On May 23, 2017, the European Space Agency's Gaia mission spotted something spectacular: a supernova so bright it would soon outshine its entire galaxy. Dubbed SN 2017egm, this stellar explosion unfolding 440 million light-years away in the constellation Ursa Major has now revealed a stunning secret that astronomers have been hunting for nearly two decades—the first clear detection of gamma rays streaming from a superluminous supernova, captured by NASA's Fermi Gamma-ray Space Telescope.
The discovery matters because it solves a puzzle that has captivated astrophysicists since the early 2000s. Nearly 400 exceptionally bright core-collapse supernovae have been identified, each producing 10 or more times the visible light of ordinary stellar explosions. But what powers them? For 20 years, researchers have sifted through Fermi data from thousands of supernovae searching for gamma-ray signals—hints that might reveal the engine driving these cosmic fireworks. "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," said study lead Fabio Acero at the University of Paris-Saclay in France. The answer, the international team concluded, points to something exotic: a newborn magnetar.
A magnetar is a type of neutron star born from stellar collapse, squeezed to city-size proportions, and magnetized to an almost incomprehensible degree. The magnetic fields surrounding a magnetar reach intensities 10 trillion times stronger than a refrigerator magnet—or up to 1,000 times more powerful than typical neutron stars. When the core of a massive star runs out of fuel and collapses under its own weight, the resulting explosion can create one of these extreme objects, which then spins hundreds of times per second. That frenzied rotation generates a roaring outflow of electrons and their antimatter twins, positrons, creating what theorists call a magnetar wind nebula—a vast cloud of energetic particles.
Researchers led by Guillem Martí-Devesa at the Institute of Space Sciences in Barcelona examined six of the closest superluminous supernovae observed during Fermi's first 16 years of operation. Only SN 2017egm showed clear evidence of gamma rays. To understand how, co-authors Indrek Vurm at the University of Tartu in Estonia and Brian Metzger at Columbia University developed a theoretical model tracing how light and particles produced by a newborn magnetar would propagate outward and interact with the supernova's expanding debris. Within the magnetar wind nebula, various interactions fuel the production and absorption of gamma rays—the most energetic form of light in the universe.
"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," Martí-Devesa said. The findings, published in Astronomy & Astrophysics, arrive at a moment when the field is primed to learn more. SN 2017egm remains one of the closest superluminous supernovae to Earth, making it an ideal laboratory for understanding how magnetars can transform stellar death into some of the brightest events in the cosmos. As new observatories come online and Fermi continues its mission, astronomers expect to detect more gamma rays from these spectacular explosions, deepening our understanding of how the universe's most extreme objects are born and behave.