The Fourth Fermi LAT Source Catalog (4FGL)

The fourth Fermi Large Area Telescope gamma-ray source catalog (4FGL) is nearly complete. Based on the first eight years of science data from the Fermi Gamma-ray Space Telescope mission covering the whole sky in the 50 MeV-1 TeV energy range, it is the deepest yet in this energy band. Relative to the 3FGL catalog, the 4FGL catalog has twice as much exposure as well as a number of analysis improvements, including an updated model for Galactic diffuse gamma-ray emission. The 4FGL catalog includes more than 5000 sources with at least 4σ significance. Seventy-five sources are modeled explicitly as spatially extended. Overall about 2/3 of the sources are associated with known gamma-ray-emitting objects, and many are firmly identified based on angular extent or correlated variability (periodic or flaring) observed at other wavelengths. For the remaining sources we have not found plausible counterparts at other wavelengths. Most of the identified or associated sources are active galaxies of the blazar class, with pulsars being the second largest class.