Astronomers have witnessed the birth of a rapidly rotating, highly magnetized neutron star or “magnetar” for the first time.
The observation of this event, triggered by the death of a massive star, confirms the link between the creation of magnetars and super-bright supernova explosions. These superluminous supernovae can be as much as ten times brighter and last much longer than the typical supernova explosions that occur when massive stars run out of nuclear fuel and undergo gravitational collapse, or “core collapse,” to give birth to neutron stars or black holes.
Almost since they were first discovered in the early 2000s, scientists have theorized that the birth of magnetars, which have the most powerful magnetic fields in the known universe, is linked to superluminous supernovae, but smoking gun confirmation of this connection has been lacking.
“What’s really exciting is that this is definitive evidence that a magnetar forms as a result of a supernova core collapse,” team member Alex Filippenko of the University of California, Berkeley, said in a statement.
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The magic trick behind supersupernovae
The theory linking magnetars and superluminous supernovae was first proposed by Dan Kasen and Lars Bildsten of UC Berkeley and independently by Stanford Woosley of UC Santa Cruz. It suggests that when a star that has a strong magnetic field and is around 25 times the mass of the Sun collapses, the magnetic field is strengthened. The result is a magnetar with a magnetic field 100 to 1000 times as strong as the magnetic field of a “standard” neutron star.
The collapse of the core of a massive star to a width of about 12 miles (20 kilometers) has another consequence. Just as a speed skater at the Winter Olympics pulls her arms to increase her spin speed, the rapid reduction in diameter of a neutron star increases its rotation.
As a result, some newborn neutron stars can spin at a rate of 700 times per second or more. These objects can blast beams of radiation from their poles that sweep across the universe like light from a cosmic lighthouse. In these cases, neutron stars and magnetars are referred to as pulsars.
When magnetars spin rapidly, their rotating magnetic fields accelerate particles and then eject them into material progenitors of the progenitor star during their supernova deaths. It causes this debris to increase in brightness.
The team behind this research confirmed this connection when they analyzed data from a supernova discovered in 2024 and called SN 2024afav. This investigation revealed strange “chirps” in the light curve from this supernova, indicating general relativistic effects caused by a magnetar.
“The basis of Dan Kasen and Stan Woosley’s model is that all you need is the energy of the magnetar deep inside, and a good portion of it will be absorbed, and that will explain why the thing is superluminous,” Filippenko said. “What was not demonstrated was that a magnetar actually formed in the middle of the supernova.”
The researcher added that this is what this research, published on Wednesday (March 11) in the journal Nature, finally 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 chirping of this supernova signal is like the engine that pulls back the curtain and reveals that it’s really there.”
Supernova smoking gun
First discovered by the 27-telescope network of the Las Cumbres Observatory in December 2024, the brightness of SN 2024afav was tracked by astronomers for 200 days. What the team noticed was that this supernova, which took place about 1 billion light-years from Earth, did not gradually fade like a typical supernova.
After peaking at the 50-day mark, the brightness of SN 2024afav gradually declined, with a series of four noticeable “bumps” in brightness resembling a sound increasing in frequency. Therefore, these functions were labeled chirp.
Similar irregularities have been seen in the light curves of other supernovae, with scientists linking them to shocks that shake out from the central stellar body and hit previously ejected material. However, no previous supernova had demonstrated as many as four of these chirps.
This team theorizes that material from the explosion seen as SN 2024afav actually fell back into the central magnetar after it was ejected, forming a swirling flat cloud called an accretion disk around this powerful stellar remnant.
Because material ejected in the supernova is unlikely to be symmetric, the accretion disc is also unlikely to be symmetric. This causes the spin axis of the magnetar and that of the accretion disk to be misaligned.
Einstein’s theory of gravity, known as general relativity, suggests that as objects with high mass spin, they drag space itself along, a process called “frame-dragging” or the Lense-Thirring effect. This effect will cause the accretion disk to wobble, and a wobbly accretion disk will occasionally block light from the magnetar and occasionally reflect it. This creates a strobing effect that turns the entire system into a cosmic “lighthouse”.
As the disk contracts and falls to the magnetar, this wobble speeds up, generating the chirping seen in the light curve of SN 2024afav.
“We tested several ideas, including pure Newtonian effects and precession driven by the magnetar’s magnetic field, but only Lense-Thirring precession matched the timing perfectly,” said lead author of the paper, Joseph Farah of UC Berkeley. “It is the first time general relativity has been needed to describe the mechanics of a supernova.”
The team was also able to determine that this central object spins around 238 times every second and has a magnetic field around 300 trillions times more powerful than the Earth’s magnetosphere, confirming this as a magnetar. It’s the smoking gun astronomers have been looking for to link magnetars and superluminous supernovae.
“He (lead author Joseph Farah) has linked the shocks to the magnetar model and explained everything with the best-tested theory in astrophysics – general relativity. It’s incredibly elegant.” Filippenko added. “Seeing 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.”
