . . . . . . . . . . .17

GRB 981220 . . . . . . . . . . . . . . . . . . . .17

GRB 981226 . . . . . . . . . . . . . . . . . . . .17

GRB 990217 . . . . . . . . . . . . . . . . . . . .17

GRO J1655-40 . . . . . . . . . . . . . . . . . . . 2

GRS 1915+105 . . . . . . . . . . . . . . . . . 2, 8

GX 1+4 . . . . . . . . . . . . . . . . . . . . . 9, 26

galactic center . . . . . . . . . . 7, 22, 26, 44

gamma ray burst (also GRB) . . . . 1, 16-18,

21, 23-25, 35-36, 40-41, 56, 58-59

Gamma-Ray Burst Coordinate Network

(also GCN) . . . . . . . . . . . . . .12, 23, 58

Geminga . . . . . . . . . . . . . . . . . . . .2, 115

Germanium . . . 22, 25, 32, 37-39, 46, 48

HEAO-1 . . . . . . . . . . . . . . . . . . . . 34, 57

HEASARC . . . . . . . . . . . . . . . . . . . . . . 53

HEGRA . . . . . . . . . . . . . . . . . . 47, 52, 59

HESS . . . . . . . . . . . . . . . . . . . . . . 47, 52

HESSI . . . . . . . . . . . . . . . . . . 224-25, 59

HETE-II . . . . . . . . . . . . . . . . . 24, 457-58

HXT . . . . . . . . . . . .0 , 29, 31-32, 36, 56

High-Resolution Spectroscopic Imager

(also HSI) . . . . . . . . . . . . . . . 32-33, 57

IBIS . . . . . . . . . . . . . . . . . 22, 24, 44, 59

INTEGRAL . . . . . . . . 2-3, 6-9, 11, 22, 24,

336-37, 44, 48, 57, 59

intergalactic medium . . . . . . . . . . . . . 56

Interplanetary Network

(also IPN) . . . . . . . . . . . 17, 23-24, 458

interstellar medium . . . . . . . 8, 21, 38, 55

IR background

(or infrared background) . . . . . . 35, 56

LOTIS . . . . . . . . . . . . . . . . . . . . . . . . 24

Large Magellanic Cloud

(also LMC) . . . . . . . . . . . . . . . . 6, 7, 30

liquid Xenon . . . . . . . . . . . . . . . . . 48-49

Line Emission . . . . . . . . 3, 5-6, 8, 112-13,

19-222, 33, 37-38, 41-42, 56

long duration balloon (LDB)

 . . . . . . . . . . . . . . . . 26-27, 36, 41, 58

MAGIC . . . . . . . . . . . . . . . . . . 26, 47, 52

MIDEX . . . . . . . . . 24, 26, 35, 449, 56-58

MKN 421 . . . . . . . . . . . . . . . . . . . . . . 14

MO&DA . . . . . . . . . . . . . . . . . . . . . . 51

MRK 501 . . . . . . . . . . . . . . . . . . . . . . . 2

magnetar . . . . . . . . . . . . . . . . . . . . 9-11

Mars Surveyor . . . . . . . . . . . . . . . . . . 23

massive star . . . . . . . . . . . . . . . 3, 6-7, 27

Milagro . . . . . . . . . . . . . . . . . 26, 47, 52

Molecular cloud . . . . . . . . . . . . . . . . . . 8

multilayer mirror . . . . . . . . . . . . . . . . 57

NEAR (Near Earth Asteroid Rendezvous)

 . . . . . . 8-11, 15, 19, 21, 23, 32, 39, 45

NGC 4945 . . . . . . . . . . . . . . . . . . 13, 31

neutron star (also NS)

