Astronomers capture birth of a magnetar, confirming link to some of universe’s brightest exploding stars

(Press-News.org) 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, coauthored 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 coauthor 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. It was a natural explanation for the extraordinary brightness of these explosions, but we couldn’t see it directly,” Kasen said. “The chirp in this supernova signal is like that engine pulling back the curtain and revealing that it’s really there.”

Distant discovery After SN 2024afav was discovered in December 2024, Las Cumbres Observatory — a network of 27 telescopes around the world — tracked it and measured its brightness for more than 200 days. The exploding star was located about a billion light-years from Earth.

Farah, working with UCSB astronomer Andy Howell, noticed that after the brightness peaked about 50 days after the explosion, it didn’t gradually fade away like typical supernovae. Instead, its brightness slowly oscillated downward, with the period of the oscillations gradually shortening, producing a series of four bumps. He compared this to a sound gradually increasing in frequency, sounding much like a bird chirp.

Previous superluminous supernovae were known to have a couple of bumps in their decaying light curve, which some interpreted as the supernova shock colliding with layers of gas clumped around the star, briefly brightening it. But no one had observed as many as four.

According to Farah’s model, some material from the SN 2024afav explosion fell back toward the magnetar, forming a disk of matter called an accretion disk. Since material around the magnetar is unlikely to be symmetric, the accretion disk would not be symmetric about the spinning neutron star either, leading to a misalignment of the magnetar spin axis and the spin axis of the accretion disk.

Because general relativity states that a spinning mass drags space-time with it, the spinning magnetar would produce an effect known as Lense-Thirring precession — that is, it would make the misaligned disk wobble. A wobbling disk could periodically block and reflect light from the magnetar, turning the whole system into a strobing cosmic lighthouse. The time for this to repeat decreases with the radius of the disk, so as the disk slides inward toward the magnetar, it wobbles faster, causing the light to oscillate more rapidly as it fades, creating the "chirp" observed by telescopes on Earth.

"We tested several ideas, including purely Newtonian effects and precession driven by the magnetar’s magnetic fields, but only Lense-Thirring precession matched the timing perfectly," Farah said. "It is the first time general relativity has been needed to describe the mechanics of a supernova."

The astronomers also used observational data to estimate the neutron star’s spin period — 4.2 milliseconds — and magnetic field: about 300 trillion times that of Earth. Both are hallmarks of a magnetar.

“I think Joseph has found the smoking gun,” said Howell, a senior scientist at LCO and UCSB adjunct professor of physics. “He’s tied the bumps into the magnetar model and explained everything with the best-tested theory in astrophysics — general relativity. It is incredibly elegant.” Filippenko added, “To see a clear effect of Einstein’s general theory of relativity is always exciting, but seeing it for the first time in a supernova is especially rewarding.”

Filippenko cautioned that Farah’s conclusion does not mean that all superluminous supernovae are powered by magnetars. There’s also the alternative theory: that the shock wave from the exploding star hits material surrounding it, bumping its brightness up a bit. Moreover, Kasen has  proposed that if the core collapse of a star results in a black hole, that could also power a brighter supernova and, if it had a misaligned accretion disk, produce bumps in the light curve.

“We don't know what fraction of Type I superluminous supernovae might be powered by circumstellar material, but it’s definitely a smaller fraction than we previously thought, because this discovery clearly accounts for some of them,” Filippenko said.

Farah expects to find dozens more of these "chirping" supernovae as the Vera C. Rubin Observatory prepares to come online and begin the most comprehensive survey of the night sky to date.

"This is the most exciting thing I have ever had the privilege to be a part of. This is the science I dreamed of as a kid," Farah said. "It’s the universe telling us out loud and in our face that we don’t fully understand it yet, and challenging us to explain it."

Howell, Logan Prust, now at the Flatiron Institute in New York, and Yuan Qi Ni of UCSB contributed equally to the work. Filippenko acknowledges financial support from Christopher R. Redlich and many other donors.

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