A Supernova's 'Chirp' Reveals a Magnetar Being Born Inside the Blast

University of California - Santa Barbara

The light curve should have been smooth. When a massive star explodes, its brightness typically rises to a peak and then fades in a predictable arc. But SN 2024afav, a superluminous supernova spotted in December 2024 roughly a billion light-years from Earth, refused to cooperate. After peaking about 50 days post-explosion, its brightness did not simply decline. It oscillated, producing four distinct bumps, each arriving faster than the last, generating a pattern that Joseph Farah, a fifth-year graduate student at UC Santa Barbara, recognized as something extraordinary: a chirp.

Not a chirp in the acoustic sense, but a signal whose frequency increases over time, much like the gravitational wave signatures produced by merging black holes. Except this chirp was written in visible light, across months of observations, in the death throes of a star. And it told Farah exactly what was hiding inside the expanding debris.

A spinning neutron star with a trillion-gauss magnetic field

Superluminous supernovae are 10 to 100 times brighter than ordinary supernovae and have puzzled astronomers since their discovery in the early 2000s. They stay bright far longer than the energy from a core collapse alone can explain. One leading hypothesis, proposed in 2010 by UC Berkeley theoretical astrophysicist Dan Kasen, Lars Bildsten, and Stanford Woosley, suggested that a magnetar, a rapidly spinning neutron star with an enormously amplified magnetic field, could act as an internal engine, pumping energy into the expanding supernova debris from within.

The theory was elegant but unproven. Magnetars embedded in supernova debris are invisible to direct observation. And while the magnetar model could explain the overall brightness of superluminous supernovae, it could not account for the mysterious bumps that some of these explosions displayed in their light curves.

Farah's breakthrough came from an unlikely source: a general relativity class he was auditing at the time with UCSB Professor Gary Horowitz. He hypothesized that some material from the explosion fell back toward the newborn magnetar, forming an accretion disk that was tilted relative to the magnetar's spin axis. Because general relativity dictates that a spinning mass drags space-time along with it, an effect known as Lense-Thirring precession, the tilted disk would wobble. As the disk spiraled inward, it wobbled faster. And as it wobbled, it periodically blocked and reflected the magnetar's energy output, producing the oscillating brightness pattern visible from Earth.

Why only this model fits the data

The chirp is the critical detail. Previous superluminous supernovae had shown one or two bumps in their declining light curves, which could be attributed to the supernova shock slamming into clumps of gas surrounding the star. But SN 2024afav showed four bumps with a clearly sinusoidal, periodic shape, and that period was getting shorter. No random interaction with surrounding material could produce such a structured signal.

Working with theorist Logan Prust, Farah tested several alternative mechanisms: purely Newtonian effects, magnetic field-driven precession, and circumstellar interaction models. Only Lense-Thirring precession matched both the period and the rate at which the period changed. The team used the observational data to estimate the neutron star's spin period at 4.2 milliseconds and its magnetic field at roughly 300 trillion times the strength of Earth's, both consistent with magnetar properties.

200 days of global telescope coverage

Confirming the chirp required sustained, high-cadence observations. After the ATLAS survey detected the initial flash in December 2024, Las Cumbres Observatory (LCO), a network of 27 telescopes distributed around the world, tracked SN 2024afav for more than 200 days. The network's ability to observe targets nearly continuously, regardless of which hemisphere is facing the sun, proved essential. The team adjusted observation parameters on the fly to capture even the faintest brightness oscillations, and used their model to predict future bumps before they appeared.

When those predictions proved correct, the case became difficult to dispute.

Two firsts in a single supernova

The paper, accepted to Nature, establishes two precedents simultaneously. It is the first observed chirp in a supernova's light curve, identifying a new class of observational phenomena in stellar explosions. And it provides the first direct evidence that magnetars power superluminous supernovae, converting the magnetar model from a plausible hypothesis into an observationally confirmed mechanism.

Andy Howell, Farah's advisor and a senior scientist at LCO who was part of the original discovery of superluminous supernovae nearly 20 years ago, noted that the bumps had long resisted explanation within the magnetar framework. Farah's model ties them in by invoking general relativity, arguably the most rigorously tested theory in physics.

Not the whole story

The result does not mean every superluminous supernova is magnetar-powered. Alex Filippenko of UC Berkeley, a co-author, cautioned that circumstellar interaction likely still accounts for some fraction of these events. Kasen himself has proposed that a black hole, rather than a magnetar, could also power a brighter supernova and, if accompanied by a misaligned accretion disk, produce light curve bumps.

This is also a single event. While the model's predictive success is compelling, the chirp signature needs to be identified in additional superluminous supernovae before the mechanism can be considered general. The Vera C. Rubin Observatory, expected to begin its ten-year survey of the night sky soon, will produce 10 terabytes of data per night and should reveal dozens more candidates if they exist.

The study also relies on electromagnetic observations alone. Gravitational wave detectors are not yet sensitive enough to detect signals from magnetar formation at these distances, so the general relativistic interpretation, while strongly supported by the light curve data, lacks independent confirmation from a second messenger.

But as a first detection, SN 2024afav delivers something rare in astrophysics: a signal structured enough to distinguish between competing theories. The universe, it turns out, left a clear signature inside this particular explosion. The challenge now is to find out how many others it has signed the same way.