GLAST BURST MONITOR

Narayana P. Bhat1, Valerie Connaughton1, Andreas von Kienlin3, Michael S. Briggs1, Charles A. Meegan2, Roland Diehl3, Gerald J. Fishman2, Jochen Greiner3, Marc Kippen4, Chryssa Kouveliotou2,5, Giselher G. Lichti3,William S. Paciesas1, Robert D. Preece1, Volker Schönfelder3 and Robert B. Wilson2.

1. The University of Alabama in Huntsville 2. NASA/Marshall Space Flight Center3. Max-Planck-Institut für extraterrestrische Physik 4. Los Alamos National Laboratory5. Universities Space Research Association

The Gamma Ray Large Area Space Telescope (GLAST) mission is a follow-up on the successful EGRET experiment onboard the CGRO. It will provide a high-sensitivity survey of the sky in high-energy -rays, and will perform detailed observations of selected persistent and transient sources. There are two experiments onboard GLAST - a Large Area Telescope (LAT) and the GLAST Burst Monitor (GBM). The primary mission of the GBM instrument is to support the LAT in observing -ray bursts (GRB's) by providing low-energy measurements with high time resolution and rapid burst locations over a large (> 8 sr) field of view. The GBM will complement the LAT measurements by observing GRBs in the energy range 10 keV to 30 MeV, the region of the prominent spectral turnover of GRB's. An important objective of GBM is to compute the locations of GRB sources on-board the spacecraft and quickly communicate them to the LAT and to the ground to allow rapid follow-up observations at space- and ground-based observatories. This information may be used to re-point the spacecraft towards particularly interesting burst sources that occurred outside the LAT field of view.The GBM consists of 14 uncollimated scintillation detectors coupled to phototubes to measure -ray energies and arrival times. Two different detector types are used to obtain spectral information over a wide energy range: 12 NaI detectors (10 keV to 1 MeV), and 2 BGO detectors (150 keV to 30 MeV). The detectors are distributed around the GLAST spacecraft to provide a large unobstructed field of view. The 12 NaI detectors are mounted with different orientations to provide directional information for use in locating GRB sources.

ABSTRACT

IntroductionIntroduction Gamma Ray Bursts (GRB’s), regarded as astrophysical mysteries,

are energetically comparable to supernovae (assuming significantbeaming).

GRB energy spectra have been measured from ~ 2 keV to 18 GeVwith no evidence for spectral cut-off at high energies.

The GRB energy spectra when plotted in Funits, make it clear that the peak of energy output during the burst is indeed at ray energies.

GRB940217 showed emission of GeV photons in prompt as wellas extended emission phases. The distinct higher energy component in GRB941017, for e.g., require extended energy range studies.

The high energy component of GRB’s could be related to relativistic electrons either through synchrotron emission or their interactionwith the surrounding cloud.

There are a significant number of “X-ray rich” GRB’s (with -raysabove 20 keV) as well as “X-ray flashes”.

Hence it is important to study the GRB spectra over an extended energy range in order to understand the emission mechanisms.

• The goal of the GLAST Burst Monitor (GBM) is toenhance the science return of the GLAST mission in the study of gamma-ray bursts by providing low-energy context measurements with high time resolution (2s);

• GBM will provide spectra for bursts from 10 keV to 30 MeV, connecting LAT high-energy measurements with more familiar energy domain;

• LAT will provide ground-breaking new GRBobservations, but without GBM it would be difficult to evaluate them in the context of prior observations & current knowledge;

The Role of GLAST Burst MonitorThe Role of GLAST Burst Monitor

LAT Fo V

GBM Fo V

• GBM will provide a wide sky coverage (8 sr) --enabling autonomous re-point requests for interesting bright bursts that occur outside LAT field-of-view (FoV) for high-energy afterglow studies (an important question from EGRET)

• GBM will also provide improved burst locations to the ground for follow-up studies at other wavelengths.

Improved GBM+LAT wide-band spectral sensitivity

Compare low-energy vs. high-energy temporal variability

Continuity with current GRB knowledge-base (GRO-BATSE)

• GBM will provide rapid GRB timing & location triggers with a field of view > LAT field of view

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

Trigger

S/CGBM LAT

Classification, Location, Hardness, Initial Flux

Flux, Fluence, Hardness(Running Updates)

Parameters

Science Re-point Candidate

S/C Re-point Decision

Mode Change?

Immediate Trigger Signal

< 2 s Begin R/T downlink

Continue R/T,5 - 10 min.

GBM LATGBM/LATS/C

Information packet

Re-point request

< 5ms

2 to ~ 60 s

GBM Burst AlertsGBM Burst Alerts

Instrument Functional Diagram

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)

Prospects of GBM + LAT in understanding the High Energy -ray Emission from GRB’s

Prospects of GBM + LAT in understanding the Prospects of GBM + LAT in understanding the High Energy High Energy --ray Emission from ray Emission from GRB’sGRB’s

High energy emission from High energy emission from GRB’s GRB’s is generally consistent with is generally consistent with Relativistic Shock Model. However there are notable exceptions Relativistic Shock Model. However there are notable exceptions e.g.e.g. GRB 941017 which showed GRB 941017 which showed multimulti--MeVMeV component with component with flatter spectral slope flatter spectral slope ((--1, >200 1, >200 MeVMeV),), a slower decay time and a a slower decay time and a higher higher fluence fluence compared those of the lower energy component.compared those of the lower energy component.

