The Second Catalog of Low-energy Fermi-LAT Sources

We used 14 yr of Fermi-Large Area Telescope (LAT) data to derive a catalog of sources detected between 20 MeV and 200 MeV. For each source, we provide localization, spectral, and association information. The Second Catalog of Low-Energy Fermi-LAT sources (2FLE) contains 450 sources detected at TS > 25, with median energy flux of F 20−200 MeV = 1.9 × 10 −11 erg cm −2 s −1 . Using a Bayesian and likelihood-ratio method, we associate 377 2FLE sources with a known counterpart. Of the 325 objects associated with extragalactic sources, the dominant source class represented in the 2FLE is flat-spectrum radio quasars (FSRQs). Additionally, 52 sources are associated with Galactic sources. Three associated sources have not been previously detected in any γ -ray catalog and represent new γ -ray sources of known origin. We find that 387 2FLE sources have a positional cross-match with the 4FGL-DR4 sources. For the sources with a 4FGL counterpart, we compare the spectrum of the 2FLE source to the 4FGL spectral energy distributions. Of the 73 2FLE sources without multiwavelength associations, 13 have a 4FGL counterpart that lacks classification, while 60 are newly discovered γ -ray sources. Among these 60 sources, 51 are found at high latitudes (∣ b ∣ > 10°), and 38 are detected at >10 σ . Additionally, we find that out of the 450 2FLE sources, 45 are TeV emitters, and six are spatially coincident with a 1LHAASO source. The 2FLE helps us pave the way for reducing the “MeV gap,” and provides the most sensitive catalog between 20 and 200 MeV to date.