Fermi Gamma-ray Space Telescope

LAT 2-year Source Catalog

The Fermi Gamma-ray Space Telescope (Fermi) Large Area Telescope (LAT) is a successor to EGRET, with greatly improved sensitivity, resolution, and energy range. This web page presents the second full catalog of LAT sources, based on the first 24 months of survey data. For a full explanation about the catalog and its construction see the LAT 2-year Catalog Paper (also available on arxiv).

The source designation is 2FGL JHHMM.m+DDMM(c,e) where the 2 refers to the second catalog (one was released at 1 year, and another is planned for after 5 years) and FGL represents Fermi Gamma-ray LAT. The optional "c" and "e" designators are explained in the caveats below.

LAT Catalog Data Products

The LAT 2-year Source Catalog is currently available as a FITS file, as an XML model file to be used for data analysis within the Fermi Science Tools, and as a BROWSE table. Supporting tools and documentation have been provided and are linked below.

It is important that all users of this catalog review the caveats listed below the catalog links. These describe the content of the catalog at a high level, as well as some cautions for the user.

We emphasize several important points and caveats regarding use of the 2FGL catalog:

  • This work is a summary of the LAT results covering the time period August 4, 2008, to July 31, 2010.
  • The analysis is based on data in the 100 MeV to 100 GeV energy range.
  • Source detection is based on the integrated data set, i.e., sources are detected according to their average fluxes.
  • The catalog adopted multiple spectral shapes, depending on the observed source properties. Pulsars are modeled by an exponentially cutoff Power Law. Others are modeled as a LogParabola if statistically significant curvature (according to Signif_Curve) is detected, or as a simple Power Law otherwise.
  • After computing the maximum likelihood, the threshold for inclusion is likelihood Test Statistic TS > 25, corresponding to somewhat more than 4 sigma significance. The catalog has 1873 sources.
  • The corresponding flux limit varies widely according to the hardness of the source spectrum and the brightness of the background Galactic diffuse emission.
  • The 2FGL information in five energy bands is not appropriate for detailed spectral modeling. Upper limits are indicated by 0 flux uncertainty; in these cases the corresponding flux entry has a 2-sigma upper limit. Note that some sources have only upper limits in each of the 5 bands; these sources have been detected based on their integrated fluxes.
  • The variability index indicates that the source is variable on a time scale of months. It does not address shorter or longer time variations. An index > 41.6 indicates a >99% confidence probability that the source is variable.
  • The diffuse model used in the analysis is still subject to improvements. Matching the model to the large-scale emission is an iterative process. The diffuse model is particularly important for sources from low to mid Galactic latitudes.
  • Catalog entries include two important quality flags that indicate potential issues: a "c" following the 2FGL name indicates that the source is found in a region with bright and/or possibly incorrectly modeled diffuse emission; any non-zero entry in the "Flags" column indicates that some inconsistency was seen during the analysis (see the table in the preprint for details). Sources with either of these indicators should be used with great care. They correspond to significant excesses of photons, but such excesses can result from residual extended emission or confused source pile-up.
  • The catalog analysis assumed point source emission for all but twelve extended sources, which were modeled using spatial templates and are indicated by an "e" following the name. Results for extended sources such as the LMC and Cen A are likely to not be representative of the total emission, and additional catalog point sources may appear near such objects.
  • Associations listed in the catalog are not in general to be taken as firm identifications. Associations are positional coincidences that are statistically unlikely to be due to chance alignment between known or candidate gamma-ray producing objects with 2FGL sources. Except for those cases with correlated variability or spatial extent, a physical relationship is not established. The designators for some associated sources may be updated in a future release to use more common or more complete names.

Again, for more information about the catalog see the LAT 2-year Catalog Paper (also on arxiv). Feedback about the catalog and paper should be directed to the paper contact authors. Ongoing efforts to understand the calibration and improve the analysis techniques may lead to updates to the 2FGL catalog. Any significant updates will be advertised to the fermi-news mailing list. All changes will be documented in the change log below.

LAT 2-year Catalog Change Log

Rev Date Filename Change Description
6 18 May 2015 gll_psc_v09.fit, gll_psc_v09_assoc.reg Added TUCDnn keywords to the main FITS file.
Removed extra associations documented in the 2LAC erratum.
5 7 August 2012 gll_psc_v08.fit, gll_psc_v08_assoc.reg Updated six association names that were not previously recognized by NED.
4 22 February 2012 gll_psc_v07.fit Corrected the names of two pulsars. Corrected the error ellipse for 2FGL J0001.7-4159.
3 26 August 2011 gll_psc_v06.fit Corrected a number of errors in the names of associations. Changed length of ASSOC columns.
2 9 August 2011 gll_psc_v05.fit Updates to some associations and classes
1 11 July 2011 gll_psc_v04.fit Initial Release