Fermi LAT Performance
(archived: Pass6_v1)

This page summarizes the top-level performance of the GLAST LAT observatory as of its launch.

Updates are expected as a result of the validations performed during the Launch and Early Operations phase, the 60-day commissioning period immediately following launch, particularly from the background measurement and subsequent rejection analysis.

The LAT performance is governed primarily by three things:

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

Thus, although the hardware integration and testing are now complete, as the event selection algorithms are optimized the performance must be updated. In other words, there is not a single, intrinsic, set of science performance parameters, but rather results of choices. These are based on detailed Monte Carlo simulations, along with beam tests and ground-level muon tests to check the characterization in the simulation. Standard sets of analysis choices, and their resulting performance characteristics, will be maintained by the LAT team. Earlier performance descriptions are archived here: v2, v3,pass4v2. Science performance requirements for the LAT are given in Table 1 of the Science Requirements Document.

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. The plots below represent the work of many people on the LAT team. They correspond to the status of the analysis known as Pass6 version 2, optimized for the study of point-like sources and the production of the LAT catalog. The full set of response functions and Science Analysis Tools can be downloaded from the GSSC site.

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 these sections.

Effective Area, Point Spread Function and Energy Resolution

Effective Area: Left for normal incidence photons (defined here as "cos(theta)>0.975"); right 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 the instrument deadtime, the SAA anomaly 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. Left for normal incidence photons (defined as "cos(theta)>0.9") - right 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 as defined above.

Energy resolution: 68% containment of the reconstructed incoming photon energy. Left for normal incident photons (defined as "cos(theta)>0.9") - right 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 is a single-energy-bin sensitivity plot, showing the 5-sigma sensitivity to a high-latitude source whose spectrum is integrated over 1/4 decade in energy centered on the energy shown on the horizontal axis. Sensitivity is defined as the flux such that the log of the expected likelihood ratio for detection is 25/2 (or 5 sigma in the Gaussian case) and at least 5 photons. Thus, this plot shows the point source sensitivity using only the photons in each energy bin separately. The assumptions are

  • 0.8 year livetime, i.e. one calendar year all-sky survey including effects of the SAA and deadtime
  • signal source with powerlaw index 2.0
  • spectrum used for uniform diffuse background: 1.5e-5, index 2.1. Also show 10x and 100x, the latter corresponding to 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 in two ways. First, the bowtie plot below, which shows the minimum needed for a 20% determination of the flux after a 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 is significant variations of the sensitivity in the sky 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. We show here an update for the GLAST 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 which corresponds to a detection (y-axis) vs the 68% localization accuracy. The dotted lines are the result of the formula on the canvas and allows to compute the localization at a given fluence (for normal incidence).

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

Useful links:

updated 11/6/2008, maintained by Steve Ritz