The GLAST Burst Monitor

Charles Meegan^\*^, Narayana Bhat^¶^, Valerie Connaughton^¶^, Michael
Briggs^¶^, Roland Diehl^§^, Gerald Fishman^\*^, Jochen Greiner^§^, R.
Marc Kippen^†^, Andreas von Kienlin^§^, Chryssa Kouveliotou^\*^,
Giselher Lichti^§^, William Paciesas^¶^, Robert Preece^¶^, Helmut
Steinle^§^, and Colleen Wilson-Hodge^\*^

^\*^Marshall Space Flight Center, VP62, Huntsville, AL 35812 USA

^¶^University of Alabama, NSSTC, 320 Sparkman Drive, Huntsville, AL
35805 USA

^§^Max-Planck-Institute for Extraterrestrial Physics,Giessenbachstrasse
85748 Garching, Germany

^†^Los Alamos National Laboratory, ISR-1, MS B244, Los Alamos, NM 87545
USA

**Abstract.** The GLAST Burst Monitor (GBM) comprises an array of NaI
and BGO scintillation detectors designed to enhance the scientific
return from GLAST in the study of gamma-ray bursts (GRBs). By observing
in the 10 keV to 30 MeV energy range, GBM extends the spectral coverage
of GRBs more than 3 decades below the LAT energy threshold. GBM computes
burst locations on-board, allowing repointing of the GLAST Observatory
to place strong bursts within the LAT field-of-view to observe delayed
high-energy emission.

Keywords: Gamma-ray bursts, Instrumentation.

PACS: 95.55.Ka

# introduction

The Large Area Telescope (LAT) on the GLAST Observatory will make
observations of high-energy emission (\>20 MeV) from GRBs with
unprecedented sensitivity. This energy range has been largely
unexplored, although some tantalizing observations from EGRET on the
Compton Gamma Ray Observatory showed delayed high-energy emission \[1\]
\[2\]. The purpose of the GLAST Burst Monitor (GBM) is to extend the
energy range of GRB observations from the LAT threshold down to the
range covered by previous burst instruments. This capability will tie
the new observations to the large existing GRB database and illuminate
the energy-dependent changes in the temporal structure. GRB spectra are
usually characterized by three parameters\[3\]: the low-energy slope α,
a break energy E~b~, and a high-energy slope β. Since E~b~ always lies
below the LAT energy threshold (as far as we know), GBM observations are
necessary to measure α and E~b~, and thereby determine the total burst
fluence.

Another contribution of GBM is detection and localization of bursts over
a wider field-of-view than the LAT. This will allow the observatory to
be repointed to particularly interesting bursts, enabling LAT
observations of delayed high-energy emissions. We currently plan to
perform such repointings only for the one or two strongest bursts each
year.

# Flight hardware

GBM comprises 12 Sodium Iodide (NaI) scintillation detectors, 2 Bismuth
Germanate (BGO) detectors, a Data Processing Unit (DPU), and a Power
Supply Box (PSB). The NaI detectors are ½ inch thick by 5 inch in
diameter, with a thin Beryllium entrance window, coupled to a 5 inch
photomultiplier tube (PMT). They cover the energy range of 8 keV to 1
MeV and are used to determine GRB localizations. The BGO detectors are 5
inch thick by 5 inch diameter, viewed by two PMTs. They cover the 150
keV to 30 MeV energy range, providing overlap with both the NaI
detectors and the LAT. The detectors are oriented and positioned on the
spacecraft so as to provide approximately uniform coverage of the
unocculted sky. Bursts will typically illuminate three or more of the
NaI detectors and one of the BGO detectors.

The DPU processes the detector inputs, constructs pulse height
histograms, formats data for output, and processes instrument commands.
It provides the interface to the spacecraft for science and housekeeping
data.

The PSB generates regulated voltage for the DPU and the detectors, as
well as the high voltage for the detector photomultipliers.

Figure 1 is a block diagram of the instrument showing the
interconnections among the hardware components.

![](media/image1.jpeg){width="5.31875in" height="2.4347222222222222in"}

Figure 1. Block diagram of GBM flight hardware.

# data types

GBM normally produces two types of data, called CTIME and CSPEC. These
are histograms of spectra from each detector, with different temporal
and spectral resolution. Time-tagged event (TTE) data are transmitted
when a burst occurs. These data encode the time of arrival and energy of
each photon event in each detector. A ring buffer of 500,000 TTE events
is frozen at the time of a trigger and later transmitted. These data
types are summarized in Table 1.

When a burst occurs, additional data (referred to as TRIGDAT) are
generated and transmitted to the ground in near-real time to facilitate
ground-based follow-up observations. TRIGDAT data are also used by the
Burst Alert Processor (BAP) to autonomously compute and disseminate
burst locations to higher accuracy than can be achieved on-board.