GBM DetectorsGBM Detectors

Characteristics– 5-inch diameter, 0.5 inch thick

[NaI (TlTl)]– One 5-inch diameter PMT per

Detector.– Placement to maximize FoV– Thin beryllium entrance window– Energy range: ~10 keV to 1 MeV

Major Purposes– Provide low-energy spectral

coverage in the typical GRB energy regime over a wide FoV

– Provide burst locations over a wide FoV

Characteristics– 5-inch diameter, 5-inch thick– High-Z (BGO), high-density– Two 5-inch diameter PMTs per

Detector.– Energy range: ~150 keV to 30 MeV

Major Purpose– Provide high-energy spectral

coverage to overlap LAT range over a wide FoV

12 Sodium Iodide (NaI)Scintillation Detectors

2 Bismuth Germanate (BGO)Scintillation Detectors

Location and orientation of GBM Location and orientation of GBM detectors in the spacecraftdetectors in the spacecraft

Param eter Requirem ent G oal Current Capability Energy range 10 keV – 25 MeV 5 keV – 30 M eV ~10 keV – 30 M eV

Energy resolution < 23.5% FW H M (10 keV – 25 MeV)

< 7% ~12% FW H M @ 511 keV

T im e resolution 10 s 2 s 2 s

O n-board G R B locations

W ith in 2 s 15º w ith in 1 s <15º; 1 .8 s

R apid ground G R B locations

5º accuracy (1 radius) w ith in 5 s

3º w ith in 1 s TBD by analysis (scattering influenced)

F ina l GR B locations

3º accuracy (1 radius) w ith in 1 day

(no stated goal) TBD by analysis (scattering influenced)

G R B sensitivity (on ground)

0.5 photons cm -2 s -1 (peak flux, 50–300 keV )

0.3 photons cm -2 s -1 (peak flux, 50–300 keV )

0.35 photons cm -2 s -1 (peak flux, 50–300 keV )

G R B on-board trigger sensitivity

1.0 photons cm -2 s -1 (peak flux, 50–300 keV )

0.75 photons cm -2 s -1 (peak flux, 50–300 keV )

0.76 photons cm -2 s -1 (peak flux, 50–300 keV

F ield of view 8 sr 10 sr 8.8 sr

D ead-tim e < 10 s/count <3 s/count ~2.5 s/count

G BM Design CapabilitiesG BM Design CapabilitiesG BM Design Capabilities

Burst Spectra GBM can provide time resolved energy spectra of GBM can provide time resolved energy spectra of GRB’s GRB’s in in

the range 10 the range 10 keVkeV –– 30 30 MeVMeV. . oo to explore the possible relation between to explore the possible relation between keVkeV--MeVMeV--GeVGeV

emissionsemissionsoo to explore if those to explore if those GRB’s GRB’s detected by the LAT form a detected by the LAT form a

separate classseparate class++ detection of high energy emission alone is not adequate detection of high energy emission alone is not adequate

for such classification for such classification oo additional benefit of a continuation of the BATSE archiveadditional benefit of a continuation of the BATSE archive

GBM can measure the temporal behavior & distribution of spectGBM can measure the temporal behavior & distribution of spectralralparameters: parameters: EEpp, , etcetc. .

Possible relation between Possible relation between EEp p and high energy emission.and high energy emission. GBM + LAT can measure the GRB spectra spanning 8 decades inGBM + LAT can measure the GRB spectra spanning 8 decades in

in in --ray energy (10 ray energy (10 keVkeV –– 300 300 GeVGeV) ) oo One can detect possible high energy cutOne can detect possible high energy cut--off in GRB spectraoff in GRB spectra

GBM can also measure energy resolved light curves so that onGBM can also measure energy resolved light curves so that one cane canlook for possible time delays between them. look for possible time delays between them. oo 128 channel spectra every 8 128 channel spectra every 8 ssoo 4 channel spectra every 0.256 4 channel spectra every 0.256 ss andandoo ~ 15 ~ 15 ss of Preof Pre--burst data of Time Tagged Events (TTE) with 2 burst data of Time Tagged Events (TTE) with 2 ss

time resolution.time resolution.oo During the burst TTE data is available for > 300 During the burst TTE data is available for > 300 ss at a peak rateat a peak rate

of 350 kHz.of 350 kHz.

Burst Light-curves

One would expect ~150 GRB triggers/year of which ~ 50One would expect ~150 GRB triggers/year of which ~ 50--100100triggers are expected to be detected by LATtriggers are expected to be detected by LAT

Prior location provided by GBM would enable LAT to detect Prior location provided by GBM would enable LAT to detect weaker weaker GRB’s GRB’s by limiting the search area which, in turn, reducesby limiting the search area which, in turn, reducesthe background.the background.

Burst Triggers

University of AlabamaUniversity of Alabamain Huntsvillein Huntsville

NASANASAMarshall Space Flight CenterMarshall Space Flight Center

MaxMax--PlanckPlanck--InstitutInstitut für für extraterrestrischeextraterrestrische PhysikPhysik

National Space Science & Technology Center

Michael BriggsWilliam PaciesasRobert PreeceNarayana BhatValerie Connaughton

Charles Meegan (PI)Gerald FishmanChryssa KouveliotouRobert WilsonColleen Wilson-Hodge

GiselherLichti (Co-PI)Andreas von KienlinVolker SchönfelderRoland DiehlJochenGreiner

On-board processing, flight software, systems engineering, analysis software, and management

Detectors, power supplies, calibration and analysis software

Los AlamosLos AlamosNational LaboratoryNational Laboratory

Detector Response

Marc Kippen