Detector/Observation TypeFermi has three types of detectors: the Large Area Telescope (LAT), the 12 Fermi Gamma-ray Burst Monitor (GBM) NaI detectors and the two GBM BGO detectors. The LAT detector sensitivity depends on the diffuse background emission underlying the source, which varies predominantly with Galactic latitude. During the first year the LAT will be in survey mode. The GBM detectors' sensitivity depends on the orientation of a gamma-ray burst to the detector. Therefore, GLASTspec provides a number of detector/observation types for each type of Fermi detector. This tool uses updated LAT instrument response functions. Background spectra are based on pre-launch modeling. Also, this generalized spectrum simulation by necessity simplifies an actual observation: the diffuse background in the LAT energy band varies spatially; the GBM backgrounds vary over an orbit; and the GBM will also observe burst photons that scatter off of the spacecraft and the atmosphere. Because of the LAT's relatively large point-spread-function (<3.5º at 100 MeV, <0.15º at >10 GeV) most LAT data should be analyzed with spatial-spectral methods. Only very strong sources should properly be analyzed with a purely spectral tool such as XSPEC, the software behind GLASTspec. Nonetheless, GLASTspec provides an approximate measure of the spectra that the LAT will observe and how well spectral parameters can be determined. Please record the date you ran GLASTspec and the response functions' version information. While we value stability during the proposal preparation period, we wish to represent the detectors' capabilities accurately, and therefore may need to update the response functions used by GLASTspec. The following detector/observation type combinations are currently available through GLASTspec:
Fermi LAT, survey, Galactic poleThis detector/observation type combination simulates an orbit averaged, orbit-precession averaged, survey mode observation near one of the Galactic poles (i.e., the diffuse emission is near the minimum value). Because the LAT is assumed to be in survey mode, the 'exposure' is the same as the actual elapsed time. The exposure will be incorrect for timescales less than 2 orbits (approximately 3 hours).Given the approximate nature of this calculation, you can model a pointed observation by multiplying the pointed observation time by a factor to get the equivalent survey mode observation time (in this tool the 'exposure'). For example, between SAA passages and earth occultations the pointed observation duty cycle might be 2/3. Between SAA passages and the varying inclination angles (the angle between the source and LAT normal) in survey mode, the average on-source effective area is about 1/6 of the on-axis effective area. Thus you can multiply the pointed observation time by a factor of ~4 to get the equivalent survey mode observation time (i.e., the exposure). Version: P6_V1_DIFFUSE
Fermi LAT, survey, 45 deg. Galactic latitudeSame as 'Fermi LAT, survey, Galactic pole' case, except that the region is at a Galactic latitude of 45 degrees, and thus has a moderate level of diffuse background.Version: P6_V1_DIFFUSE
Fermi LAT Detector, survey, Galactic planeSame as 'Fermi LAT, survey, Galactic pole' case, except that the region is on the Galactic plane (but not at the Galactic Center), and thus has a high level of diffuse background.Version: P6_V1_DIFFUSE
Fermi GBM, NaI, 0 degrees inclinationThis detector/observation type combination models a gamma-ray burst on the axis of a NaI detector. The response is for the direct component only, without the burst gamma rays that scatter off the spacecraft or the Earth's atmosphere.Version: Direct_070323
Fermi GBM, NaI, 30 degrees inclinationSame as the 'Fermi GBM, NaI, 0 degrees inclination' case, but for a NaI detector at 30 degrees incidence.Version: Direct_070323
Fermi GBM, NaI, 60 degrees inclinationSame as the 'Fermi GBM, NaI, 0 degrees inclination' case, but for a NaI detector at 60 degrees incidence.Version: Direct_070323
Fermi GBM, BGO, 0 degrees inclinationSame as the 'Fermi GBM, NaI, 0 degrees inclination' case but for a BGO detector at 0 degree incidence.Version: Direct_070323
Fermi GBM, BGO, 30 degrees inclinationSame as the 'Fermi GBM, BGO, 0 degrees inclination' case, but for a BGO detector at 30 degree incidence.Version: Direct_070323
Fermi GBM, BGO, 60 degrees inclinationSame as the 'Fermi GBM, BGO, 0 degrees inclination', but for a BGO detector at 60 degrees incidence.Version: Direct_070323
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- HEASARC Director: Dr. Andy Ptak
- NASA Official: Dr. Andy Ptak
- Web Curator: J.D. Myers
- Last Modified: 5-Apr-2022