# Presentation: 105756

## gammaray.msfc.nasa.gov/GBM

## Abstract

**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*

**T**he 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

**NASA’s next major gamma-ray mission**
**Follow-on to successful *****Compton*****-EGRET**
**Primary scientific mission: high-energy gamma-ray astronomy (20 MeV to >300 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 > GeV**
**Fading multiwavelength afterglow**

**One case of prompt optical emission**
**Measured redshifts *****z***** ~ 1–5**
**Associated with “Normal” host galaxies**
**E******** ~10****52-53**** 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)

- Little is known about GRB emission in the >50 MeV energy regime
- EGRET detected ~5 high-energy bursts, but suffered from:
- Small field of view (~40°), so few bursts were detected
- 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:     > 102–103
- Extended or delayed GeV emission may require more than one emission mechanism

- Composite spectrum of 5 EGRET Bursts

- Dingus et al. 1997

- Hurley et al. 1994

**No evidence **
**of cutoff**

**Extended/Delayed**
**emission**

## GLAST and Gamma-Ray Bursts (Future)

**The GLAST LAT will have:**
- Large ~2 sr field of view, so more detected bursts (~50–100/yr)
- >10 EGRET effective area, so more photons per burst
- 105 lower deadtime, so more detected prompt photons
- Improved sensitivity E>10 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 <10 MeV, where there is the most knowledge of GRBs
- Sensitivity outside its FoV
- Fast trigger for weak bursts

**GLAST LAT**

**Baring 1996**

**Norris et al. 1998**

**GLAST LAT**
**GRB Location Accuracy**

## Role of the GLAST Burst Monitor (GBM)

**GBM will enhance GLAST GRB science by providing low-energy context measurements with high time resolution**
**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 w/FoV > 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**

**LAT will provide ground-breaking new GRB observations, but it will be difficult to evaluate them in the context of current GRB knowledge**

## GBM Collaboration

**University of Alabama**
** ****in Huntsville**

**NASA**
**Marshall Space Flight Center**

**Max-Planck-Institut für **
**extraterrestrische Physik**

**National Space Science & Technology Center**

**Michael Briggs**
**Marc Kippen**
**William Paciesas**
**Robert Preece**

**Charles Meegan (PI)**
**Gerald Fishman**
**Chryssa Kouveliotou**

**Giselher Lichti (Co-PI)**
**Andreas von Keinlin**
**Robert Georgii**
**Volker Schönfelder**
**Roland Diehl**

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

*Detectors, power supplies, calibration, and analysis software*

## Instrument Requirements

**Available Resources:**
**Mass: <70 kg        Power: <50 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 signals
- 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 NaI Detector**

**Prototype BGO Detector**

**Prototype detectors being tested at MPE**

## Detector Placement Concept

**Low-Energy NaI(Tl)**
**Detectors (3 of 12)**

**High-Energy BGO**
**Detector (1 of 2)**

**LAT**

**Top View**

**Side View**

## Expected Detector Performance

**Expected Detector Performance**

## Expected Detector Performance

**Detector Response Matrices (DRMs)**
**from GEANT3 Monte Carlo Simulations**

**Estimated Background from**
**Simulations and BATSE Extrapolations**

## GRB Spectral Performance (GBM+LAT)

**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 cm****2**** LAT @ 30o**
- (actual LAT now expected to be ~10000 cm2)

**Good spectral response over 5.3 decades in energy!**
**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 GRB~990123**
**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 ********
**LAT+GBM can detect evolution of ********

## GRB Sensitivity & Location Performance

**Trigger sensitivity assumes BATSE-like trigger (>5******** in 2 or more NaI detectors)**
**Total on-board burst trigger rate 150–225 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

**Multiple data types to maximize science return for given telemetry allocation**

**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**

## Mission/Instrument Operations

**GBM **
**Instrument **
**Operations**
**Center (IOC)**

**GLAST **
**Science**
**Operations**
**Center (SOC)**

- 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

- Archive processed science data
- Distribute data to user community

**GLAST **
**Mission**
**Operations**
**Center (MOC)**

**LAT **
**Instrument **
**Operations**
**Center (IOC)**

**Science**
**User Community**

- Including Inst. Team members
- Scientific data analysis
- Including final GRB locations and joint GBM/LAT spectral & timing analysis