Fermi LAT Performance
(archived: Pass6_V3)

This page summarizes the top-level performance of the Fermi-LAT observatory that are currently used by the collaboration for science analysis and are available to the general public.

The LAT performance is governed primarily by three things:

  • LAT hardware design
  • Event reconstruction algorithms
  • Background selections and event quality selections

A result of the analysis is the production of full Instrument Response Functions (IRFs), describing the performance as a function of photon energy, incidence angle, conversion point within the instrument, and other important parameters.

Pass6 version 3 Instrument Response Functions

The plots below represent the work of many people in the LAT team. They correspond to the status of the analysis known as Pass6 version 3, also called Pass6_V3 or P6_V3, which has been optimized for the study of point-like sources and the production of the LAT catalog.

These IRFs 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 (Pass6_V1), primarily pile-up and accidental coincidence effects in the detector subsystems leaving ghost signals in the detector in coincidence with good photon triggers.

An appropriate sampling of flight data periodic triggers was overlayed as a background to standard simulations of gamma-rays, and the resulting performance was derived by applying pre-launch event analysis to such updated simulations.

The on-axis effective area reported here is about 7000 cm2 at 1 GeV and is energy dependent; this is approximately 10% lower at 1 GeV than the pre-launch effective area corresponding to the same event selection.

For more details, please refer to the paper Post-launch performance of the Fermi Large Area Telescope.

Important caveats are associated with these IRFs and should be kept in mind when performing analysis of the publicly distributed Fermi-LAT data.

Effective Area, Point Spread Function and Energy Resolution

Note: Starting from the front of the instrument, the LAT tracker (TKR) has 12 layers of 3% radiation length tungsten converters (THIN or FRONT section), followed by 4 layers of 18% r.l. tungsten converters (THICK or BACK section). These sections have intrinsically different PSF due to multiple scattering, and the performance plots are presented for both of these sections.

Effective Area: The plot on the left is for normal incidence photons (defined here as "cos(theta)>0.975"); the one on the right is for 10 GeV photons as a function of incidence angle.

Acceptance: Defined here as the effective area integrated over the solid angle. Notice that the acceptance has a slower turn-on wrt the effective area, highlighting the dependence of the FOV on energy. That is the intrinsic acceptance regardless of the orbital characteristics. To obtain the effective acceptance, the curve has to be scaled by a constant factor which takes into account the instrument deadtime; the South Atlantic Anomaly (SAA), where the LAT does not take data; and details of the observation strategy (about 20% for standard survey and according to current simulations).

Point Spread Function (PSF): Angles for 68% and 95% containment of the reconstructed incoming photon direction. The plot on the left is for normal incidence photons (defined as "cos(theta)>0.9"); the one on the right is for 10 GeV photons as a function of incidence angle.

The ratio PSF95% / PSF68% is a useful indicator of the magnitude of the tails of the distribution. Plots are as defined above.

Energy resolution: 68% containment of the reconstructed incoming photon energy. The plot on the left is for normal incident photons (defined as "cos(theta)>0.9"); the one on the right is for 10 GeV photons as a function of incidence angle.

Point Source Sensitivity Plots

Using the above instrument performance characterization, we have produced additional plots related to point source sensitivity. The first plot shows the differential sensitivity for an isolated, high-latitude point source. The differential sensitivity is evaluated from the 5-sigma sensitivity limit for 1/4-decade ranges of energy. Quantitatively, the 5-sigma limit is taken as the likelihood test statistic of 25. Limits are derived in each energy range independently and are based on a live time of 0.8 yr, which is the approximate value for a 1-year data set; this takes into account the loss of observing time due to: SAA passages and the instrumental deadtime that the observations are in survey mode; and that the diffuse background is isotropic and has an intensity of 1.5 x 10^-5 cm-2 s-1 sr-1 (>100 MeV) and a photon index of 2.1. Also shown are curves for background intensities that are 10x and 100x greater; the latter roughly approximating the intensity near the Galactic center.

The point source sensitivity using the information in all energy bins is much better than the individual energy bin sensitivities above. We therefore provide the integral sensitivity measures as follows:

The bowtie plot shown below depicts the minimum needed for a 20% determination of the flux after one day, one month, and one year of operation in all-sky survey for a 1/E^2 source. The resulting significance at each of these levels is about 8-sigma; the spectral index is determined to about 6%; and the bowtie shape indicates the energy range that contributes the most to the sensitivity. To make a measurement at that level or better, a flat spectral energy density curve must lie above the axis of the bowtie.

There are also significant variations of the sensitivity due to the spatial structure in the diffuse galactic gamma-ray background. These are summarized in the map below, which shows the sensitivity across the sky in Galactic coordinates.

Finally, experiments are often compared using an integral sensitivity plot (5-sigma sensitivity for E>E0), assuming a 1/E^2 spectrum source at high latitude. Below, we show an update for the Fermi LAT:

Gamma-ray Bursts Sensitivity

An estimator for the Localization power as a function of the fluence is plotted in the graph below.

Each marker corresponds to a different inclination angle and a different high energy spectral index, and represents the minimum fluence (in the standard 50 keV-300 keV energy band), which corresponds to a detection (y-axis) vs the 68% localization accuracy (x-axis). The solid and dotted lines are the result of the formula shown on the canvas, and allow computation of the localization at a given fluence (for normal incidence and for 60 degrees off-axis).

If there is any additional information you would find useful to have posted on this page, please contact Julie McEnery and/or the LAT Analysis Coordinator, Nicola Omodei.

Fermi LAT performance updates and archive

Performance updates are expected in the second year for:

  • Mapping the orbital dependence of the pileup effect, which varies with the incoming particle trigger rate.
  • Recovering the instrument acceptance after proper correction of the event reconstruction and background rejection analysis based on update simulations as described above.

Earlier performance descriptions are archived; see: Pass6_V1. Science performance requirements for the LAT are given in Table 1 of the Science Requirements Document.

The Fermi data and the full set of response functions and Science Analysis Tools can be downloaded from the Fermi Science Support Center website's Data Section.

Useful links:

Updated 3/22/2010, maintained by Riccardo Rando (riccardo.rando@pd.infn.it), Eric Charles (echarles@slac.stanford.edu), and Seth Digel (digel@slac.stanford.edu).