NASA’s Fermi Glimpses Power Source of Supercharged Supernovae

An international team studying data from NASA’s Fermi Gamma-ray Space Telescope concludes that the mission detected a rare and unusually bright supernova. Researchers say it likely received its power from a supermagnetized neutron star born during the stellar collapse that triggered the explosion.

The Fermi mission is part of NASA’s fleet of observatories that monitor the cosmos to help humanity better understand how the universe works.

“For nearly 20 years, astronomers have searched the Fermi data for gamma-ray signals from thousands of supernovae, and although some intriguing clues have been reported, none have been definitive until now,” study leader Fabio Acero at the Center national de la recherche scientifique (CNRS) and Université Paris-Saclay.

An article describing the results published Wednesday in the journal Astronomy & Astrophysics.

Core-collapse supernovae occur when the energy-producing center of a star with a mass several times that of our Sun runs out of fuel, collapses under its own weight, and explodes. During the collapse, a neutron star the size of a city or an even smaller black hole could form. A blast wave carries away the rest of the star, which rapidly expands as a hot, dense cloud of ionized gas.

Over the past two decades, nearly 400 exceptional supernovae with core collapse have been identified. Each of these events, called superluminous supernovae, produced 10 times or more the amount of visible light normally observed.

In 2024, a study led by Li Shang of Anhui University in Hefei, China, noted that the Fermi Large Area Telescope could have seen gamma rays – the most energetic form of light – coming from a superluminous supernova that occurred years earlier.

Dubbed SN 2017egm, this supercharged explosion occurred in the galaxy NGC 3191, located about 440 million light-years away in the constellation Ursa Major. Even at this distance, the explosion remains one of the closest of its type on Earth.

“We searched for gamma rays from the six closest superluminous supernovae observed during the first 16 years of the Fermi mission,” said Guillem Martí-Devesa, a former researcher at the University of Trieste in Italy and now a researcher at the Institute of Space Sciences in Barcelona, ​​Spain. “Only SN 2017egm shows evidence of gamma rays, confirming previous indications that some supernovae can be as bright in gamma rays as in visible light. This opens a new window for studying these fascinating events.”

Theorists have debated the possible energy sources that give these explosions their extra oomph. Topping the list is the formation of a magnetar, a type of neutron star with the strongest magnetic fields known – up to 1,000 times the intensity of typical neutron stars. That’s 10 trillion times stronger than a refrigerator magnet.

The team undertook a more in-depth analysis of the observed optical and gamma-ray characteristics of the supernova to compare how different theoretical models reproduced them. A model developed by co-authors Indrek Vurm of the University of Tartu in Estonia and Brian Metzger of Columbia University in New York traced how light and particles produced by a newborn magnetar would move outward and interact with the expanding supernova debris.

Scientists expect a newly formed magnetar to rotate several hundred times per second. This rapid rotation produces a strong flow of electrons and positrons, their antimatter counterparts, which forms a vast cloud of energetic particles.

Within this cloud – called the Magnetar Wind Nebula – various interactions fuel the production and absorption of gamma rays. For example, an electron and a positron can annihilate into a pair of gamma photons, or two gamma rays can collide and produce the particles. In these and other ways, gamma rays interact with supernova debris. Unable to escape directly, they are reprocessed and downgraded to lower energy visible light which provides the supernova with its further increase in brightness.

“About three months after the collapse, as the supernova debris expands and cools, gamma rays can begin to escape,” Acero said. “This magnetar model best reproduces the supernova’s brightness and the arrival time of its gamma rays in the first few months, but we see opportunities for improvement later, when visible light fades quite irregularly.”

Acero and colleagues suggest that additional processes likely played a contributing role during the long fade-out of SN 2017egm. These include debris falling on the magnetar and interactions between the blast wave and material ejected by the star in the centuries before its disappearance.

The team also examined how well a new ground-based gamma-ray facility, the Cerenkov Telescope Observatory, could detect events such as SN 2017egm. With about 50 hours of observation, they say, a similar supernova could be detected up to about 500 million light years away. Our understanding of phenomena such as SN 2017egm will improve through cooperation between these facilities and NASA’s fleet of space observatories that monitor rapid changes in the universe.

“The magnetar central motor mechanism discussed in this paper builds on many observational and theoretical advances in magnetars over the past 20 years,” said Judy Racusin, 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. »

By Francis Reddy
NASA Goddard Space Flight Center, Greenbelt, Maryland.

Media Contact:
Claire Andreoli
301-286-1940
claire.andreoli@nasa.gov
NASA Goddard Space Flight Center, Greenbelt, Maryland.