Caveats About Analyzing LAT Data
Prescriptions for event selection for analysis of Fermi-LAT data are provided in the data preparation section of the Cicerone.
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, with overlays of periodic trigger events to realistically simulate the effects of residual signals from off-time or non-triggering particles. 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.
- The "Transient" event class (class 1 and up) has large non-photon backgrounds and is only appropriate for the study of short transient events with a duration of less than 200s. Beyond these timescales the level on non-photon backgrounds can compromise the quality of the data analysis.
- We recommend using the "Diffuse" (class 3 and up) photons for all point source analyses as well as for the analysis of bright diffuse sources up to 20 GeV. When integrating over large regions of the sky, and particularly at high energies, the residual non-photon contamination in this event class may result in spurious spectral features, and so we recommend using only the "DataClean" (class 4) photons for studies of diffuse emission at these high energies.
- Data below 100 MeV can not be used for spectral analysis because of the rapid change of effective area with energy and because of residual uncertainty in the instrument response. Furthermore, when using the "Diffuse" (class 3 and up) and "DataClean" (class 4) we recommend starting at 200 MeV for the most robust results. Inclusion of data outside the recommended energy range can 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 (Abdo A. A. et al. 2009, PRL, 103 251101) and Post-launch performance of the Fermi Large Area Telescope.
- 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, from 70 MeV to 1 GeV, but have no effect on spectral analysis. The Fermi-LAT team is testing corrections for these behaviors 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. For best results, we recommend starting above 200 MeV. As a practical matter, given the current limited statistics at hundreds of GeV, we recommend limiting spectral analysis to energies less 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 and differ slightly for each event class. For "Transient" and "Diffuse" classes these numbers are 10% at 100 MeV, decreasing to 5% at 560 MeV, and increasing to 20% at 10 GeV, see Fermi LAT Measurements of the Diffuse Gamma-Ray Emission at Intermediate Galactic Latitudes (Abdo A. A. et al. 2009, PRL, 103 251101). For "DataClean" these numbers are 8% at 100 MeV, decreasing to 5% at 560 MeV, and increasing to 20% at 10 GeV, see The Spectrum of the Isotropic Diffuse Gamma-Ray Emission Derived From First-Year Fermi Large Area Telescope Data (Abdo A. A. et al. 2010, JCAP, 04, 014).
The Vela pulsar does not provide sufficient statistics to measure the systematic error above 10 GeV. However, studies using the photons from the limb of the Earth confirm a 20% systematic error from 10 GeV up to 100 GeV. Furthermore, the limited statistics we do have suggest that the 20% error extrapolates unchanged to 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.
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 when the various sources were visible.
Update: These effects have been corrected in the Pass6 V11 IRFs. The phi dependence has been taken into account, and correction have been made using flight data for calibration sources.
- 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 Fermi Large Area Telescope Bright Gamma-ray Source List (Abdo, A. A. et al. 2009, ApJS, 183, 46, 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 (1FGL, Fermi Large Area Telescope First Source Catalog (Abdo, A. A. et al. 2010, ApJS, 188, 405), a factor of 1.1 was applied.
The on-orbit psf delivered with P6V11 irfs addresses the discrepancy described above by deriving a new estimate of the psf by observed point sources above 1 GeV. The improved description of the LAT psf in P6_V11 irfs ensures that the 95% containment of the P6_V11 psf matches what can be derived by observed point sources within 20%. The limitations of this approach are mostly due to the small statistics available, especially at higher energies: the main consequence is that there is no description of the dependence of the psf on the photon inclination with respect to the LAT boresight and only a value averaged over the two-year orbital history is available. For very short time intervals where the theta distribution of collected photons can be significantly different from the long-term average some small deviation is expected: this can cause some small apparent variability of strong sources on very small time periods. A dedicated paper describing in detail the derivation of the on-orbit psf and the related systematics will be available soon.
- The absolute LAT energy scale 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 (Abdo, A. A. et al. 2009 PRL, 102, 181101).
- 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 not feasible. The livetime value resets often and this column should be ignored. The LIVETIME accumulations in the spacecraft data files are correct and complete.
- Please see the diffuse model page to access 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. Documentation is also provided.
- 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.
- All the released diffuse models were derived for point sources and compact extended sources studies only, and are not suited for studies of extended sources and/or large-scale diffuse emissions. There is no standard background model for extended diffuse analyses, nor a generic treatment of extended sources and diffuse extended emissions. The approach for such studies depends on the aim and the objective of the study and should be evaluated, treated and tested on a case by case basis.
- Each diffuse model should be used with the corresponding Event Selection and IRF. For example using P6 background models with P7 dataset or P7 background models with P6 dataset is strongly discouraged, because each model was fitted to the specific Event Selection and IRF. Ideally the resulting diffuse emission model is independent of IRFs used to derive it, but in practice the model will have built into it the low-level systematics associated with our imperfect knowledge of the IRFs and the approximations that we make regarding neglecting the finite energy resolution. However, as the diffuse emission is bright over most regions of the sky, these small imperfections may result in large systematic effects for sources. For this reason we do not recommend using diffuse emission models for analyses with event classes for which their validity has not been tested and established.
Furthermore, the templates for the isotropic spectrum include misclassified cosmic rays. The probability of such misclassifications differs between event classes as well as between front-converting and back-converting events. Therefore, it is important to use the isotropic template corresponding to the data selection. This applies both in terms of the event class and conversion class, i.e., if "Diffuse" front-converting events are selected, then the corresponding isotropic template for "Diffuse" front-converting events should be used. If both front-converting and back-converting events are selected then the combined template for front+back events is appropriate.
- It is recommended to 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 to not use data below 100 MeV for spectral analysis since the instrument response is not validated in the lower energy band.
- It is recommended to use the "Diffuse" class to search for GRB afterglows since the "Transient" class only adds lower energy photons which many not help given the increased backgrounds.
- It is recommended to 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 when updated calibrations become available over the coming months.
Last updated by: Elizabeth Ferrara 6/16/2014