Fermi Releases Most Sensitive Low‑Energy Gamma‑Ray Catalog, Identifying 60 New Sources

Astronomers have released the most sensitive catalog yet of one of the sky’s least well-mapped forms of light: low-energy gamma rays. The new catalog, called 2FLE, lists 450 sources seen by NASA’s Fermi Large Area Telescope between 20 and 200 megaelectron volts, including 60 gamma-ray sources not present in previous gamma-ray catalogs.

The Fermi-LAT Collaboration posted the paper, “The Second Catalog of Low-Energy Fermi-LAT Sources,” on arXiv on Oct. 13, 2026, as arXiv:2610.18972v1. The arXiv record says the paper has been accepted for publication in the Astrophysical Journal Supplement Series. In the paper’s abstract, the collaboration said, “The 2FLE helps us pave the way for reducing the ‘MeV gap’, and provides the most sensitive catalog between 20-200 MeV to date.”

That matters because this part of the electromagnetic spectrum has long been harder to chart than neighboring bands. The so-called MeV gap refers to the historical difficulty of mapping gamma rays in the megaelectron-volt range, where instruments have generally had lower sensitivity and poorer source localization than in better-covered X-ray and higher-energy gamma-ray surveys. A denser, more reliable source list gives astronomers a clearer map of that underexplored sky.

The catalog is based on 14 years of Fermi data, spanning Aug. 4, 2008, to Aug. 2, 2022. After excluding interruptions, the analysis used about 10.3 years of observations. The collaboration reported 450 sources detected above its significance threshold of TS greater than 25.

Of those 450 sources, 377 were associated with known objects using Bayesian and likelihood-ratio methods. The paper says 325 of the associated sources are extragalactic and 52 are Galactic. Among the extragalactic identifications, the dominant class is flat-spectrum radio quasars, or FSRQs, a type of active galaxy with a jet pointed roughly toward Earth. The catalog includes 218 such FSRQs.

The remaining 73 sources do not yet have a multiwavelength association. Of those, 13 line up with previously unclassified sources in 4FGL, Fermi’s main source catalog. The other 60 are newly discovered gamma-ray sources not present in earlier gamma-ray catalogs. Most of those new detections, 51, lie at high Galactic latitude, meaning away from the crowded plane of the Milky Way. And 38 of the 60 were detected at more than 10 sigma, indicating especially strong statistical significance.

The new release also marks a clear step up from the first Fermi low-energy catalog, 1FLE, published in 2018. That earlier catalog covered a narrower 30 to 100 MeV range, used roughly 8.7 years of LAT data and reported about 198 sources. The new catalog has just over twice as many sources, spans a broader 20 to 200 MeV band and reaches fainter detections, giving a fuller picture of the low-energy gamma-ray sky.

The paper says 387 of the 2FLE sources have a positional cross-match in 4FGL-DR4, the latest major Fermi source catalog. It also identifies 45 sources that are TeV emitters and six sources spatially coincident with a 1LHAASO source, linking the low-energy catalog to objects already known at much higher energies. Public data products, including the catalog and supporting files, are available through the Fermi Science Support Center, which listed the release in October 2026.