A new view of the gamma-ray gap: 14 years of Fermi data reveal around 450 sources in a largely unexplored energy range

The universe is filled with light at many different energies. Some of it is invisible to our eyes, including extremely energetic gamma rays, which can only be produced in the most extreme environments in our universe, for example supermassive black holes, exploding stars or astrophysical jets. To “see” gamma rays, astrophysicists use particle experiments in space, such as the Fermi-Large Area Telescope (Fermi-LAT). This instrument was launched into space by NASA in 2008, and it can detect photons from 20 megaelectronvolts (MeV, a unit of energy) to beyond 1 teraelectronvolt (TeV).

However, due to technical challenges, one particularly underexplored region of the universe’s energy output is the MeV range. To start bridging this gap while awaiting the launch of a mission that can detect photons only in the MeV band, an international team presents the Second Catalog of Low-Energy Fermi-LAT Sources (2FLE) in a new study, using 14 years of Fermi-LAT photons from 20 MeV to 200 MeV. The work is published in The Astrophysical Journal Supplement Series.

“We are trying to understand how our universe works, which physical processes produce the highest-energy photons in the universe, how particles are accelerated in our galaxy and beyond—and without information at all energies, our picture will always remain incomplete,” says Lea Marcotulli, a Helmholtz investigator group leader at DESY. The new catalog provides another piece of the puzzle, revealing what is happening in a largely unexplored part of the gamma-ray sky.

A long-hidden window

Gamma rays are produced by some of the most energetic processes in the universe. These include black holes and particles accelerated to high energies in cosmic accelerators. Studying the MeV range can provide researchers with clues about how such particles are accelerated and how they release their energy as radiation.

Detecting MeV photons is very difficult, however. At these energy levels, photons most often interact with matter through a process called Compton scattering, which makes it difficult to determine where the original photon came from. In addition, background interference is significant, and the technology needed to overcome sensor limitations has only recently become available.

At the same time, radiation from different astronomical objects overlaps with diffuse background emission. Weak signals can therefore easily merge with one another or disappear into the background.

The 2FLE content

In the 2FLE, the team analyzed 14 years of Fermi-LAT data, considering an energy range from 20 to 200 MeV and looking for MeV-bright objects across the entire sky. The result is around 450 statistically significant sources.

This catalog not only doubles the number of known objects in this energy range, but it will also remain the most sensitive catalog in the MeV range for years to come. Interestingly, some of the detected sources had never previously been detected in gamma rays. The catalog therefore provides a much broader basis for investigating the processes behind this high-energy radiation.

How do we know the signals are real?

Distinguishing genuine sources from background emission and statistical fluctuations is particularly challenging at these energies. Scott Joffre, one of the lead researchers on the analysis, led much of the work needed to test the results.

The team used extensive simulations and repeated checks to make sure that the signals they identified were not simply artifacts of the analysis. The researchers also had to account for the unusual shape of the positional uncertainties at low energies and for issues such as blending of MeV signals from two separate objects.

Are the sources real?

Around 15% of the gamma-ray signals identified in the catalog cannot be linked to any known astronomical object. Are they genuine sources—or is something else behind them? Marcotulli explains that the first step is to establish whether the signals really originate from astrophysical objects.

Further observations with other telescopes should help identify the true nature of these unknown objects. If confirmed, they could, for example, come from black holes with powerful jets, or from young pulsars and supernova remnants in our galaxy.

A resource for future research

The catalog is intended to do more than document the results of the study. It will be made publicly available at the same time as the study, giving other researchers access to the results for their own investigations.

The published spectral energy distributions, for example, can be used to test models of how high-energy radiation is produced or to compare the objects with observations from other telescopes.

“What Lea and Scott have done with 2FLE echoes what astronomers have done for thousands of years: chart the sky. The difference is that they are charting a part of the universe our eyes could never see. By mapping this largely unexplored gamma-ray sky, they have created a foundation on which many future discoveries can be built,” says Marco Ajello, co-lead researcher of this work and professor at Clemson University.

The 2FLE catalog will also provide an important reference for upcoming MeV missions. One of these is the Compton Spectrometer and Imager (COSI), a space telescope that will explore the MeV range in a different energy window and is scheduled to launch in August 2027. COSI is designed, among other things, to measure characteristic lines in gamma-ray emission.

These can provide clues about which elements are produced in energetic processes and where particles are accelerated. Other examples include proposed larger mission concepts such as newASTROGAM and AMEGO-X.

The 2FLE catalog therefore connects existing observations with the next generation of MeV missions. It shows which signals can already be detected in this challenging energy range—and which mysteries are still waiting to be solved.

Who’s behind this story?

Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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