Fermi Science Support Center

LAT Instrument Response Functions for Pass 6 Analysis—Overview

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.

IRF selection

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:

  • Improved effective area: to reduce known systematics by comparing Monte Carlo distributions with the same derived by collected data one evident cause of 10-percent-level systematics on the estimated LAT efficiency become apparent; effective area is now rescaled to cancel this effect to the best of our knowledge; further analyses so not indicate any similar inconsistency between data and Monte Carlo
  • second order effects: inclination- and livetime-dependence of effective area are now included
  • the point spread function we derived from Monte Carlo was soon observed to be an incorrect description of the LAT performance at high energies: P6_V11 IRFs feature above 1 GeV a point spread function derived by fitting the width of bright point sources

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

Additional Information

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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