 . . . . . . . . . . . . 2, 8-12, 15, 23, 28, 48

Next Generation Gamma-Ray Burst

Mission (also NGGRB) . . . . . . . . . . 457

Next Generation High-Energy Gamma-Ray

Mission (also NGHEG) . . . . . . 41, 57-58

normal galaxies . . . . . . . . . . . . . . .15, 30

Nova . . . . . . . . . . . . . . . . . . . . . 6, 8, 56

Nova Muscae 91 . . . . . . . . . . . . . . . . . 8

Nuclear Astrophysics . . . . . .32, 36-37, 56

nucleosynthesis

 . . . . . . 3-8, 21-22, 27, 32, 37, 53, 56-57

OSSE . . . . 3, 6, 13, 15, 19, 21-22, 34, 37

optical telescope . . . . . . . . . . . . . . . . 58

PHEBUS . . . . . . . . . . . . . . . . . . . . . . . 19

PKS 0528+134 . . . . . . . . . . . . . . . . . . 14

pair production . . . 14, 25, 29, 39, 43, 46
62

pulsar . . . . . . 2-3, 6, 9-12, 23, 25, 35-36,

49, 57

RE J0317-853 . . . . . . . . . . . . . . . . . . . 10

ROSAT . . . . . . . . . . . . . . . . . 113, 31, 34

ROTSE . . . . . . . . . . . . . . . . . . 17, 24, 41

RXTE (or Rossi X-Ray Timing Explorer)

 . . . . . . . . . . . . . . 9, 13, 17, 22-23, 57

SAS-2 . . . . . . . . . . . . . . . . . . . . . . . 2, 47

SGR 1900+14 . . . . . . . . . . . . . . . . . 9-10

SIGMA . . . 22, 26-27, 333-34, 36, 44, 59

SMEX . . . . . . . . . . . . . . . . . . . . . . . . 27

SMM . . . . . . . . . . . . . . . . . . . . . . 15, 19

SN 1987A . . . . . . . . . . . . . . . 5-6, 32, 42

SN 1998bw . . . . . . . . . . . . . . . . . . . . 17

SPI . . . . . . . . . . . . . . . . . . . . . . . . . 7, 22

STACEE . . . . . . . . . . . . . . . . . . . . 47, 52

scintillating fiber . . . . . . . . . . . . . . . . . 29

Seyfert . . . . . . . . 3, 13, 15, 33, 39, 56-57

Silicon strips . . . . . . . . . . . . . . . . . . . . 48

Small Magellanic Cloud (also SMC) . . .7, 30

Soft Gamma Repeater (also SGR) . . . 9-10

Solar . . . . . . . 5-7, 19-222, 24-25, 52, 55

Solar flare . . . . . . . . . . . . . . . . . . . 19-20

Space Station (also International

Space Station) . . . . . . . . . 35, 42, 57-58

star formation . . . . 6-8, 17, 27, 35-38, 56

Super-Cangaroo . . . . . . . . . . . . . . . . . 52

Supernova (also SN) . . . . . 2-3, 5-7, 115,

17-18, 21, 26, 30-31, 33, 36-38, 

40-42, 56

supernova remnant (also SNR)

 . . . . . . . . . 6, 15, 26, 31-32, 36, 38, 40

Swift . . . . . . 24, 36, 40-41, 44, 48, 56-58

TAROT . . . . . . . . . . . . . . . . . . . . . . . . 24

TeV Astronomy . . . . . . . . . . . . . . . . . . 58

TeV telescope

 . . . . . . . . .2, 11, 14, 18, 25-26, 35, 58

theory . . . . . . . 11, 18, 27-28, 31, 51, 58

transients . . . . . . 1-2, 12, 15, 21, 23, 25,

35, 56, 58

Tycho . . . . . . . . . . . . . . . . . . . . . . . . 31

ULDB . . . . . . . . . . . . . . . . . . . 27, 36, 58

UNEX . . . . . . . . . . . . . . . . . . . 27, 41-42

Ulysses . . . . . . . . . . . . . . . . . . . . . . . . 23

VERITAS . . . . . . . . . . . . . . . 35, 47, 51-52

VLSI . . . . . . . . . . . . . . . . . . . . 48-49, 58

VLSI . . . . . . . . . . . . . . . . . . . . 48-49, 58

Vela pulsar . . . . . . . . . . 3, 6, 126, 38, 40

Vela Region . . . . . . . . . . . . . . . . . . . . . 3

Virgo . . . . . . . . . . . . . . . . . . . . 5, 33, 37

WIND . . . . . . . . . . . . . . . . . . . 7, 11, 23

Whipple Observatory . . . . . . . 26, 47, 59

white dwarf . . . . . . . . . . . . . . . . . 5-6, 10

XMM . . . . . . . . . . . . . . . . . . . . . . . . . 31