Preliminary results fromgamma-ray observations with theCALorimeteric Electron Telescope (CALET)
Y.Asaoka for the CALET Collaboration
RISE, Waseda University
2016/12/15 CTA-Japan Workshop“The extreme Universe viewed in very-high-energy gamma rays 2016”

CALET Collaboration

Contents
1. Introduction– What’s CALET

2. Gamma-ray Observation with CALET– Electromagnetic Counterpart Search ofGravitational Wave Event GW151226 in GeV
Gamma Rays

3. Summary & Prospects

What’s CALET• highest energy region
accessible from space– long duration
observation onboard theinternational spacestation (ISS)– instrument featuring thethick calorimeter of 30
radiation length– specialized to electron
measurements– continuous observation
since October 2015

• Direct and indirect
measurements of cosmic
rays are complementary
to each other.

CALET System Overview(CAL)(*) JEM stands for Japan Experiment Module

CALET-CAL Detector
Fully active thick calorimeter (30X0) optimized for electron
spectrum measurements well into TeV region
448mm

Charge Detector

CHD

plastic scintillator hodoscope,absolute charge measurement(including charge zero)

ImagingCalorimeter

IMC

SciFi + tungsten plate (3X0),
reconstruction of arrival direction
and initial shower development

Total Absorption
Calorimeter

1TeV electron shower isfully contained in TASCTASCPWO hodoscope (27X0),
energy measurements and particle
identification

CALET-CAL Flight Model

CALET-CAL Shower Imaging Capability (MC)gammaelectronpion• Proton rejection power of 105 can be achieved by taking advantage ofshower imaging capability of IMC and TASC
• Angular resolution of ~0.2o for 10GeV gamma rays

Intrinsic Advantage of CALET Instrument
EM Shower Energy Measurement =TASC Energy Sum × “small” Correction• Active and thick calorimeter absorbs most of the electromagnetic energy (~95%)
up to the TeV region• In principle, energy measurement with very small systematic error is possible.
• We can use penetrating particles to do absolute calibration of the instrument(so called “MIP” calibration).
– NO worries about quantum efficiency or collection efficiency“MIP”
peakin ADU

Flight Data
Penetrating helium“MIP”
peakmatch

Equivalent histograms inADC unit and in Energy“MIP”
peakin GeV

MC Data
Penetrating helium

Resultant Energy Resolution
for electrons

preliminary• All the calibration including
position/temperaturedependence, dynamic range
calibration are applied andaccuracy of each calibration isestimated.
• Considering the calibration errors
and instrument noise, energy
resolution is estimated as afunction of energy• Space based direct detection with CALET– event-by-event particle identification– excellent energy resolution– limited effective area

• Ground based indirect detection method– very large effective area
– decent energy resolution

Complementary
to each otherCALET Scientific Objectives
While CALET is optimized for observation of TeV electrons, theinstrument is sensitive to gamma-rays, protons and nuclei.

Science Objectives

Observation Targets

Nearby Cosmic-ray Sources

Electron spectrum in trans-TeV region

Dark Matter

Signatures in electron/gamma energy
spectraOrigin and Acceleration of Cosmic Rays

p-Fe over several tens of GeV,
Ultra-heavy ions

Cosmic-ray Propagation in the Galaxy

B/C ratio up to several TeV/nucleon,
Diffuse gamma-ray observation up to TeV

Solar Physics

Electron flux below 10 GeV

Gamma-ray Transients

X-rays/Gamma-rays in 7 keV－20MeV in
CGBM and GeV gamma-rays in CAL

Objectives of CALET Gamma-Ray Observation• Diffuse Component => High energy region– Galactic– Extragalactic• Point Source– Calibration of pointing accuracy– Confirmation of angular resolution– Cross check of energy measurements/efficiency• Transient Object– GRB– Gravitational wave
– Other transients

Objectives of CALET Gamma-Ray Observation• Diffuse Component => High energy region– Galactic– ExtragalacticStatistics is the KEY• Point Source– Calibration of pointing accuracy– Confirmation of angular resolution– Cross check of energy measurements/efficiency• Transient Object– GRB– Gravitational wave
– Other transientsGW Counterpart Search with CALET
Publishedin ApJ Letters

GW 151226 [B. P. Abbott et al., PRL 116 (2016) 241103]• GW trigger Time: 2015/12/26 3:38:53.647 UT- gravitational-wave signal produced by the coalescenceof two stellar-mass black holes at a luminosity distanceof ~440Mpc.

CALET Observation• CGBM HV-on (3:20 – 3:40 UT)- No on-board trigger• CAL: low-energy gamma-ray mode (> 1GeV) 3:30-3:43UTOverview of CALET Trigger System

ISS Orbit and CALET Operations

CAL Limit Calculation Procedure
Gamma-ray EventSelection

Effective Area andSensitivity Calculation

Exposure Calculation

Upper Limit
Calculation

Check Overlap with
LIGO Localization Map

Lower energy is important
Seff as a function of zenith
and azimuth

Seff(zen,azi) Tlive projected tosky event by event along
CALET trajectory
If no gamma-rays are observed

