A UC Berkeley theorist proposed that highly magnetized, spinning neutron stars were the power source behind superluminous supernovae. A recent supernova provided the smoking gun.
Astronomers have for the first time seen the birth of a magnetar — a highly magnetized, spinning neutron star — and confirmed that it’s the power source behind some of the brightest exploding stars in the cosmos.
The finding corroborates a theory proposed by a UC Berkeley physicist 16 years ago and establishes a new phenomenon in exploding stars: supernovae with a “chirp” in their light curve that is caused by general relativity. A paper describing the phenomenon was published today (March 11) in the journal Nature.
Superluminous supernovae — which can be 10 or more times brighter than run-of-the-mill supernovae — have puzzled astronomers since their discovery in the early 2000s. They were thought to result from the explosion of very massive stars, perhaps 25 times the mass of our sun, but they stayed bright much longer than would be expected when a star’s iron core collapses and its outer layers are subsequently blown off.
In 2010, Dan Kasen, now a UC Berkeley theoretical astrophysicist and professor of physics, was the first to propose that a magnetar was powering the long-lasting glow. According to the theory, co-authored with Lars Bildsten and suggested independently by Stanford Woosley of UC Santa Cruz, when a massive star collapses at the end of its lifetime, it crushes much of its mass into a very compact neutron star — a fate just short of collapsing to a black hole. If the star originally had a very strong magnetic field, it would have been amplified during magnetar formation, producing a field 100 to 1,000 times stronger than that of normal spinning neutron stars — so-called pulsars. Pulsars and their highly magnetized big brothers, magnetars, are only about 10 miles in diameter but, in their youth, can spin more than 1,000 times per second.
As the magnetar spins, the spinning magnetic field can accelerate charged particles that slam into the debris from the expanding supernova, increasing its brightness. Magnetars are also thought to be the source of fast radio bursts.
Graduate student Joseph Farah of UC Santa Barbara and Las Cumbres Observatory (LCO), who will come to UC Berkeley this fall as a Miller Postdoctoral Fellow in Kasen’s group, confirmed the connection between magnetars and Type I superluminous supernovae (SLSNe-I) after analyzing data from a 2024 supernova dubbed SN 2024afav. In the Nature paper, Farah and his colleagues proposed a general relativistic explanation for unusual bumps in the light curve of this supernova — what they call a chirp — that conclusively connect it to a magnetar.
“What’s really exciting is that this is definitive evidence for a magnetar forming as the result of a superluminous supernova core collapse,” said Alex Filippenko, a UC Berkeley distinguished professor of astronomy who is a co-author of the paper and one of Farah’s soon-to-be mentors. “The basis of Dan Kasen and Stan Woosley’s model is that all you need is the energy of the magnetar deep within and a good fraction of it will get absorbed, and that’ll explain why the thing is superluminous. What had not been demonstrated was that a magnetar did in fact form in the middle of the supernova, and that’s what Joseph’s paper shows.”
“For years the magnetar idea has felt almost like a theorist’s magic trick — hiding a powerful engine behind layers of supernova debris,” Kasen said.“It was a natural explanation for the extraordinary brightness of these explosions, but we couldn’t see it directly. The chirp in this supernova signal is like that engine pulling back the curtain and revealing that it’s really there.”