The Second Catalog of Low-Energy Fermi-LAT Sources

We used 14 years 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 $F_{20-200\text{MeV}}=1.9\times {10}^{-11}$ 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 are flat-spectrum radio quasars (FSRQ). Additionally, 52 sources are associated with Galactic sources. Three associated sources have not been previously detected in any $\gamma$-ray catalog and represent new $\gamma$-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 multi-wavelength associations, 13 have a 4FGL counterpart that lacks classification, while 60 are newly discovered $\gamma$-ray sources. Among these 60 sources, 51 are found at high latitudes ($\mathrm{\mid }b\mathrm{\mid }>{10}^{\circ }$), and 38 are detected at $>10\sigma$. Additionally, we find that out of the 450 2FLE sources 45 are TeV emitters, and 6 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 20-200 MeV to date.