For many years, astronomers have relied on distant supernovae as cosmic beacons to study the universe and test the laws of physics. But while analyzing one particular stellar explosion, Joseph Farah, a fifth year graduate student at UC Santa Barbara, noticed something entirely unexpected. The supernova appeared to produce a strange signal that sped up over time, something he described as a "chirp."
In a new study accepted by the journal Nature, Farah and an international team of researchers report the discovery of a superluminous supernova (SN 2024afav) with highly unusual behavior. The group includes Farah's advisor Andy Howell, who leads the supernova research team at Las Cumbres Observatory (LCO). By applying ideas from general relativity to the aftermath of a massive star's explosion, the researchers were able to explain the strange signals seen in this extraordinarily bright event.
The Mystery Behind Supernova Brightness Surges
When a massive star exhausts its nuclear fuel, its core collapses and triggers a dramatic explosion known as a supernova. Most supernovae follow a fairly smooth pattern, gradually brightening before slowly fading away. Even typical supernovae can outshine entire galaxies for a time.
However, astronomers have recently identified a rare group known as superluminous supernovae that shine 10 to 100 times brighter than normal ones. Scientists still do not fully understand what powers these extreme explosions. Many of them display puzzling fluctuations in brightness, brief increases in light that interrupt the expected smooth curve and hint that complex processes are unfolding within the expanding debris.
Researchers have proposed several explanations for these brightness surges. One possibility is that the energy source lies at the center of the explosion. In this scenario, the collapse of the star forms a neutron star, an incredibly dense remnant that injects energy into the surrounding debris and boosts the supernova's brightness. Another idea suggests the brightness spikes occur when the blast wave from the explosion slams into dense shells of gas surrounding the star. These collisions could temporarily intensify the light coming from the expanding material.
A Strange Signal From a Distant Supernova
Scientists at LCO closely monitored SN 2024afav, which lies about a billion light years from Earth. During their observations, they noticed a series of repeating bumps in the supernova's brightness.
Farah realized the pattern was far too structured to be explained by random interactions. The variations followed a smooth, wave like rhythm, and the time between each bump was shrinking rapidly. This meant the signal was occurring more and more frequently.
For the first time, astronomers had observed a supernova producing a quasi periodic signal that increased in frequency, forming a "chirp." The phenomenon is similar to the signals detected in gravitational waves when two black holes spiral together.
"There was just no existing model that could explain a pattern of bumps that get faster in time," said Farah. "I started thinking about ways this could happen, because the signal seemed too structured to be due to random interactions."
A Magnetar at the Center
The idea that ultimately explained the signal came from an unexpected source. At the time, Farah was auditing a General Relativity course taught by UCSB physicist Gary Horowitz.
Farah proposed that the supernova left behind a magnetar, a type of neutron star that spins extremely rapidly and has an extraordinarily powerful magnetic field. In current models, a magnetar can act like an energy source that feeds power into a supernova, making it exceptionally bright and shaping its overall light curve.
But existing magnetar models could not explain the repeating bumps. Those fluctuations might arise from interactions with surrounding gas or from irregularities in the magnetar's energy output.
Farah suggested a different mechanism. In his model, some of the exploded material falls back toward the magnetar and forms a tilted accretion disk. Because of a general relativity effect known as Lense-Thirring precession, the spinning magnetar twists the surrounding space-time, causing the disk to wobble.
As the disk precesses, it periodically blocks and reflects light coming from the magnetar. This makes the system behave like a flashing cosmic lighthouse. As the disk gradually moves inward toward the magnetar, its wobble speeds up. The result is the accelerating pulses of light detected from Earth, producing the distinctive "chirp."
Testing the Relativity Explanation
Lense-Thirring precession is not the only process that could cause a disk to wobble. To test their explanation, Farah and colleagues worked with theorist Logan Prust (a former postdoctoral scholar at UCSB's Kavli Institute for Theoretical Physics) to examine several other possibilities.
SN 2024afav turned out to be a powerful laboratory for testing these ideas because any model had to match both the period of the signal and the rate at which the period changed.
"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 explained. "It is the first time General Relativity has been invoked to describe the mechanics of a supernova."
A Global Telescope Effort
Capturing the discovery required rapid coordination across a worldwide network of telescopes. The initial flash of the explosion was first detected in December 2024 by the ATLAS survey. Observatories in the Las Cumbres Observatory network, based in Goleta, then tracked the event for more than 200 days.
During this extended campaign, researchers used LCO's full range of instruments to monitor the supernova almost continuously. They also adjusted observing strategies in real time to ensure even the smallest fluctuations in brightness were recorded.
"This is a major victory for LCO," said Farah. "The uniquely pristine and high-cadence LCO data allowed us to predict future bumps and the ability to dynamically adjust the campaign on a dime let us check our predictions in real-time. When the predictions started coming true, we knew we were watching something special."
The study represents a major advance for two reasons. First, it identifies the first known example of a "chirp" in a supernova, revealing a new type of observable behavior in stellar explosions. Second, it provides the clearest evidence yet that magnetars power superluminous supernovae, turning what had been a theoretical explanation into a confirmed mechanism.
Looking Ahead to Future Discoveries
Farah will defend his Ph.D. thesis at UCSB this May and plans to continue studying these phenomena as a Miller Fellow at the Miller Institute for Basic Science at UC Berkeley. There he will work with Professor Dan Kasen, the scientist who originally proposed the magnetar powered supernova model.
Farah's advisor Andy Howell highlighted the importance of the discovery.
"I was part of the discovery of superluminous supernovae almost 20 years ago, and at first we didn't know what they were. Then the magnetar model was developed and it seemed like it could explain the astounding energies needed, but not the bumps.
"Now, I think Joseph has found the smoking gun," Howell continued, "and 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."
Farah believes astronomers will soon detect many more of these "chirping" supernovae. The upcoming Vera C. Rubin Observatory in Chile will soon begin an unprecedented survey of the night sky, generating about 10 terabytes of data every night during a decade long program.
"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."
