An international group of astrophysicists has presented the second catalog of low-energy gamma-ray sources, derived from data from the Fermi-LAT space telescope. The work was published in The Astrophysical Journal Supplement Series. The NASA telescope has been operating since 2008 and detects photons with energies from 20 megaelectronvolts (MeV) to more than 1 teraelectronvolt (TeV).
Gamma rays are born in the most extreme conditions - for example, near supermassive black holes, during stellar explosions or in astrophysical jets (narrow beams of particles ejected at enormous speed). The energy range around a few MeV remains the least studied: such photons are strongly scattered by matter, and it is very difficult to understand where they came from. In addition, weak signals are easily lost against the background of diffuse glow.
To partially close this gap, researchers analyzed 14 years of Fermi-LAT observations in the range from 20 to 200 MeV and identified about 450 statistically significant sources across the entire sky. This is twice as many as were previously known in this range. The catalog, named 2FLE (Second Catalog of Low-Energy Fermi-LAT Sources), will remain the most sensitive in the MeV region for years to come. Some of the objects found had never been observed in gamma rays before.
Distinguishing real sources from background and random fluctuations is especially difficult at such energies. Scott Joffre, one of the lead authors of the analysis, did extensive work to verify the results: large-scale simulations and repeated tests were used to rule out processing artifacts. The scientists also had to take into account the unusual shape of the source position uncertainty region and cases where signals from two objects merge into one.
About 15% of the signals in the catalog cannot be identified with any known astronomical object. It is not yet clear whether they are real sources. As explained by Lea Marcotulli, head of the research group at DESY, it is first necessary to establish whether these signals are indeed produced by astrophysical objects. Further observations with other telescopes will help determine their nature. If confirmed, they could be, for example, black holes with powerful jets or young pulsars and supernova remnants in our Galaxy.
The catalog will become available to everyone at the same time as the publication. The published spectral energy distributions will make it possible to test models of high-energy emission and compare objects with data from other telescopes. In addition, 2FLE will serve as a reference for future missions operating in the MeV range, including the COSI telescope, whose launch is planned for August 2027, as well as for the newASTROGAM and AMEGO-X projects.
"What Lea and Scott have done echoes what astronomers have been doing for thousands of years: they are making a map of the sky. The difference is that they are mapping a part of the universe that our eyes will never see," noted Marco Ajello, a co-author of the work and a professor at Clemson University.