Validation ofSensitivity with
Observation ofDiffuse Component

Gamma Ray Event Selection= Electron Selection Cut + Gamma-ray ID Cut w/ Lower Energy Extension

gamma-ray

100 GeV Event Examples
electron

Charge Z=0proton

Charge Z=1

Electromagnetic Shower

Hadron Shower

well contained, constant shower development

larger spread

Gamma Ray Event Selection= Electron Selection Cut + Gamma-ray ID Cut w/ Lower Energy Extension
1. Geometry Condition- CHD-Top to TASC
Bottom (415cm2)

2. Pre selection- Offline trigger- Shower concentration- Shower starting point

3. Track quality cut- Track hits >2- matching w/ TASC

4. Electromagneticshower selection- shower shape

5. Gamma-ray ID- CHD-vetoFor quick result, not fully optimized, yet.

Gamma Ray Event Selection= Electron Selection Cut + Gamma-ray ID Cut w/ Lower Energy Extension
1. Geometry Condition
ALL VETO(E<8GeV)- CHD-Top to TASC
Bottom (415cm2)

2. Pre selection- Offline trigger- Shower concentration- Shower starting point

3. Track quality cut- Track hits >2- matching w/ TASC
⇒ reject

4. Electromagnetic
shower selection- shower shape

5. Gamma-ray ID- CHD-vetoEffective Area and Sensitivity
dy/dz

3-30 GeV
average

Effective area is estimated as a function ofincident angle (dx/dz, dy/dz) and energy.
Maximum effective area is achieved ataround 10 GeV, but lower energy is more
important for steep spectrum like E-2.

mostly axiallysymmetricdx/dz

Effective area as a function ofenergy. Three representing zenith
angle ranges are shown

Maximum sensitivity of 4x10-5 erg/cm2
per Δ(ln E) was achieved at around 1
GeV for E-2 spectrum
21Diffuse Gamma-Ray Observation• Purpose:• Data set:
• Observation Mode:

Sensitivity validation & BG estimationfrom 151013 to 160531 (232 days)
Low Energy Gamma-Ray Trigger

－180°

+180°

Black Point: Gamma-ray
event candidates

Color Map: Exposurein 10-12.6GeV in cm2sec

22

Diffuse Gamma-Ray Observation• Purpose:• Data set:
• Observation Mode:+180°

Black Point: Gamma-ray
event candidates

Sensitivity validation & BG estimationfrom 151013 to 160531 (232 days)
Low Energy Gamma-Ray Trigger－180°

exposure is limited in low latitude region, it covers ahalf of galactic plane including Geminga+Crab regionDiffuse Gamma-Ray Observation• Purpose:• Data set:
• Observation Mode:

Sensitivity validation & BG estimationfrom 151013 to 160531 (232 days)
Low Energy Gamma-Ray Trigger

-150°
30°

+180°

Geminga－180°
CrabFiltered with point spreadfunctions to see point sources

Geminga and Crab are clearly identified.

Projection to Galactic Latitude (|l|<80deg)
And comparison with diffuse radiation model

BG is estimated from |b|>20deg
as 2.4 x 10-7 event/(cm2sec sr)
by assuming isotropic
contamination of cosmic-rays ingalactic coordinates.

Considering the contribution from point sources, it will be consistent
with expectation.

25Projection to Galactic Latitude (|l|<80deg)
And comparison with Fermi-LAT’s observation

Fermi-LAT Data:
2015/10/8-2016/6/02(using weekly files)

Considering the contribution from point sources, it was actually consistent
with expectation. Therefore, it was found that current selection criteria has
a validated sensitivity and can be used to set limit on GW counterpart flux.

26

Integrated Exposurefor GW151226 counterpart search
Assuming E-2 spectrum, exposure was integrated over 1 to 100GeV.

+180°

T0: 1451101134 => REDT0-525 to T0+221 sec => CYAN－180°

1-100GeV

27

90% CL Upper limit for GW151226
counterpart search
NO event remained after applying all the selection criteria.

+180°

T0: 1451101134 => REDT0-525 to T0+221 sec => CYAN

－180°

1-100GeV

Background contamination is negligible: ~0.035 event28

90% CL Upper limit for GW151226
counterpart search+180°

Contour:
GW localization
significance Map－180°

UL = 2x10-7 erg cm-2sec-1@ 15%
1-100GeV

CALET observation constrains at least some portion of LIGO probability.

29Summary & Conclusion
1.

2.
3.

As a result of GW151226 counterpart search in GeV gamma-rays, CALET-CALobservation constrains 15% of LIGO localization map by 90% upper limit fluxof 2x10-7 erg cm-2sec-1 .
Its sensitivity was validated with diffuse gamma-ray observation.
Due to closeness of GW candidates, FOV coverage is more important thandeepness of counterpart search assuming typical short GRBs as candidates.

CGBM Upper Limits:
1.0x10-6 erg cm-2 s-1 (7-500keV) @ 33%

1.8x10-6 erg cm-2 s-1 (50-1000keV) @ 49%

LaBr3(Ce)BGOAssuming the distance of 440 Mpc, the upper limit in luminosity is3-5 x 1049 erg s-1 (typical mean luminosity of s-GRB is 1.6 x 1051 erg s-1).

Prospects• Continues observation of transient objects– Analysis flow is established through the analysis of GW151226 counterpart.
– We’re ready for LIGO/Virgo O2 and KAGRA’s alert.

• Detailed analysis of point source and diffuse components– optimizing analysis for each target– accumulating more and more statistics

232days with High Energy Trigger

VERY PRELIMINARY

Exposure =2x108cm2sec