Fermi Gamma-ray Space Telescope

LAT 8-year Source Catalog (4FGL)

The Large Area Telescope (LAT) on board NASA's Fermi Gamma-ray Space Telescope (launched June 11, 2008) surveys the entire sky each day. This web page presents the fourth full catalog of LAT sources, based on 8 years of survey data and the 50 MeV-1 TeV range. For a full explanation about the catalog and its construction see the Fermi-LAT Fourth Source Catalog Paper (4FGL) available on arxiv.

The source designation is 4FGL JHHMM.m+DDMM(c,e,i,s) where the 4 refers to the fourth catalog (1FGL was released at 1 year, 2FGL at 2 years, and 3FGL at 4 years) and FGL represents Fermi Gamma-ray LAT. The optional "e", "i" and "s" designators are explained in the caveats below.

LAT Catalog Data Products

The LAT 8-year Source Catalog is currently available as a FITS file to be used for data analysis within the Fermitools. A BROWSE table containing a searchable version of the catalog is also available. Supporting tools and documentation have been provided and are linked below.

  • Sun/Moon template model files: Only for use with the 8-year data set!
  • We emphasize several important points and caveats regarding use of the 4FGL catalog:

    • This work is a summary of the LAT results covering the time period August 4, 2008, to August 2, 2016.
    • The analysis is based on data in the 50 MeV to 1 TeV 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 5065 sources.
    • The corresponding energy flux limit varies according to the hardness of the source spectrum and the brightness of the background Galactic diffuse emission.
    • Catalog entries include quality flags that indicate potential issues: 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 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 seventy-five 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.
    • The Crab pulsar and PWN are represented by a total of three entries, two of which (designated "i" for inverse compton and "s" for synchrotron) represent spectral components of the PWN. We consider these three entries to represent two sources.
    • 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 4FGL sources. Upper case in the CLASS column means identified. 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 8-year Source Catalog Paper. 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 4FGL catalog. Any significant updates will be advertised to the fermi-news mailing list. All changes will be documented in the change log below.

    4FGL Source List Change Log

    Rev Date Filename Change Description
    1 25 February 2019 gll_psc_v17.fit Initial Release
    2 7 March 2019 gll_psc_v18.fit Improve counterpart position when VLBI is available, report positions of counterparts of extended sources, provide counterpart positions in assoc.reg file, clean up TeV associations
    3 15 May 2019 gll_psc_v19.fit Delete curved sources below detection threshold, add spectral energy distributions and light curves with 1-year bins, add low-confidence associations and counterparts from follow-up of Fermi sources