# Presentation: 881261

## GBM Trigger, Calibration, and Response Review

**Trigger and On-Board Algorithms – Chip Meegan**
**Calibrations – Jerry Fishman**
**Response Matrices – Marc Kippen**

## The GLAST Burst Monitor

**Purpose: Extend GLAST spectra of GRBs from LAT thresold down to 10 keV;**
**               ****alert LAT to strong bursts outside its field of view.**

**Energy Range:**
**	****10 keV to 30 MeV.**
**Detectors: **
**	****12 NaI, 5” diameter x 1⁄2” thick;**
**                 ****2 BGO, 5” diameter x 5” thick.**

**Notes:**
Cover entire unocculted sky when zenith pointed.  Four NaI detectors generally visible.  Always have 1 BGO visible.  

## Energy range: 50 – 300 keV
Time Scales: 64, 256, and 1024 ms
Significance: nominally 5.5 sigma in at least 2 detectors
Background: computed every ~17 s
Threshold: ~0.2 photons/cm2-s
Rate: ~300 per year
93% of triggered bursts were above the 1024 ms threshold

**Time Scales: 64, 256, and 1024 ms**
**Significance: nominally 5.5 sigma in at least 2 detectors**
**Background: computed every ~17 s**
**Threshold: ~0.2 photons/cm2-s**
**Rate: ~300 per year**
**93% of triggered bursts were above the 1024 ms threshold**

**BATSE Burst Trigger**

## Energy range: 3 programmable, incl. 50 – 300 keV
Time Scales: 16, 64, 256, 1024, 4096, 16384 ms
Significance: nominally 4.5 sigma in at least 2 detectors
Background: flexible algorithm, quadratic fit
Threshold: ~0.7 photons/cm2-s
Requirement: 1.0 photons/cm2-s
Goal: 0.75 photons/cm2-s
Rate: ~200 per year (BATSE-like trigger)

**Time Scales: 16, 64, 256, 1024, 4096, 16384 ms**
**Significance: nominally 4.5 sigma in at least 2 detectors**
**Background: flexible algorithm, quadratic fit**
**Threshold: ~0.7 photons/cm2-s**
**Requirement: 1.0 photons/cm2-s**
**Goal: 0.75 photons/cm2-s**
**Rate: ~200 per year (BATSE-like trigger)**

**GBM Burst Trigger**

## On-Board Locations

**Grid search to match relative rates of NaI detectors**
**Grid size of 1634 points – 5 degree resolution**
**Assume typical spectrum and zenith pointing**
**Systematic errors**
**Grid discreteness**
**Spectral variations**
**Off-zenith pointing**

- GBM meets accuracy goal at any zenith angle.

## On-Board Trigger Classification

**Bayesian approach**
**Algorithm developed and tested**
**Classes: GRBs, Solar Flares, Particle Precipitation (local and distant), Hard X-Ray Transients (known and generic), SGRs (known and generic), Cygnus X-1, TGFs**
**Parameters: Location, Hardness Ratio, Geomagnetic Latitude**
**Always fooled by a new hard X-ray transient**
**About 95% of GRBs classified correctly – other 5% ambiguous**

## Effective Area and Field of View

**Field of View: 9.5 steradians**
**Requirement: 8 steradians**
**Goal: 10 steradians**

- Effective area in units of detector area (front face only)

## Ground Analysis Burst Sensitivity

- Significance (sigma) of a GRB of 0.5 photons/cm2-s.
- RSS combination of all detectors with >2 sigma signal.

**Threshold Requirement: 0.5 photons/cm2-s; 5 sigma**

## Performance Simulations

**Uses reliminary response functions for typical burst spectrum; 50-300 keV**
**Includes blockage by spacecraft, LAT, radiator, but not by solar panels, antennas, other GBM detectors.**
**Background rates estimated from BATSE, extrapolated to 565 km altitude; include directional and orbital variations.**
**Spacecraft and atmospheric scattering estimates included in location accuracy tests.**