For years, astronomers have relied on distant supernovae as cosmic indicators to study the universe and test the laws of physics. But while analyzing the explosion of one particular star, Joseph Farrar, a fifth-year graduate student at the University of California, Santa Barbara, noticed something completely unexpected. The supernova appeared to emit a strange signal that accelerated over time, which he described as a “chirp.”
In a new study accepted by the journal natureFarrar and an international team of researchers report the discovery of an ultraluminous supernova (SN 2024afav) that exhibits highly unusual behavior. The group also includes Farrar’s advisor Andy Howell, who heads the supernova research team at Las Cumbres Observatory (LCO). By applying ideas from general relativity to the aftermath of the explosion of a massive star, researchers were able to explain the strange signals seen in this unusually bright event.
The mystery of the brightness of supernova explosions
When a giant star runs out of nuclear fuel, its core collapses, causing a dramatic explosion known as a supernova. Most supernovae follow a fairly smooth pattern, gradually getting brighter and then slowly fading away. Even a typical supernova can momentarily outshine an entire galaxy.
But astronomers have recently identified a rare group of supernovae known as hyperluminous supernovae, which shine 10 to 100 times brighter than normal supernovae. Scientists still do not fully understand the causes of these extreme explosions. Many of them exhibit mysterious fluctuations in brightness, temporary increases in light that interrupt the expected smooth curves, suggesting complex processes are unfolding within the expanding debris.
Researchers have proposed several explanations for these brightness spikes. One possibility is that the energy source is at the center of the explosion. In this scenario, the collapse of a star forms a neutron star. Neutron stars are incredibly dense remnants that inject energy into the surrounding debris, increasing the brightness of the supernova. Another idea suggests that the brightness spike occurs when the blast wave from the explosion hits the dense shell of gas around the star. These collisions can temporarily intensify the light from the expanding material.
Strange signal from a distant supernova
LCO scientists carefully monitored SN 2024afav, which is about 1 billion light-years from Earth. During their observations, they noticed a series of repeating bumps in the supernova’s brightness.
Farrar realized that this pattern was too structured to be explained by random interactions. The changes followed a smooth wave-like rhythm, and the time between each bump was rapidly decreasing. This meant that the signals would occur more and more frequently.
Astronomers have observed for the first time a supernova producing a quasi-periodic signal that increases in frequency to form a “chirp”. This phenomenon is similar to the signal detected in gravitational waves when two black holes spiral together.
“There were no existing models that could explain the pattern of irregularities that increase in velocity over time,” Farrar said. “The signal seemed too structured to be due to random interactions, so I started thinking about how this could happen.”
magnetar in the center
The idea that ultimately explains the signal came from an unexpected source. At the time, Farrar was taking a general relativity course taught by UCSB physicist Gary Horowitz.
Dr. Farrar proposed that the supernova left behind a magnetar, a type of neutron star that rotates very fast and has a very strong magnetic field. In current models, the magnetar acts like an energy source that powers the supernova, making it so bright that it shapes its entire light curve.
However, existing magnetar models have been unable to explain the repeated uplifts. Those fluctuations can result from interactions with the surrounding gas or from irregularities in the magnetar’s energy output.
Farrar proposed a different mechanism. In his model, some of the exploded material falls toward the magnetar, forming a tilted accretion disk. Due to a general relativity effect known as Lens-Sarling precession, the rotating magnetar twists space-time around it, causing the disk to wobble.
As the disk precesses, it periodically blocks or reflects light coming from the magnetar. This makes the system behave like a blinking lighthouse in space. As the disk gradually moves inward toward the magnetar, its wobble accelerates. The result is an accelerated pulse of light that is detected from Earth, producing a distinctive “chirp.”
Testing the explanation of relativity
Lens-Sarling precession is not the only process that can cause disk wobbling. To test their explanation, Farrar and his colleagues teamed up with theorist Logan Prust, a former postdoctoral fellow at UCSB’s Kavli Institute for Theoretical Physics, to consider several other possibilities.
The SN 2024afav proved to be a powerful laboratory for testing these ideas, as all models require matching both the period of the signal and the rate of change of the period.
“We tested several ideas, including pure Newtonian effects and precession driven by the magnetar’s magnetic field, but lens-tilling precession was the only one whose timing matched perfectly,” Farrar explained. “This is the first time that general relativity has been used to describe the dynamics of a supernova.”
Global telescope efforts
Capturing this discovery required rapid coordination across a worldwide network of telescopes. The first flash of the explosion was first detected in December 2024 by the ATLAS survey. Observatories from the Goleta-based Las Cumbres Observatory Network then tracked the phenomenon for more than 200 days.
During this long period of activity, researchers monitored the supernova almost continuously using all of the LCO’s instruments. They also adjusted their observing strategy in real time to record even the smallest fluctuations in brightness.
“This is a huge win for LCO,” Farrar said. “Our unique, pristine, high-frequency LCO data allows us to predict future fluctuations, and the ability to dynamically adjust campaigns by the dime allows us to see our predictions in real time. When our predictions started to come true, we knew we were looking at something special.”
This study represents a major advance for two reasons. First, the first known example of a “chirp” in a supernova was identified, revealing a new type of observable behavior in stellar explosions. Second, it provided the clearest evidence yet that magnetars power ultraluminous supernovae, turning what was a theoretical explanation into a confirmed mechanism.
Looking to future discoveries
Farah will defend his Ph.D. She will complete her doctoral dissertation at UCSB in May of this year and will continue studying these phenomena as a Miller Fellow at the Miller Institute for Basic Science at the University of California, Berkeley. There he collaborated with Professor Dan Kasen, the scientist who first proposed the magnetar-powered supernova model.
Farrar’s advisor, Andy Howell, emphasized the importance of the discovery.
“Almost 20 years ago I was part of the discovery of a superluminous supernova and at first I didn’t know what it was. Then the magnetar model was developed and it seemed to explain the incredible energy required, but not the bulge.
“I think Joseph has found the answer,” Howell continued. “And he connected the bump to the magnetar model and explained it all in general relativity, the best-tested theory in astrophysics. It’s incredibly elegant.”
Farrar believes astronomers will soon detect more “singing” supernovae. The upcoming Vera C. Rubin Observatory in Chile will soon begin an unprecedented survey of the night sky, producing approximately 10 terabytes of data each night over a 10-year program.
“This is the most exciting thing I’ve ever had the privilege of being a part of. This is the science I dreamed about as a child,” Farrar said. “The universe is telling us out loud and to our faces things that we don’t fully understand yet, and challenging us to explain them.”
