Fermi catalog maps about 450 gamma-ray sources in 14 years

“The difference is that they are charting a part of the universe our eyes could never see,” Ajello said. “By mapping this largely unexplored gamma-ray sky, they have created a foundation on which many future discoveries can be built.”

What did 2FLE find?

The all-sky analysis produced around 450 sources that met the team’s statistical significance threshold, meaning their signals stand out from background variation more strongly than chance alone would predict.

The catalog doubles the previously known count in the 20–200 MeV range. The authors say it will remain the most sensitive MeV catalog for years to come. Some sources in it had never previously been detected in gamma rays at all.

The published spectral energy distributions show how strongly sources radiate across the measured energy range. Researchers can use those curves to test models of high-energy emission and compare catalog objects with observations from other telescopes.

How did researchers test the faint signals?

Detecting MeV photons remains difficult because they most often interact with matter through Compton scattering. In that process, a photon transfers energy to matter, obscuring clues about its original direction and source.

Diffuse background emission—faint radiation spread across the sky—can overlap with signals from separate astronomical objects. Weak signals may merge into that background or disappear. Technology that overcomes these sensor limitations has only recently become available.

Scott Joffre, one of the lead researchers, led much of the work to determine whether the detections were genuine rather than statistical fluctuations or analysis artifacts. Extensive simulations and repeated checks tested that possibility. The team also accounted for the unusual shape of errors in source locations at low energies and for blending between signals from two separate objects.

What remains unidentified?

About 15% of the catalog’s gamma-ray signals—roughly one in seven—remain unlinked to any known astronomical object. The study leaves open whether they represent real sources or whether something else produces them. Researchers must first establish whether the signals originate in astrophysical objects, meaning objects in space.

Follow-up observations with other telescopes should help identify their nature. If the signals prove astrophysical, possible counterparts include black holes with powerful jets, or young pulsars and supernova remnants in our galaxy. Pulsars are rapidly rotating neutron stars, while supernova remnants are the expanding leftovers of stellar explosions.

How will 2FLE guide upcoming missions?

The catalog will also provide a reference for forthcoming MeV missions. One is the Compton Spectrometer and Imager (COSI), a space telescope scheduled to launch in August 2027.

COSI will explore a different MeV energy window and measure characteristic gamma-ray lines, which are distinct energies that can trace particular elements. The team says those lines can provide clues about which elements energetic processes produce and where particles accelerate.

The team also identifies proposed larger mission concepts, newASTROGAM and AMEGO-X, as future opportunities. Together, 2FLE links current observations with next-generation instruments while identifying which signals remain mysterious.

A. Adelfio et al. published “The Second Catalog of Low-energy Fermi-LAT Sources” in The Astrophysical Journal Supplement Series in 2026. The paper’s DOI is 10.3847/1538-4365/aeb54d.