Clemson University-led efforts to map the gamma-ray sky at opposite ends of its energy bands will help scientists study the universe’s most extreme environments.
Gamma rays are the most energetic form of light. They can reveal extreme environments that are difficult to study in other ways, including the remains of exploding stars and jets powered by supermassive black holes. By recording where gamma rays come from and how their energy is distributed, researchers can investigate the processes that accelerate particles throughout the universe.
This is why Clemson researchers undertook the efforts of mapping the gamma-ray sky at its lowest and highest energy bands using observations from the Large Area Telescope (LAT) onboard NASA’s Fermi Gamma-Ray Space Telescope. They produced two catalogs. One catalog surveys low-energy gamma rays, from 20 to 200 million electron volts, in an underexplored energy band known as the “MeV gap.” The other maps gamma-ray sources at energies above 50 billion electron volts.
Understanding the universe
“We are trying to understand how our universe works,” said Lea Marcotulli, who earned her Ph.D. from Clemson in 2021. She is now a Helmholtz Investigator Group leader at Deutsches Elektronen-Synchrotron (DESY) in Zeuthen, Germany, and an adjunct professor at Clemson. “Without information at all energies, our picture will always remain incomplete.”
Marcotulli worked on the low-energy catalog, known as 2FLE, that identifies 450 gamma-ray sources using 14 years of Fermi data. This catalog more than doubles the sources previously known in the MeV band. It will give scientists their most sensitive view of the MeV for years to come.
Researchers found that 377 of the new catalog’s sources match an already known astrophysical object, while the nature of 73 of them remains a puzzle. Sixty of these new MeV sources had never been seen in gamma-rays and could be a new population of astrophysical MeV sources.
The project began in 2021, near the end of Marcotulli’s doctoral work. Scott Joffre further developed the catalog as a major part of his Ph.D. thesis at Clemson, where he graduated in 2025. Joffre now works as a research engineer at Auburn University. After he graduated, Marcotulli, then an adjunct professor at Clemson, completed the catalog.
Largely unexplored
The low-energy band has been largely unexplored because gamma rays at these energies can be hard to trace back to their origin. Diffuse emission from the Milky Way makes it harder for sources to stand out, and overlapping signals from nearby sources can often be detected together, confusing what an instrument is detecting.
“The first step is to establish whether the signals really originate from astrophysical objects,” Marcotulli said.
The catalog will be publicly available and provide researchers a starting point for follow-up work.
It will also help guide observations for upcoming missions, including NASA’s Compton Spectrometer and Imager (COSI), a space telescope that will explore the sub-MeV to low-MeV range and is scheduled to launch in August 2027. COSI is designed to measure characteristic lines in gamma-ray emission, which can provide clues about where elements are produced in the galaxy. Clemson researchers are a part of the COSI mission.
High-energy range
In the high-energy range, researchers used 16 years of Fermi observations to identify 673 gamma-ray sources, nearly doubling the number previously known in this energy range. About 5% of the sources are unassociated or uncertain in classification, and 20 have no counterparts in earlier Fermi catalogs, potentially revealing new cosmic particle accelerators.
The high-energy catalog , known as 4FHL, is part of Alba Rico’s doctoral research at Clemson. The work was done under the supervision of Marco Ajello, a professor in the Department of Physics and Astronomy, who co-led the study.
Most of the catalog’s sources are associated with blazars, which are the incredibly bright centers of distant galaxies powered by supermassive black holes shooting a powerful jet of energy and matter directly at Earth.
“Blazars are among the most powerful persistent sources of radiation in the universe,” Rico said. “They provide new laboratories for understanding how nature accelerates particles to extraordinary energies.”
The catalog also includes sources within our galaxy, such as supernova remnants and pulsar wind nebulae, which trace the final stages of stellar evolution, where particles are accelerated to the highest energies by stellar explosions and rapidly rotating neutron stars.
Impossible to reproduce
“By studying them at these extreme energies, we can investigate how particles are accelerated in environments that are impossible to reproduce on Earth,” said Alberto Domínguez, a professor at the Universidad Complutense de Madrid, who also co-led the study.
The work will support the new Cherenkov Telescope Array Observatory (CTAO), which will be the world’s largest and most powerful ground-based observatory for gamma-ray astronomy at very high energies, Ajello said.
“What Lea and Alba have done with their catalogs 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,” Ajello said.
The Astrophysical Journal Supplement Series published the low-energy (2FLE) catalog article. The high-energy catalog (4FHL) article was published in The Astrophysical Journal.
