NASA’s Fermi Gamma-ray spacecraft has observed a super-bright, supercharged supernova explosion that may have been powered up by a highly magnetic dead star, a type of neutron star called a magnetar. This magnetar would have actually been born in the supernova itself, forced into existence when the core of a star that was much more massive than the sun underwent gravitational collapse at the end of its life.
During these core-collapse supernovas, stellar cores with between one and two times the mass of the sun crush down to a radius of around 12 miles (20 kilometers) to create a neutron star, just like scientists say they see here. Not only does this rapid compression mean that neutron stars are made of material so dense that one teaspoon of it brought to Earth would weigh around 10 million tons (think 350 Statues of Liberty sitting on a teaspoon), but it also causes them to spin at rates as rapid as 700 times every second. The magnetic field lines of these dead stars are also forced together, intensifying the strength of neutron stars’ magnetic fields, which makes magnetars the most powerful magnetic objects in the known universe.
“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,” team leader Fabio Acero of the University of Paris-Saclay said in a statement.
A superbright supernova
Over the last few decades, astronomers have observed around 400 core collapse supernovas, which, depending on the initial mass of the dying star involved, can also birth a black hole. Some of these stellar explosions are described as “superluminous” because they produce in excess of 10 times as much visible light as other core-collapse supernovas.
In 2024, scientists revealed they had successfully used Fermi to spot gamma-rays, the most energetic form of light, emitted from a supercharged supernova designated SN 2017egm. This supernova erupted around 440 million light-years away in the galaxy NGC 3191. Though that distance is so vast it took gamma-rays from the event 440 million years to reach Earth and Fermi, it is still one of the closest core-collapse supernovas to Earth ever seen.
“We searched for gamma rays from the six nearest superluminous supernovas seen during the first 16 years of Fermi’s mission,” Guillem Martí-Devesa, of the Institute of Space Sciences in Barcelona, Spain, said in the statement. “Only SN 2017egm shows evidence for gamma rays, confirming earlier hints that some supernovas 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 are keen to discover what it is about superluminous supernovas that lets them pack such a powerful punch. One theory suggests this extra energy comes from the fact that these events birth a magnetar with magnetic fields 1,000 times stronger than those of “ordinary” neutron stars.
This team observed the optical and gamma-ray radiation emitted by SN 2017egm and compared this data to theoretical models of the flow of light and particles from a newborn magnetar. The models specifically reproduced how said particles would interact with the expanding shell of material shrugged off by the supernova’s dying progenitor star. Of particular interest was a cloud of electrons and positrons in addition to their antimatter counterpart particles.
Scientists believe those particles were thrown out by the rapidly spinning newborn magnetar and call the cloud a magnetar wind nebula. The magnetar wind nebula is believed to boost the production and absorption of gamma-rays. One of the processes that would allow it to do this is the annihilation of particles and the release of energy as gamma-rays that occurs when a matter particle and its antimatter counterpart meet. These gamma-rays strike the outer shell of supernova debris and are turned into lower-energy optical light, explaining why these superluminous supernovas are so bright in visible light.
“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.”
Acero and colleagues have a theory of what may be causing this gradual fade-out, suggesting it could be the result of debris ejected by the destroyed star hundreds of years prior to its supernova destruction falling back onto the magnetar.
The team also had one eye on the future, assessing how efficient the new ground-based gamma-ray observatory, the Cerenkov Telescope Array Observatory, will be at spotting events like SN 2017egm. They found that in 50 hours of observing time, the telescope array, located at the Paranal Observatory and on the island of La Palma, Spain, should be able to spot similar cosmic blasts up to a distance of around 500 million light-years.
That could help scientists finally understand these super-powerful supernovas.
“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,” team member Judy Racusin, at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said. “Observing gamma rays from supernovae will give us a new way to explore their inner workings.”
The team’s results were published on Wednesday (May 20) in the journal Astronomy & Astrophysics.