New gamma-ray observations from NASA’s Fermi Space Telescope suggest ultra-magnetic neutron stars called magnetars could be fueling superluminous supernovae, a rare class of stellar explosions with peak luminosities 10-100 times greater than those of standard core-collapse supernovae.
The superluminous supernova SN 2017egm was discovered by ESA’Gaia mission on May 23, 2017; it exploded in a massive barred spiral galaxy known as NGC 3191, shown on the left before the eruption; the image at right, taken on July 1, 2017, shows the supernova outshining the entire galaxy. Image credit: SDSS / PS1 / NOT+ALFSOC / Bose et al.
Core-collapse supernovae occur when the energy-producing center of a star many times our Sun’s mass runs out of fuel, collapses under its own weight, and explodes.
During the collapse, a city-sized neutron star or an even smaller black hole may form.
A blast wave blows away the rest of the star, which rapidly expands as a hot, dense cloud of ionized gas.
In the last couple of decades, nearly 400 exceptional core-collapse supernovae have been identified.
Each of these events, dubbed superluminous supernovae, produced 10 or more times the amount of visible light normally seen.
According to a 2026 paper, Fermi’s Large Area Telescope may have detected gamma rays from a superluminous supernova called SN 2017egm.
This event occurred in NGC 3191, a barred spiral galaxy located about 440 million light-years away in the constellation of Ursa Major.
“We searched for gamma rays from the six nearest superluminous supernovae seen during the first 16 years of Fermi’s mission,” said Dr. Guillem Martí-Devesa, a researcher 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.”
Theorists have debated the possible energy sources that give these explosions their extra punch.
High on the list has been 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.
The astronomers undertook a deeper analysis of the SN 2017egm’s observed optical and gamma-ray features to compare how well different theoretical models reproduced them.
Their model traced how light and particles produced by a newborn magnetar would move outward and interact with the supernova’s expanding debris.
They expect a newly formed magnetar to spin a few hundred times a second.
This rapid rotation produces a strong outflow of electrons and positrons, their antimatter counterparts, that forms a vast cloud of energetic particles.
Within this cloud — called a 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-ray photons, or two gamma rays can collide and produce the particles.
In these and other ways, gamma rays interact with the supernova debris.
Unable to escape directly, they become reprocessed, downshifted into lower-energy visible light that provides the supernova with its extra boost in luminosity.
“About three months after the collapse, as the supernova debris expands and cools, the gamma rays can begin to leak out,” said Dr. Fabio Acero, a researcher at the University of Paris-Saclay and CNRS.
“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.”
“Additional processes likely played contributing roles during SN 2017egm’s long fade-out.”
“These include debris falling back onto the magnetar and interactions between the blast wave and matter ejected by the star in the centuries prior to its demise.”
The team’s paper was published today in the journal Astronomy & Astrophysics.
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F. Acero et al. 2026. Gamma-ray signature of superluminous supernovae: Fermi-LAT GeV detection of SN 2017egm and evidence of a central engine. A&A 709, A229; doi: 10.1051/0004-6361/202558547
