NASA’s Fermi Gamma-ray Space Telescope may have finally figured out what’s driving the explosion of the brightest star ever observed. After studying years of data, an international team of researchers has found strong evidence that rare ultraluminous supernovae were energized by polar magnetic neutron stars formed during star collapse.
The Fermi mission is part of NASA’s network of observatories designed to track changing events across the universe and help scientists better understand how cosmic phenomena work.
“For nearly 20 years, astronomers have been searching for Fermi data for gamma-ray signals from thousands of supernovae. Although some interesting hints have been reported, so far nothing has been conclusive,” said Fabio Acero, head of research at France’s National Center for Scientific Research (CNRS) and the University of Paris-Saclay.
The research results were published in a magazine astronomy and astrophysics.
Rare supernova emits powerful gamma rays
A nuclear collapse supernova occurs when a massive star runs out of the fuel needed to support its core. Without that energy source, the core would collapse under gravity, causing a violent explosion. Depending on conditions, the collapse could leave behind either a neutron star or a black hole. The rest of the star is blown into space as an expanding cloud of very hot gas.
Over the past two decades, astronomers have identified nearly 400 unusually powerful examples of what are known as hyperluminous supernovae. These rare explosions can shine at least 10 times brighter in visible light than a typical supernova.
In 2024, researchers 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 events years after the explosion occurred.
The object, called SN 2017egm, erupted in the galaxy NGC 3191 in the constellation Ursa Major, about 440 million light-years away. Even with its enormous distance, this remains one of the closest superluminous supernovae observed from Earth.
“We looked for gamma rays from the six closest superluminous supernovae observed during the first 16 years of the Fermi mission,” said Guillem Martí Debesa, formerly a researcher at the University of Trieste in Italy and now a researcher at the Institute of Space Sciences in Barcelona, Spain. “Only SN 2017egm shows evidence of gamma rays, confirming previous hints that some supernovae can be as bright in gamma rays as they are in visible light. This opens a new window to study these interesting phenomena.”
It may be an engine with a hidden magnetar.
Scientists have long debated what gives ultraluminous supernovae their unusual brightness. One likely explanation involves magnetars, neutron stars with the strongest magnetic fields known in the universe. Its magnetic field is up to 1,000 times stronger than that of a typical neutron star, and about 10 trillion times stronger than a refrigerator magnet.
To investigate further, the team took a closer look at both the visible light and gamma-ray signals from SN 2017egm and compared their observations to various theoretical models.
The model, created by co-authors Indrek Wurm of the University of Tartu in Estonia and Brian Metzger of Columbia University in New York City, tracked how radiation and particles from a newborn magnetar moved through the expanding supernova debris.
Researchers believe that newly formed magnetars can rotate hundreds of times per second. Its incredible speed generates a powerful stream of electrons and positrons, the antimatter version of electrons. Together, these particles form huge clouds of high-energy matter called magnetar wind nebulae.
Inside this nebula, gamma rays are produced by particle interactions in several ways. Electrons and positrons can collide and turn into gamma ray photons, but gamma rays themselves can also collide and create new particles. As these interactions continue, much of the gamma-ray energy is trapped inside the supernova debris and converted to lower-energy visible light, helping to make the explosion so bright.
Gamma rays escape after a few months
“About three months after the collapse, as the supernova debris expands and cools, gamma rays may begin to leak out,” Acero said. “This magnetar model best reproduces the supernova’s brightness and gamma-ray arrival time during the first few months, but we think there is room for improvement in the later stages, when visible light disappears very erratically.”
The researchers suggest that additional processes may have influenced the supernova during its long-term decline in brightness. These could include material falling toward the magnetar, or the colliding of the expanding blast wave with material ejected centuries before the star exploded.
The research team also investigated whether future observatories could detect similar phenomena. They found that the upcoming Cherenkov Telescope Array Observatory should be able to detect supernovae like SN 2017egm from distances up to about 500 million light-years away in about 50 hours of observation time.
Scientists say future collaboration between ground-based observatories and NASA’s space telescopes will reveal more about these violent stellar explosions and the extreme objects hidden within them.
“The magnetar central engine mechanism discussed in this paper builds on observational and theoretical advances in magnetars over the past 20 years,” said Judy Raksin, deputy project scientist for the Fermi mission at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Observing gamma rays from supernovae will provide a new way to explore the inner workings of supernovae.”
