Caveats on Use of LAT Data (adapted from the Cicerone)

Event Selection

The Fermi-LAT performance associated to the released Pass6_V3 Instrument Response Functions (IRF) are documented on the LAT Performance Page. These IRFs were derived using MonteCarlo generated samples of photons between 18 MeV and 562 GeV. However, the validity of these IRFs when performing analysis of LAT data, in terms of usable event classes and energy range, is determined by the caveats discussed below.

• Use "Diffuse" class for diffuse, extended, and point source analysis. Other event classes have higher charged-particle background contamination and may result in spurious spectral features.

• Data below 100 MeV cannot be used for spectral analysis because of the rapidly changing effective area with energy, and because of residual uncertainty in the instrument response. Inclusion of these data will result in the creation of spurious spectral features under the current Instrument Response Functions. For additional details about this effect, see Fermi LAT Measurements of the Diffuse Gamma-Ray Emission at Intermediate Galactic Latitudes and Post-launch performance of the Fermi Large Area Telescope.

• Data above 10 GeV currently have non-negligible background contamination from charged particles. However, it is fine to use all released data in combination with the released diffuse model and its associated isotropic component template, which models such residual instrumental background for the purpose of performing point-source, extended and Galactic diffuse analysis of LAT data.

Note: The isotropic component template consists of both residual cosmic-rays and diffuse isotropic gamma-ray emission; thus, at present these data and background models do not support an analysis of the isotropic gamma-ray component by itself.

• The photon energy assignment algorithm has been found to have some biases at high energies. The most prominent effect is a tendency to concentrate events near 300 GeV into a relatively narrow feature just below 300 GeV. This effect has been observed in large Monte Carlo studies, and is unlikely to affect point or stacked source studies with the statistics available in the data. Weaker structures in the shape of very narrow spikes appear at lower energies, Ê from 70 MeV to 1 GeV, but have no effect on spectral analysis. The Fermi-LAT team is testing corrections for these behaviours that will be distributed in our next release.

All the effects described above currently limit the spectral analysis of LAT data to energies greater than 100 MeV. As a practical matter, given the current limited statistics at hundreds of GeV, we recommend limiting spectral analysis to energies smaller than 300 GeV.

Systematic effects and uncertainties

• The systematic uncertainties on the effective area, evaluated by comparing the efficiencies of analysis cuts for data and simulation of observations of Vela, are energy dependent: 10% at 100 MeV, decreasing to 5% at 560 MeV, and increasing to 20% at 10 GeV and above, see Fermi LAT Measurements of the Diffuse Gamma-Ray Emission at Intermediate Galactic Latitudes.

Measurements of the systematic errors above 10 GeV will be made when sufficient statistics are available for high energy sources. So, the systematic errors above 10 GeV have not been fully validated. It is unlikely that the current estimate of 20% systematic error at 10 GeV extrapolates unchanged to significantly higher energies like hundreds of GeV.

• While the current Instrument Response Functions describe the average effect on the acceptance due to the pile-ups and chance coincidences in the detector, residual effects remain on short time scales. More details can be found in Post-launch performance of the Fermi Large Area Telescope.

When the spacecraft is in orbital regions with high particle backgrounds, the increased levels of activity in the LAT reduce the photon selection efficiency, especially in the lower parts of the Fermi energy range. The two plots linked below show the ratio of the efficiency for the LAT photon selection as a function of orbital position relative to the mean efficiency for long time scales at 100 MeV and 3 GeV, respectively. Variability studies, and studies involving data taken over time periods less that one orbit, should directly account for these effects. Studies comparing sources in different regions of the sky should allow for slight differences in mean efficiency during the time periods the various sources were visible.

• Studies of the point-spread function (PSF) with on-orbit data are ongoing. A stacking analysis combining data for many blazars that are confidently associated with LAT sources as well as bright pulsars suggest that the width of the PSF at high energies (>5 GeV) may be underestimated in the Monte Carlo simulations that were used to generate the response functions for the high-level analysis. At energies >32 GeV the width may be underestimated by a factor of ~2. The LAT team is continuing to base its analyses on the Monte Carlo-generated PSFs, which were partly validated with beam test data of photons up to 2.5 GeV and electrons up to 282 GeV.

The effects of underestimating the width of the PSF at high energies are primarily in the precision of the source location regions for bright or hard-spectrum sources. For the Bright Source List (Abdo et al. 2009, http://arxiv.org/abs/0902.1340, eqn. 1), the LAT team applied a conservative factor of 1.4 scale factor to the source location regions. In the analysis for the first-year Catalog, a factor of 1.1 is being applied.

• The absolute LAT energy scale, at this early stage of the mission, is determined with an uncertainty of +5% -10%. For additional details, see Measurement of the Cosmic Ray e+ + e- spectrum from 20 GeV to 1 TeV with the Fermi Large Area Telescope.

• The LIVETIME column in the photon FITS file should be ignored. It was intended to contain the livetime accumulated since the start of the current run (data-taking interval, typically ~1 orbit in duration), but technical difficulties made this unfeasible. The livetime value resets often, and this column should be ignored. The LIVETIME accumulations in the spacecraft data files are correct and complete.

Diffuse Model

• From the Fermi LAT U33 Disk Browser , you can obtain the currently-recommended model of Galactic diffuse emission and the corresponding spectrum of the isotropic emission, including residual background from cosmic rays misclassified as gamma rays.

• The model of Galactic diffuse emission is defined between 50 MeV and 100 GeV. To derive the isotropic spectrum at higher energies, we extrapolated the Galactic diffuse emission model. Analyses at energies >100 GeV are likely to be limited primarily by photon statistics, but the accuracy of the modeling of diffuse emission at those high energies is also reduced.

GRB analysis

Recommendations:

• Use the "Transient" class to maximize the statistics for spectral analysis of GRB prompt emission, since it is more-or-less background free due to the short duration of the GRB prompt emission. When using the "Transient" class, it is recommended that you not use data below 100 MeV for spectral analysis, since the instrument response is not validated in the lower energy band.

• Use the "Diffuse" class to search for GRB afterglows, since the "Transient" class only adds lower energy photons, which may not help given the increased backgrounds.

• Use data above 500 MeV to localize the GRB, since low energy events suffers from the so-called "Fish eye" effect, where the reconstructed direction points toward smaller inclination angles due to selection bias. It is also recommended to use the "Diffuse" class for GRB localization, since the "Transient" class does not improve statistics at the high energy band.

LAT Monitored Source List

• These early flux estimates do not include systematic uncertainties, and do not have an absolute flux calibration. Use of these data as absolute flux measurements for constraining models, or for comparison to other data, is strongly discouraged at this time. In addition to overall normalization uncertainties, source fluxes may have variations of up to 10% due to currently-uncorrected dependencies of the gamma-ray detection efficiency on variations of the particle background in orbit. Please note that these results are produced using preliminary instrument response functions and calibrations. The quality and stability of these results will improve as updated calibrations become available over the coming months.