First Magnetar Birth Observed in a Superluminous Supernova's Light Curve

University of California - Berkeley

What powers the brightest explosions in the universe? For 16 years, one answer stood above the rest on theoretical grounds: a magnetar, a neutron star spinning hundreds of times per second with a magnetic field strong enough to warp atomic structure. The model was mathematically compelling. It was also unproven, because no one had directly observed a magnetar forming inside a supernova. That changed with SN 2024afav.

The magnetar hypothesis, from 2010 to now

When Dan Kasen, now a UC Berkeley professor of physics, first proposed in 2010 that magnetars could power superluminous supernovae (SLSNe), the idea solved an energy problem. Ordinary core-collapse supernovae are already bright enough to outshine their host galaxies, but SLSNe are 10 to 100 times brighter still, and they stay luminous far longer than a collapsing star's gravitational energy can explain. Kasen's model, co-authored with Lars Bildsten and developed independently by Stanford Woosley of UC Santa Cruz, proposed that the collapsing core forges a neutron star with an amplified magnetic field, 100 to 1,000 times stronger than an ordinary pulsar. As this magnetar spins, its field accelerates charged particles that slam into the expanding debris, inflating the supernova's brightness like a battery that keeps charging.

The theory explained the energetics. But it could not explain the bumps, the brief, mysterious brightness surges that some superluminous supernovae displayed as they faded.

Four bumps and a chirp

SN 2024afav was discovered by the ATLAS survey in December 2024, located roughly a billion light-years from Earth. Las Cumbres Observatory, a network of 27 telescopes spanning the globe, tracked it for more than 200 days. About 50 days after the explosion, the supernova's brightness peaked and began to decline. But instead of fading smoothly, it oscillated downward through four distinct bumps, each arriving faster than the one before.

Joseph Farah, a graduate student at UC Santa Barbara working with LCO astronomer Andy Howell, noticed the pattern immediately. Previous SLSNe had shown one or two bumps, which astronomers attributed to the shock wave colliding with gas clumps around the star. But four bumps with a clearly periodic, sinusoidal shape and an accelerating frequency? That was something new.

Farah, who happened to be auditing a general relativity course with UCSB's Gary Horowitz at the time, built a model that fit the data precisely. In his scenario, fallback material from the explosion forms a tilted accretion disk around the newborn magnetar. General relativity predicts that a spinning mass drags the fabric of space-time with it, an effect called Lense-Thirring precession, causing the tilted disk to wobble. As the disk spirals inward, it wobbles faster. The wobbling disk periodically blocks and reflects the magnetar's energy output, creating a strobing light pattern whose frequency increases over time.

The team tested alternatives, including Newtonian precession effects and magnetic field-driven wobble, but only Lense-Thirring precession matched both the oscillation period and its rate of change.

Pinning down the magnetar's properties

The chirp pattern did more than confirm the magnetar's existence. It allowed the researchers to estimate its physical properties. The data pointed to a neutron star spinning once every 4.2 milliseconds with a magnetic field approximately 300 trillion times the strength of Earth's. Both values fall squarely within the expected range for magnetars and are consistent with theoretical predictions for what a core collapse should produce.

The predictive power of the model proved particularly convincing. After identifying the chirp pattern in early observations, Farah and his team used it to predict when future brightness bumps would appear. LCO's global telescope network allowed them to adjust their observing campaign in real time to capture these predicted features. When the predictions held, the evidence became difficult to dismiss.

What this settles, and what it does not

The result transforms the magnetar model from one of several competing hypotheses for SLSNe into a confirmed mechanism, at least for SN 2024afav. Alex Filippenko of UC Berkeley, a co-author, was careful to note that this does not mean all superluminous supernovae are magnetar-powered. Circumstellar interaction, where the supernova shock hits surrounding material, likely still explains some fraction of these events. Kasen himself has proposed that black holes, rather than magnetars, could also power brighter supernovae and produce light curve bumps through similar disk precession.

This is also a study of a single object. The chirp signature needs to be found in additional SLSNe to establish how common the phenomenon is. The Vera C. Rubin Observatory, set to begin the most comprehensive survey of the night sky to date, will generate 10 terabytes of data per night over a ten-year campaign and should reveal many more candidates.

The general relativistic interpretation also relies entirely on electromagnetic observations. Gravitational wave detectors currently lack the sensitivity to confirm Lense-Thirring precession at cosmological distances, so the model, while strongly constrained by the light curve data, awaits independent verification from a different observational channel.

Farah, who will defend his Ph.D. thesis at UCSB in May before joining Kasen's group at UC Berkeley as a Miller Fellow, is already looking ahead. If the chirp turns out to be common among SLSNe, it would mean general relativity has left its fingerprint on an entire class of stellar explosions, hiding in plain sight in data that astronomers have been collecting for two decades.