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The GLAST Burst Monitor (GBM)
R. M. Kippen, M. S. Briggs, W. S. Paciesas, R. D.
Preece UAH/NSSTC C. A. Meegan, G. J. Fishman, C.
Kouveliotou NASA/NSSTC G. G. Lichti, V.
Schönfelder, A. von Kienlin, R. H. Georgii, R.
Diehl MPE
The study of gamma-ray bursts (GRBs) is one of
the primary scientific objectives of the
Gamma-ray Large Area Space Telescope (GLAST)
mission. With its high sensitivity to prompt and
extended 20 MeV to 300 GeV burst emission,
GLAST's Large Area Telescope (LAT) is expected to
yield significant progress in the understanding
of GRB physics. To tie these breakthrough
high-energy measurements to the known properties
of GRBs at lower energies, the GLAST Burst
Monitor (GBM) will provide spectra and timing in
the 10 keV to 25 MeV energy range. The GBM will
also have the capability to quickly localize
burst sources to 20 over more than half the
sky, allowing the LAT to re-point at particularly
interesting bursts which occur outside its field
of view. With combined LAT/GBM measurements
GLAST will be able to characterize the spectral
behavior of many bursts over six decades in
energy. This will allow the unknown aspects of
high-energy burst emission to be explored in the
context of well-known low-energy properties. In
this paper, we present an overview of the GBM
instrument, including its technical design,
scientific goals, and expected performance.
The GLAST Mission
  • NASAs next major gamma-ray mission
  • Follow-on to successful Compton-EGRET
  • Primary scientific mission high-energy gamma-ray
    astronomy (20 MeV to gt300 GeV), including
  • Active Galactic Nuclei
  • Diffuse Galactic Cosmic Emission
  • Gamma-ray bursts
  • Solar flares
  • Pulsars, Neutron stars, BHCs
  • Molecular clouds, SNR
  • Dark matter, particle physics...
  • 5-year mission starting 2006
  • International participation

  • Primary instrument Large Area Telescope (LAT)
    95 of total instrument resources
  • Secondary instrument GLAST Burst Monitor (GBM)
    5 of total instrument resources

Gamma-Ray Bursts
  • Properties
  • Isotropic inhomogeneous spatial distribution
  • Fast, chaotic, variability
  • Bimodal prompt duration distribution
  • Characteristic non-thermal, evolving gamma-ray
    spectra extending E gt GeV
  • Fading multiwavelength afterglow
  • One case of prompt optical emission
  • Measured redshifts z 15
  • Associated with Normal host galaxies
  • Eg 1052-53 erg
  • Wide luminosity distribution
  • Most energetic explosions in the universe
  • Extreme physics, highly relativistic outflow
  • Cosmological probes of early universe

GLAST and Gamma-Ray Bursts (Present)
Composite spectrum of 5 EGRET Bursts
  • Little is known about GRB emission in the gt50 MeV
    energy regime
  • EGRET detected 5 high-energy bursts, but
    suffered from
  • Small field of view (40), so few bursts were
  • Small effective area (1000 cm2), so few detected
    photons per burst
  • Large deadtime (100 ms/photon), so few prompt
    photons were detected
  • Prompt GeV emission with no high-energy cutoff
    (combined with rapid variability) implies highly
    relativistic bulk motion at source G gt
  • Extended or delayed GeV emission may require more
    than one emission mechanism

Dingus et al. 1997
No evidence of cutoff
Extended/Delayed emission
Hurley et al. 1994
GLAST and Gamma-Ray Bursts (Future)
  • The GLAST LAT will have
  • Large 2 sr field of view, so more detected
    bursts (50100/yr)
  • gt10? EGRET effective area, so more photons per
  • 105? lower deadtime, so more detected prompt
  • Improved sensitivity Egt10 GeV, for better
    locations and spectral range
  • 5? better angular resolution, for arc-min GRB
    locations and better afterglow sensitivity
  • On-board computing for providing rapid GRB
    locations to afterglow observers
  • The GLAST LAT will not have
  • Sensitivity lt10 MeV, where there is the most
    knowledge of GRBs
  • Sensitivity outside its FoV
  • Fast trigger for weak bursts

Baring 1996
GLAST LAT GRB Location Accuracy
Norris et al. 1998
Role of the GLAST Burst Monitor (GBM)
  • LAT will provide ground-breaking new GRB
    observations, but it will be difficult to
    evaluate them in the context of current GRB
  • GBM will enhance GLAST GRB science by providing
    low-energy context measurements with high time
  • Improved GBMLAT wide-band spectral sensitivity
  • Compare low-energy vs. high-energy temporal
  • Continuity with current GRB knowledge-base
  • GBM will provide rapid GRB timing location
    triggers w/FoV gt LAT FoV
  • Improve LAT sensitivity and response time for
    weak bursts
  • Re-point GLAST/LAT at particularly interesting
    bursts for afterglow observations
  • Provide rapid locations for ground/space
    follow-up observations IPN timing

