LAT Background Models
| Galactic diffuse model |
Event Selection/ IRF Name |
Isotropic spectral template |
|---|---|---|
| gll_iem_v02_P6_V11_DIFFUSE.fit | Pass 6 (V11) Diffuse (front+back) P6_V11_DIFFUSE |
isotropic_iem_v02_P6_V11_DIFFUSE.txt |
| Pass 6 (V11) Diffuse (front only) P6_V11_DIFFUSE::FRONT |
isotropic_iem_front_v02_P6_V11_DIFFUSE.txt | |
| Pass 6 (V11) Diffuse (back only) P6_V11_DIFFUSE::BACK |
isotropic_iem_back_v02_P6_V11_DIFFUSE.txt | |
| gll_iem_v02.fit | Pass 6 (V3) Diffuse (front+back) P6_V3_DIFFUSE |
isotropic_iem_v02.txt |
| Pass 6 (V3) Diffuse (front only) P6_V3_DIFFUSE::FRONT |
isotropic_iem_front_v02.txt | |
| Pass 6 (V3) Diffuse (back only) P6_V3_DIFFUSE::BACK |
isotropic_iem_back_v02.txt | |
| Pass 6 (V3) DataClean (front+back) P6_V3_DATACLEAN |
isotropic_iem_p6v3dataclean_v02.txt |
Many analyses of LAT data require models of Galactic diffuse and isotropic emission (see the likelihood analysis tutorial for an example). Find the detailed description of the models here.
The Galactic Diffuse Model
The Galactic model is a spatial and spectral template. The model for the Galactic diffuse emission was developed using spectral line surveys of H I and CO (as a tracer of H2) to derive the distribution of interstellar gas in Galactocentric rings. Infrared tracers of dust column density were used to correct column densities in directions where the optical depth of H I was either over or under-estimated. The model of the diffuse gamma-ray emission was then constructed by fitting the gamma-ray emissivities of the rings in several energy bands to the LAT observations. The fitting also required a model of the inverse Compton emission calculated using GALPROP and a model for the isotropic diffuse emission.
Isotropic Spectral Templates
An isotropic spectral template provides the spectral form from a fit to the all-sky emission (|b|>30 deg) that is not represented in the Galactic diffuse model and therefore includes both extragalactic diffuse gamma rays and remaining residual (misclassified) cosmic-ray emission. The Pass 6 Diffuse class contains a significant background contribution at high energies. An isotropic model is determined under the assumption of a specified Galactic diffuse model and for one of the LAT event class selections. The resulting spectral template should be used with the same Galactic diffuse model and event class selection that were used to derive it. The two columns in the isotropic template are: the central energy (in log MeV) for the band, the differential flux in that band (in ph/cm2/s/MeV/sr).
The Pass 6 Source class does not have an isotropic spectral template because it is not in common use. Analysis using the Pass 6 Transient class typically requires case-specific estimates of the background and so no general template is provided.