Fermi Detects Delayed Gamma Rays from Superluminous Supernova SN 2017egm, Supporting Magnetar Power Source

NASA’s Fermi Gamma-ray Space Telescope has detected a delayed burst of high-energy gamma rays from SN 2017egm, an unusually bright stellar explosion, giving rare evidence that this superluminous supernova was powered by a newborn magnetar — a rapidly spinning, highly magnetized neutron star. Researchers stress that the finding applies to this event, not to all superluminous supernovae.

The result was highlighted in a NASA mission summary published Tuesday. It is based on the peer-reviewed paper “Evidence for GeV emission of the superluminous supernova SN 2017egm,” by Shang Li and colleagues, published March 20 in Physical Review Letters. SN 2017egm was discovered by the European Space Agency’s Gaia mission on May 23, 2017, in the barred spiral galaxy NGC 3191, about 440 million light-years away in the constellation Ursa Major.

The finding matters because astronomers have long argued over what powers superluminous supernovae, a rare class of explosions that can shine about 10 to 100 times brighter in visible light than ordinary core-collapse supernovae. One leading explanation says the blast is energized from within by a magnetar formed in the collapse. Another says the explosion is brightened when debris slams into dense material around the star. Delayed gamma rays offer a particularly useful test because magnetar models predicted that such high-energy light should emerge months after the explosion, once the expanding debris thins enough to let it escape.

That is roughly what the Fermi team says it saw. In the paper’s words, “There is a transient γ-ray source appearing about 2 months after this event and lasting a few months.” NASA said photons detected by Fermi’s Large Area Telescope, or LAT, were concentrated between about July 5 and Oct. 25, 2017, roughly 43 to 155 days after the supernova was discovered. The proposed picture is that the magnetar blows a wind of energetic particles into the wreckage, creating a kind of nebula inside the explosion. Early on, the debris is too thick for GeV gamma rays — photons carrying billions of electron volts of energy — to get out. As the ejecta expands and becomes more transparent, those gamma rays can leak into space.

The signal is notable, but it is not the last word. The paper reports a global significance of at least about 4 sigma, strong enough to attract attention but below the standard often used for a discovery claim in physics. And NASA said that when researchers searched the six nearest superluminous supernovae seen during Fermi’s first 16 years, only SN 2017egm showed evidence for gamma rays. A broader 2026 Fermi-LAT population study of 223 hydrogen-poor superluminous supernovae also found no population-level detections above 5 sigma and treated SN 2017egm as a suggestive outlier.

That makes the result an important clue rather than a settled answer for the whole class of explosions. Still, it gives astronomers one of their clearest tests yet of the magnetar idea in a real event. As Guillem Martí-Devesa said in NASA’s summary, “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.”