GBM Collaboration
National Space Science Technology Center
University of Alabama in Huntsville
NASA Marshall Space Flight Center
Max-Planck-Institut für extraterrestrische Physik
Michael Briggs Marc Kippen William
Paciesas Robert Preece
Charles Meegan (PI) Gerald Fishman Chryssa
Giselher Lichti (Co-PI) Andreas von
Keinlin Robert Georgii Volker Schönfelder Roland
On-board processing, flight software, systems
engineering, analysis software, and management
Detectors, power supplies, calibration, and
analysis software
Instrument Requirements
  • Available Resources
  • Mass lt70 kg Power lt50 Watts (average)
    Cost to NASA 5M

Instrument Design Major Components
12 Sodium Iodide (NaI) Scintillation Detectors
2 Bismuth Germanate (BGO) Scintillation Detectors
Data Processing Unit (DPU)
  • Characteristics
  • 5-inch diameter, 0.5-inch thick
  • One 5-inch diameter PMT per Det.
  • Placement to maximize FoV
  • Thin beryllium entrance window
  • Energy range 5 keV to 1 MeV
  • Major Purposes
  • Provide low-energy spectral coverage in the
    typical GRB energy regime over a wide FoV
  • Provide rough burst locations over a wide FoV
  • Characteristics
  • 5-inch diameter, 5-inch thick
  • High-Z, high-density
  • Two 5-inch diameter PMTs per Det.
  • Energy range 150 keV to 30 MeV
  • Major Purpose
  • Provide high-energy spectral coverage to overlap
    LAT range over a wide FoV
  • Characteristics
  • Analog data acquisition electronics for detector
  • CPU for data packaging/processing
  • Major Purposes
  • Central system for instrument command, control,
    data processing
  • Flexible burst trigger algorithm(s)
  • Automatic detector/PMT gain control
  • Compute on-board burst locations
  • Issue r/t burst alert messages

Instrument Design Functional Diagram
Prototype Detectors
Prototype detectors being tested at MPE
Prototype NaI Detector
Prototype BGO Detector
Detector Placement Concept
Low-Energy NaI(Tl) Detectors (3 of 12)
High-Energy BGO Detector (1 of 2)
Top View
Side View
Expected Detector Performance
Expected Detector Performance
Detector Response Matrices (DRMs) from GEANT3
Monte Carlo Simulations
Estimated Background from Simulations and BATSE
GRB Spectral Performance (GBMLAT)
  • Simulated GBM and LAT response to time-integrated
    flux from bright GRB 940217
  • Spectral model parameters from CGRO wide-band fit
  • 1 NaI (14 º) and 1 BGO (30 º)
  • Baseline 8000 cm2 LAT _at_ 30º
  • (actual LAT now expected to be 10000 cm2)
  • Good spectral response over 5.3 decades in
  • In addition to providing low-energy parameters,
    combined fit yields better constraints on
    high-energy power-law index than LAT-only fit

Time-Resolved Spectroscopy Performance
  • Simulation of bright GRB990123
  • Model parameters taken from BATSE fits
  • Same detector response as previous example
  • GBM detectors can easily measure evolution of
    low-energy spectral parameters
  • LAT alone cannot detect evolution of high-energy
    index b
  • LATGBM can detect evolution of b

GRB Sensitivity Location Performance
  • Trigger sensitivity assumes BATSE-like trigger
    (gt5s in 2 or more NaI detectors)
  • Total on-board burst trigger rate 150225 yr-1,
    depending on GLAST pointing schedule
  • Statistical GRB location errors
  • 9º for 1 ph cm-2 s-1 burst (100 yr-1)
  • 1.5º for 10 ph cm-2 s-1 burst (10 yr-1)

Map of GRB Statistical Location Error for Peak
Flux 1 ph s-1 cm-2
Data Types
  • Continuous data for weak (post-facto) burst
    triggering background estimation
  • Triggered data for best possible temporal
    spectral resolution during bursts
  • Length of trigger data readout computer controlled
  • Multiple data types to maximize science return
    for given telemetry allocation

Mission/Instrument Operations
LAT Instrument Operations Center (IOC)
GLAST Mission Operations Center (MOC)
GLAST Science Operations Center (SOC)
Science User Community
GBM Instrument Operations Center (IOC)
  • Including Inst. Team members
  • Scientific data analysis
  • Including final GRB locations and joint GBM/LAT
    spectral timing analysis
  • Archive processed science data
  • Distribute data to user community
  • GBM/LAT commands
  • GBM/LAT monitoring
  • Compute rapid GRB locations LAT/GBM
  • Distribute GRB alerts via GCN
  • Monitor instrument operation and performance
  • Flight s/w updates
  • Generate Inst. commands
  • Routine science processing