NASA’s Fermi telescope may have finally uncovered the magnetic powerhouse behind the universe’s brightest supernovae.
An international team of astronomers analyzing observations from NASA’s Fermi Gamma-ray Space Telescope has found what appears to be the first convincing detection of gamma rays from a rare type of extraordinarily bright stellar explosion known as a superluminous supernova.
Their findings suggest the explosion was powered by a newly formed magnetar, an extremely magnetized neutron star created when a massive star collapsed. The discovery provides fresh insight into some of the most energetic explosions in the universe.
NASA’s Fermi mission is part of a fleet of observatories that monitor changes across the cosmos, helping scientists better understand how the universe evolves.
“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 results were published in the journal Astronomy & Astrophysics.
How Massive Stars End Their Lives
Core-collapse supernovae occur when a massive star, many times larger than the Sun, exhausts the fuel needed to support itself. Without that energy source, the star’s core collapses under gravity and triggers a powerful explosion.
The collapse can leave behind either a neutron star roughly the size of a city or an even more compact black hole. Meanwhile, a powerful shock wave blasts the outer layers of the star into space, creating a rapidly expanding cloud of hot, ionized gas.
Over the past two decades, astronomers have identified nearly 400 unusually bright examples of these explosions. Known as superluminous supernovae, they can shine more than ten times brighter in visible light than typical supernovae.
In 2024, a study led by Li Shang of Anhui University in Hefei, China, suggested that Fermi’s Large Area Telescope may have detected gamma rays from one of these exceptional explosions years after it occurred.
That event, called SN 2017egm, erupted in the galaxy NGC 3191, about 440 million light-years from Earth in the constellation Ursa Major. Despite its great distance, it remains one of the nearest superluminous supernovae ever observed.
A Rare Gamma-Ray Detection
“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.”
Scientists have long debated what supplies the enormous energy behind superluminous supernovae. One leading explanation involves the birth of a magnetar, a neutron star possessing the strongest magnetic fields known in nature.
These magnetic fields can be up to 1,000 times stronger than those of ordinary neutron stars, reaching roughly 10 trillion times the strength of a refrigerator magnet.
To investigate, researchers conducted a detailed analysis of both the optical and gamma-ray observations of SN 2017egm. They compared the data with several theoretical models designed to explain the explosion’s behavior.
One model, developed by co-authors Indrek Vurm of the University of Tartu in Estonia and Brian Metzger of Columbia University in New York City, simulated how particles and radiation from a newly born magnetar would interact with the expanding material ejected by the supernova.
How a Magnetar Can Supercharge a Supernova
Scientists expect newborn magnetars to rotate extremely rapidly, completing hundreds of spins every second. This rapid rotation generates a powerful flow of electrons and positrons, which are the antimatter counterparts of electrons.
These particles create a huge region known as a magnetar wind nebula, filled with highly energetic particles.
Within this environment, numerous interactions can create and absorb gamma rays. For example, an electron and a positron can annihilate each other and produce gamma-ray photons. Gamma rays can also collide and create new particle pairs.
As these interactions continue, gamma rays become trapped within the expanding supernova debris. Rather than escaping immediately, much of their energy is converted into lower-energy visible light. This process helps explain why superluminous supernovae shine so intensely.
“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.”
Additional Forces May Be Involved
The researchers believe other processes likely contributed to the supernova’s behavior as it gradually dimmed.
Possible factors include material falling back onto the magnetar and interactions between the expanding blast wave and matter expelled by the star during the centuries before its final collapse.
The team also explored whether future facilities could detect similar events. Their analysis suggests the new ground-based Cerenkov Telescope Array Observatory could identify a supernova like SN 2017egm from as far away as about 500 million light-years with roughly 50 hours of observations.
Scientists expect that combining observations from facilities like the Cerenkov Telescope Array Observatory with data from NASA’s space-based telescopes will significantly improve understanding of these extraordinary explosions.
“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.”
Reference: “Gamma-ray signature of superluminous supernovae: Fermi-LAT GeV detection of SN 2017egm and evidence of a central engine” by F. Acero, A. Acharyya, A. Adelfio, M. Ajello, E. Aviano, L. Baldini, J. Ballet, C. Bartolini, D. Bastieri, J. Becerra Gonzalez, R. Bellazzini, E. Bissaldi, R. Bonino, P. Bruel, S. Buson, R. A. Cameron, P. A. Caraveo, F. Casaburo, F. Casini, E. Cavazzuti, C. C. Cheung, N. Cibrario, G. Cozzolongo, P. Cristarella Orestano, F. Cuna, S. Cutini, F. D’Ammando, D. Depalo, S. W. Digel, N. Di Lalla, A. Dinesh, L. Di Venere, P. Fauverge, A. Fiori, A. Franckowiak, Y. Fukazawa, S. Funk, P. Fusco, F. Gargano, C. Gasbarra, D. Gasparrini, S. Germani, F. Giacchino, N. Giglietto, M. Giliberti, F. Giordano, M. Giroletti, I. A. Grenier, M.-H. Grondin, S. Guiriec, R. Gupta, E. Hays, J. W. Hewitt, A. Holzmann Airasca, D. Horan, X. Hou, T. Kayanoki, M. Kerr, M. Kuss, A. Laviron, M. Lemoine-Goumard, A. Liguori, J. Li, I. Liodakis, P. Loizzo, F. Longo, F. Loparco, S. López Pérez, L. Lorusso, M. N. Lovellette, P. Lubrano, S. Maldera, A. Manfreda, G. Martí-Devesa, R. Martinelli, M. N. Mazziotta, M. Michailidis, P. F. Michelson, N. Mirabal, T. Mizuno, P. Monti-Guarnieri, M. E. Monzani, A. Morselli, I. V. Moskalenko, M. Negro, N. Omodei, M. Orienti, E. Orlando, G. Panzarini, M. Persic, M. Pesce-Rollins, R. Pillera, T. A. Porter, G. Principe, S. Rainò, R. Rando, B. Rani, M. Razzano, A. Reimer, O. Reimer, M. Sánchez-Conde, P. M. Saz Parkinson, D. Serini, C. Sgrò, E. J. Siskind, G. Spandre, P. Spinelli, D. J. Suson, H. Tajima, D. J. Thompson, D. F. Torres, Z. Wadiasingh, K. Wood, G. Zaharijas, W. Zhang, E. Chatzopoulos, B. D. Metzger, P. J. Pessi and I. Vurm, 20 May 2026, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202558547
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