+-----+--------------+--------------------------+---------------------+
| *   |              |                          |                     |
| *TA |              |                          |                     |
| BLE |              |                          |                     |
|  1. |              |                          |                     |
| D   |              |                          |                     |
| ata |              |                          |                     |
| Ty  |              |                          |                     |
| pes |              |                          |                     |
| **. |              |                          |                     |
+-----+--------------+--------------------------+---------------------+
| **  | **Purpose**  | **Temporal Resolution**  | **Energy            |
| Nam |              |                          | Resolution**        |
| e** |              |                          |                     |
+-----+--------------+--------------------------+---------------------+
| CS  | Continuous   | Nominal: 8.192 seconds   | 128 energy channels |
| PEC | high         |                          |                     |
|     | spectral     | During Bursts: 1.024     | (adjustable channel |
|     | resolution   | seconds                  | boundaries)         |
|     |              |                          |                     |
|     |              | Adjustable Range: 1.024  |                     |
|     |              | -- 32.768 s              |                     |
+-----+--------------+--------------------------+---------------------+
| CT  | Continuous   | Nominal: 0.256 seconds   | 8 energy channels   |
| IME | high time    |                          |                     |
|     | resolution   | During Bursts: 0.064     | (adjustable channel |
|     |              | seconds                  | boundaries)         |
|     |              |                          |                     |
|     |              | Adjustable Range: 0.064  |                     |
|     |              | -- 1.024 s               |                     |
+-----+--------------+--------------------------+---------------------+
| TTE | Time-tagged  | 2 microsecond time tags  | 128 energy channels |
|     | events       | for 300 s after trigger; |                     |
|     | during       | 500K events before       | (adjustable channel |
|     | bursts       | trigger. Max. rate, all  | boundaries)         |
|     |              | detectors: 375 kHz.      |                     |
+-----+--------------+--------------------------+---------------------+

# burst triggerS

GBM flight software will implement an on-board burst trigger that will
initiate an increase in data transmission. A trigger occurs if the count
rates in two or more of the NaI detectors exceed a specified statistical
significance above the background rate. The required significance is
separately adjustable for five different time scales (16 ms, 64 ms, 256
ms, 1.024 s, and 4.096 s) in up to five adjustable energy ranges. The
nominal trigger, which will always be enabled, uses the 1.024 s time
scale and the 50-300 keV energy band. The nominal significance threshold
is 4.5 .

When a burst trigger occurs, GBM begins transmitting time-tagged event
data for 300 seconds. A ring buffer of 500,000 pre-trigger time-tagged
events is also transmitted. On-board software also computes the
direction to the event, the classification likelihood (GRB, solar flare,
particle precipitation, etc.), and peak flux and fluence estimates.
These parameters are sent to the LAT and to the ground in near-real
time. Trigger information will be distributed to ground-based observers
via the GCN. The predicted rate of GRB triggers is 200 per year.

Trigger events are localized on-board by comparing the relative count
rates of the NaI detectors to a table of predicted rates corresponding
to 1634 directions (in spacecraft coordinates).

Trigger events are categorized on-board by computing the Bayesian
probability for several classes of events, including GRB, SGR, solar
flare, or particle precipitation. Parameters considered are the event
localization, spectral hardness, and the geomagnetic latitude of
observatory.

# summary

GBM will enhance the scientific return from GLAST in the study of GRBs
by extending the energy range down to 8 keV and by providing real-time
burst locations over the entire unocculted sky. Table 2 lists the
expected values for several key parameters of the instrument. More about
the GBM and its performance can be found in Lichti et al. \[4\] and
references therein.

The primary science investigations that the GBM team will conduct will
be computing and disseminating burst locations, generating a catalog of
GBM-triggered bursts, and, in collaboration with LAT team members,
performing joint spectral fits of GRBs that are observed by both LAT and
GBM. The production of continuous background data as well as triggers on
other types of transient events enable several other projects that are
appropriate for Guest Investigations. These include studies of solar
flares, long period X-ray pulsars, SGRs, and transient hard X-ray
sources.

  -------------------------------- --------------------------------------
  **TABLE 2. GBM Performance.**    

  **Parameter**                    **Expected Performance**

  Energy Range                     8 keV to 30 MeV

  Energy Resolution                \~14% at 100 keV; \~9% at 1MeV

  Dead time per event              2.6 microseconds

  High Rate Performance            \< 2 % distortion at 50,000 counts/s

  On-board Trigger Threshold       0.7 photons cm^-2^ s^-1^

  On-board Burst Location Error    \< 15

  Burst Trigger Rate               200 per year
  -------------------------------- --------------------------------------

# acknowledgements

The GBM science team appreciates the excellent engineering support
provided by our home institutions and our contractors. Crismatec
manufactured the crystal assemblies. Jena-Optronic GmbH produced the
detector electronics and integrated and tested the detector assemblies.
EADS Astrium designed, built and tested the Power Box. Southwest
Research Institute designed, built, and tested the DPU. The German part
of the GBM project was supported by the BMBF via the DLR under the
contract number 50 QV 0301.

# References

1\. K. Hurley, *Nature,* **372,** 652 (1994) .

2\. M. M. Gonzales, B. L. Dingus, Y. Kaneko, R. D. Preece, C. D. Dermer,
and M. S. Briggs, *Nature.,* **424**, 749 (2003).

3\. D. Band, J. Matteson, L. Ford, B. Schaefer, D. Palmer, B. Teegarden,
T. Cline, M. Briggs, W. Paciesas, G. Pendleton, G. Fishman, C.
Kouveliotou, et al., *Astrophys. J.,* **413**, 281 (1993).

4\. G. Lichti, et al.., "The State of the GBM Project 1½ Years Before
Launch" in *Gamma-Ray Bursts: Prospects for GLAST*, edited by M Axelsson
and F. Ryde., AIP Conference Proceedings 906, American Institute of
Physics, New York, 2007.