The Fermi mission is part of NASA's network of observatories designed to track changing events across the universe and help scientists better understand how cosmic phenomena work.

"For nearly 20 years, astronomers have searched Fermi data for gamma-ray signals from thousands of supernovae, and while a few intriguing hints have been reported, none were definitive until now," study lead Fabio Acero at the French National Centre for Scientific Research (CNRS) and the University of Paris-Saclay.

The findings were published in the journal Astronomy & Astrophysics.

Rare Supernova Emits Powerful Gamma Rays

Core-collapse supernovae occur when a massive star exhausts the fuel needed to support its core. Without that energy source, the core collapses under gravity and triggers a violent explosion. Depending on conditions, the collapse can leave behind either a neutron star or a black hole. The rest of the star is blasted outward into space as an expanding cloud of extremely hot gas.

Over the past two decades, astronomers have identified nearly 400 unusually powerful examples known as superluminous supernovae. These rare explosions can shine at least 10 times brighter in visible light than ordinary supernovae.

In 2024, researchers led by Li Shang at Anhui University in Hefei, China, suggested that Fermi's Large Area Telescope may have detected gamma rays from one of these events years after the explosion occurred.

The object, called SN 2017egm, erupted in the galaxy NGC 3191, about 440 million light-years away in the constellation Ursa Major. Even from that enormous distance, it remains one of the closest superluminous supernovae ever observed from Earth.

"We searched for gamma rays from the six nearest superluminous supernovae seen during the first 16 years of Fermi's mission," said Guillem Martí-Devesa, a researcher previously at the University of Trieste in Italy and now a fellow at the Institute of Space Sciences in Barcelona, Spain. "Only SN 2017egm shows evidence for gamma rays, confirming earlier hints that some supernovae can be as luminous in gamma rays as they are in visible light. This opens up a new window for studying these fascinating events."

Magnetars May Be the Hidden Engine

Scientists have long debated what gives superluminous supernovae their extraordinary brightness. One leading explanation involves magnetars, which are neutron stars with the strongest magnetic fields known in the universe. Their magnetic fields can be up to 1,000 times stronger than those of ordinary neutron stars, reaching strengths roughly 10 trillion times greater than a refrigerator magnet.

To investigate further, the team closely examined both the visible light and gamma-ray signals from SN 2017egm and compared the observations with different theoretical models.

A model created by co-authors Indrek Vurm at the University of Tartu in Estonia and Brian Metzger at Columbia University in New York City followed how radiation and particles from a newborn magnetar would move through the expanding supernova debris.

Researchers believe a newly formed magnetar can rotate several hundred times every second. That incredible speed generates a powerful flow of electrons and positrons, which are the antimatter versions of electrons. Together, these particles create a huge cloud of high-energy material called a magnetar wind nebula.

Inside this nebula, particle interactions can generate gamma rays in several ways. Electrons and positrons can collide and transform into gamma-ray photons, while gamma rays themselves can collide and create new particles. As these interactions continue, much of the gamma-ray energy becomes trapped inside the supernova debris and is converted into lower-energy visible light, helping make the explosion exceptionally bright.

Gamma Rays Escape Months Later

"About three months after the collapse, as the supernova debris expands and cools, the gamma rays can begin to leak out," Acero said. "This magnetar model best reproduces the supernova's luminosity and the arrival time of its gamma rays during the first months, but we see room for improvement at later times, when the visible light fades quite irregularly."

The researchers suggest that additional processes likely influenced the supernova during its long decline in brightness. These may include material falling back toward the magnetar and collisions between the expanding blast wave and matter expelled by the star centuries before it exploded.

The team also explored whether future observatories could detect similar events. They found that the upcoming Cerenkov Telescope Array Observatory should be capable of spotting supernovae like SN 2017egm from distances up to about 500 million light-years away with roughly 50 hours of observation time.

Scientists say future cooperation between ground-based observatories and NASA's space telescopes will help reveal even more about these violent stellar explosions and the extreme objects hidden inside them.

"The magnetar central engine mechanism discussed in this paper builds upon a lot of observational and theoretical advances in magnetars over the last 20 years," said Judy Racusin, a deputy project scientist for the Fermi mission at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Observing gamma rays from supernovae will give us a new way to explore their inner workings."