For the first time, astronomers witnessed the birth of a ‘magnetar’
In December 2024, astronomers watched a star with a mass about 25 times that of our sun die in a blaze of glory. Located a billion light years from Earth, SN 2024afav was a prime example of a superluminous supernova, an event at least 10 times brighter than the explosion of a large star. Researchers from around the world used the Las Cumbres Observatory’s global network of 27 telescopes to document the spectacle for more than 200 days.
As the supernova’s brightness reached its peak around day 50, astronomers noticed something strange. Instead of slowly fading as expected, the brightness oscillated downward as the time between each fluctuation shortened. Past examples of superluminous supernovae had one or two bumps, but SN 2024afav had four.
After months of calculations, as well as help from Albert Einstein’s theory of general relativity, researchers believe they have an explanation. For the first time ever, astronomers witnessed the birth of a magnetar, a rapidly rotating and extremely magnetized neutron star. The ramifications, detailed in a study published today in the journal Natureimply that such cosmic powers fuel some of the most explosive supernovae in the universe.
The role of the magnetar
The results confirm a theory first proposed 16 years ago by Dan Kasen, a theoretical astrophysicist at the University of California, Berkeley. Kasen and his colleagues hypothesized that at least some superluminous supernovae get their juice from magnetars – one of several possible outcomes when a star dies.
The mass of a star determines the end of its life. If it is not massive enough to collapse into a black hole, it will collapse into a neutron star. However, stars that have had a strong magnetic field during their lifetime do not lose it. Instead, they become magnetars with fields between 100 and 1,000 times stronger than rotating neutron stars, or pulsars. Magnetars and pulsars are only about 10 miles in diameter, but they will begin to spin more than 1,000 times per second.
Kasen’s team hypothesized that a spinning magnetar would accelerate charged particles so quickly that they would collide with debris from the expanding supernova. According to the team, this is what makes some supernovae much brighter than others.
“For years, the magnetar idea seemed almost like a theorist’s magic trick: hiding a powerful engine behind layers of supernova debris,” Kasen, who was not involved in the new study, said in a statement. “It was a natural explanation for the extraordinary brightness of these explosions, but we couldn’t see it directly.”
The latest study by a team including UC Santa Barbara astrophysicist Joseph Farah finally explains the magic trick, but it took some trial and error to get it right.
“We tested several ideas, including purely Newtonian effects,” Farah explained.
Wobbly discs
The solution did not come from Newtonian physics, but from general relativity. Farah’s model for SN 2024afav implies that material from the explosion falls inward toward the magnetar and forms what is called an accretion disk. This debris field in the disk is almost certainly asymmetric, meaning that the axes of rotation of the accretion disk and the magnetar are misaligned. General relativity says that a rotating object drags spacetime as it rotates. When applied to a magnetar, the rotation would hypothetically create what is called a Lense-Thirring precession.
To put it (very) simply: the misaligned accretion disk begins to wobble. When this is the case, it can occasionally block and reflect light from a magnetar like a flashing turn signal. As the disk gets closer to the magnetar, its radius decreases and causes it to oscillate faster. Overall, this explains the decrease in time between brightness oscillations of SN 2024afav And confirms Kasen’s magnetar theory.
“This is the first time that general relativity has been needed to describe the mechanics of a supernova,” Farah said.
“I think Joseph found the smoking gun,” said Andy Howell, senior scientist at Las Cumbres Observatory, physicist at UCSB and co-author of the study. “He linked the bumps to the magnetar model and explained it all with the best-tested theory in astrophysics: general relativity. It’s incredibly elegant.”
“The science I dreamed of as a child”
A magnetar still does not constitute a universal explanation for superluminous supernovae. Another theory proposes that the shock wave from an exploding star can sometimes hit nearby matter and increase its brightness. Kasen also suggested that a newly formed black hole with a misaligned accretion disk could also briefly fuel a bright supernova.
But even if magnetars power a small percentage of superluminous supernovae, this marks a major moment for both astronomy and general relativity.
“It’s the most exciting thing I’ve ever had the privilege of being a part of,” Farah said. “This is the science I dreamed of as a child.”
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