For nearly two decades, astronomers searching NASA's Fermi Gamma-ray Space Telescope have been hunting for a cosmic ghost—the gamma-ray fingerprint of a superluminous supernova—and after sifting through data on thousands of stellar explosions, they have finally found it. In 2017, in the galaxy NGC 3191 some 440 million light-years away in the constellation Ursa Major, a star collapsed catastrophically and unleashed one of the brightest explosions ever witnessed. Now, an international research team led by Fabio Acero at France's National Centre for Scientific Research has confirmed that Fermi detected the telltale gamma-ray signal from that event—SN 2017egm—opening a new window into what powers these extraordinary cosmic phenomena.
The discovery matters because it solves a puzzle astronomers have wrestled with for years. When a massive star exhausts its nuclear fuel, the core collapses under its own gravity in a violent implosion that triggers an outward explosion so powerful it can rival entire galaxies in brightness. Over the past two decades, researchers have identified nearly 400 of these superluminous supernovae—rare explosions that shine at least 10 times brighter in visible light than ordinary supernovae. Yet the question of what energizes them remained stubbornly unanswered until now.
The leading suspect is a magnetar, an exotic neutron star born in the star's collapse and endowed with magnetic fields so extreme they defy intuition. These fields can reach strengths roughly 10 trillion times greater than a refrigerator magnet—up to 1,000 times stronger than ordinary neutron stars. When a newly formed magnetar spins several hundred times per second, it generates a powerful outflow of electrons and positrons that creates what scientists call a magnetar wind nebula, a vast cloud of high-energy material inside the expanding supernova debris.
Inside this nebula, the physics becomes almost surreal. Electrons and positrons collide and transform into gamma-ray photons. Those gamma rays themselves collide and create new particles. Through these cascading interactions, gamma rays bounce and build up inside the supernova debris, eventually converting into lower-energy visible light that radiates outward, making the explosion dazzling to distant observers. About three months after the collapse, as the expanding debris cools and thins, those gamma rays finally leak free into space—becoming detectable by sensitive instruments like Fermi's Large Area Telescope.
The research team, which included Guillem Martí-Devesa from the Institute of Space Sciences in Barcelona and theorists Indrek Vurm at the University of Tartu and Brian Metzger at Columbia University, carefully matched observations of SN 2017egm's visible light and gamma-ray signals against models of magnetar-powered explosions. The match held, confirming that some supernovae can be as luminous in gamma rays as they are in visible light—a stunning confirmation that magnetars likely fuel these events.
The work, published in Astronomy & Astrophysics, is only the beginning. Researchers expect that the upcoming Cerenkov Telescope Array Observatory will be sensitive enough to spot similar events in distant galaxies, transforming the study of these rare cosmic fireworks from a hunt for needles in a haystack into systematic science. For the first time, astronomers have a proven method to identify when a magnetar is born inside a supernova, offering a direct window into some of the universe's most extreme physics.