This page details the Instrument response functions for the "Pass 6" version of the LAT event selections. These correspond to the data that were released up to August of 2011. People using the the newer "Pass 7" version of the events selection should read the update version of this page.
Because of the complexity of the IRFs, LAT data analysis must be performed carefully to avoid a mismatch between the selected events and the IRF used within the analysis. There are multiple IRFs delivered with the Fermi Science Tools to allow the user the flexibility necessary for the different analysis types. The LAT data has been processed using Pass 6 and so the corresponding Pass 6 IRF sets must be used to accurately represent those photons.
Pass 6 event analysis was developed prior to launch. Correspondingly, P6_V1 IRFs were based only on on-ground estimates. The LAT instrument team soon updated their Monte Carlo simulations to include the most evident on-orbit effects, like event pile-up: P6_V3 IRFs were based on this improved simulation. Since then, they have improved their knowledge of the on-orbit effects, including second order dependencies, and singled out some major sources of systematics. P6_V11 IRFs address these issues and constitute the best description of the LAT instrument the LAT team can provide while remaining in the framework of the Pass 6 event analysis. Any further improvement will be based on a brand new event analysis (“Pass7”).
The main features of P6_V11 IRFs are:
Recommendations for the appropriate selection and IRF set to be used within an analysis are provided in the data preparation section of the Cicerone. The tables below give the association between the Pass 6 IRF sets and the photon properties as provided in the LAT photon data.
P6_V11 IRF name | Event Class | Conversion Type | Description |
---|---|---|---|
P6_V11_DATACLEAN | 4 | 0+1 | Highest quality and lowest background selection - recommended for analyses that integrate large regions of the sky. Minimizes non-photon spectral features. |
P6_V11_DATACLEAN::FRONT | 4 | 0 | Front converting events |
P6_V11_DATACLEAN::BACK | 4 | 1 | Back converting events |
P6_V11_DIFFUSE | 3+4 | 0+1 | High quality selection - recommended for most analysis |
P6_V11_DIFFUSE::FRONT | 3+4 | 0 | Front converting events |
P6_V11_DIFFUSE::BACK | 3+4 | 1 | Back converting events |
P6_V11_SOURCE | 2+3+4 | 0+1 | Intermediate quality selection - not recommended for analysis |
P6_V11_SOURCE::FRONT | 2+3+4 | 0 | Front converting events |
P6_V11_SOURCE::BACK | 2+3+4 | 1 | Back converting events |
P6_V11_TRANSIENT | 1+2+3+4 | 0+1 | Lower quality selection - used for certain transient or timing analysis |
P6_V11_TRANSIENT::FRONT | 1+2+3+4 | 0 | Front converting events |
P6_V11_TRANSIENT::BACK | 1+2+3+4 | 1 | Back converting events |
P6_V3 IRF name | Event Class | Conversion Type | Description |
---|---|---|---|
P6_V3_DATACLEAN | 4 | 0+1 | Highest quality and lowest background selection - recommended for analyses that integrate large regions of the sky. Minimizes non-photon spectral features. |
P6_V3_DATACLEAN::FRONT | 4 | 0 | Front converting events |
P6_V3_DATACLEAN::BACK | 4 | 1 | Back converting events |
P6_V3_DIFFUSE | 3+4 | 0+1 | High quality selection - recommended for most analysis |
P6_V3_DIFFUSE::FRONT | 3+4 | 0 | Front converting events |
P6_V3_DIFFUSE::BACK | 3+4 | 1 | Back converting events |
P6_V3_SOURCE | 2+3+4 | 0+1 | Intermediate quality selection - not recommended for analysis |
P6_V3_SOURCE::FRONT | 2+3+4 | 0 | Front converting events |
P6_V3_SOURCE::BACK | 2+3+4 | 1 | Back converting events |
P6_V3_TRANSIENT | 1+2+3+4 | 0+1 | Lower quality selection - used for certain transient or timing analysis |
P6_V3_TRANSIENT::FRONT | 1+2+3+4 | 0 | Front converting events |
P6_V3_TRANSIENT::BACK | 1+2+3+4 | 1 | Back converting events |
Detailed descriptions of the LAT instrument, event analysis, and performance can be found in the following:
The IRFs shown here are based on updated simulations of the instrument that take into account effects measured in flight that were not considered in pre-launch performance estimates. Recent information regarding the results of post-launch testing which led to this formulation can be found in the following documentation:
The Pass 6 DataClean event selection is described in Spectrum of the Isotropic Diffuse Gamma-Ray Emission Derived from First-Year Fermi Large Area Telescope Data (Abdo, A. A., et al. 2010, Physical Review Letters, 104, 101101).
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