For decades, astronomers have used distant supernovae as cosmic lighthouses to test fundamental physics and to measure the universe. For Joseph Farah, a fifth-year graduate student at Las Cumbres Observatory (LCO) and UC Santa Barbara, one particular supernova began to signal something never seen before: a "chirp."
In a groundbreaking paper published in the journal Nature, Farah and a team of international researchers, including his advisor LCO Senior Scientist Dr. Andy Howell, announce the discovery of a superluminous supernova (SN 2024afav) whose erratic behavior has confirmed a long-standing theory of stellar death. By applying the principles of general relativity to the explosive death of a massive star, the team has provided an explanation for the unusual behavior of these ultra-bright events.
The mystery of the bumps
When a massive star runs out of fuel, its core collapses and the star dies in a spectacular explosion called a supernova. While ordinary supernovae are some of the brightest phenomena in the universe, a rare class of supernovae has been discovered in recent years which are ten to a hundred times brighter than ordinary supernovae. The power source behind these superluminous supernovae has been a mystery. Most supernovae follow a predictable evolution, brightening and fading in a smooth arc, but these hyper-bright explosions often have mysterious undulations – temporary surges that defy expectation and point to hidden physics inside the expanding supernova.
The origin of the overbrightness and surges in luminosity is hotly debated. One possibility is that superluminous supernovae are powered from within. The violent core collapse is theorized to forge a magnetar—a rapidly spinning neutron star with a massive magnetic field. A neutron star is almost, but not quite, a black hole – it is more massive than the sun, but only about 10 miles wide. As the magnetar spins, it deposits energy into the expanding supernova, increasing its brightness. Another school of thought suggests that the supernova shock slams into layers of gas clumped around the star. As the blast wave crashes into this surrounding material, it can briefly brighten the supernova again.
Scientists at LCO observed that SN 2024afav — located roughly a billion light-years away — displayed a strange sequence of "bumps" or modulations in its brightness. With SN 2024afav, Farah noticed a pattern that no random interactions could explain: the bumps had a clearly sinusoidal, periodic shape—and that period was getting rapidly shorter. For the first time, a supernova was displaying a quasi-periodic signal with an increasing frequency, generating a "chirp".
"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 under the hood
In the existing theory, the magnetar powers the supernova like a battery, pumping in energy from within, leading to an ultra-bright and smooth rise and fall. But this theory can’t explain the bumps, which could be caused by anything from interactions with surrounding material to unexplained deviations in the power output of the magnetar.
According to Farah’s model, some material from the explosion fell back toward the magnetar, forming a disk of matter called an accretion disk. Because of a General Relativity effect known as Lense-Thirring precession, the fabric of space-time itself, twisted by the spinning magnetar, caused the disk to wobble. As the disk precessed, it periodically blocked and reflected light from the magnetar, turning the whole system into a strobing cosmic lighthouse. The time for this to repeat decreases with the radius of the disk; so as the disk slides inward towards the magnetar, the disk wobbles faster, creating the "chirp" observed by telescopes on Earth.
Lense-Thirring precession isn’t the only effect that can make a disk wobble. Working with theorist Dr. Logan Prust (now at the Flatiron Institute in New York), Farah and his team investigated several alternatives. What makes SN 2024afav unique — and a particularly effective test bed for these theories — is that any model needs to explain both the period and the rate-of-change of the period observed in the data. "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 needed to describe the mechanics of a supernova."
A victory for global observation
The discovery was a "mad dash" involving a global network of telescopes. While the ATLAS survey discovered the supernova in December 2024, Las Cumbres Observatory (LCO) played a pivotal role, observing the event for over 200 days. During this extended campaign, the team took maximal advantage of the full suite of LCO’s instruments and ability to near-continuously survey any target. Observation parameters were adjusted on-the-fly to capture even the faintest bumps in SN 2024afav’s evolution.
"This is a major victory for LCO," says 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 paper is being hailed as a breakthrough for two reasons:
- First "Chirp" in a Supernova: It identifies a new class of observational phenomena in exploding stars, supernovae that “chirp”.
- Magnetar Confirmation: It provides the first unambiguous confirmation of the magnetar model for superluminous supernovae, transforming the magnetar model from a competing hypothesis into an observationally confirmed mechanism.
The next frontier
Farah, who is set to defend his PhD thesis at UCSB this May, will continue his work as a Miller Fellow of the Miller Institute for Basic Science at UC Berkeley.
Farah’s advisor, Dr. Andy Howell, emphasized the importance of the breakthrough: “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, 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.”
LCO Director Dr. Lisa Storrie-Lombardi knows that the Observatory’s network of optical telescopes is vital to ground-breaking research. Dr. Storrie-Lombardi said, “Much effort goes into designing, building, and operating astronomical facilities. While the scientific endeavor typically moves in small steps, it is enormously rewarding for everyone involved when we get to experience something that has never been seen before being revealed.” She explained the significance of this work, “I’ve been doing this for 30 years and the only thing with an impact close to Farah’s result that I’ve been able to be a part of was the discovery nine years ago of 7 earth-sized planets orbiting the star TRAPPIST-1. Farah’s result is phenomenal.”
Farah expects to find dozens more of these "chirping" supernovae as the Vera C. Rubin Observatory prepares to come online and begin the most comprehensive survey of the night sky. The new facility will produce 10 terabytes of data every night throughout a ten-year initiative. "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."
LCO and UCSB graduate student Joseph Farah will be defending his PhD thesis in May.
Photo credit: Joseph Farah.
Upcoming event
Las Cumbres Observatory and the Santa Barbara Museum of Natural History are partnering to present a public event featuring this exciting work. Please join us on March 30, when Joseph Farah will speak on “General Relativity Beats the Heart of a Dying Star”. The talk will start at 7:00pm in the Fleischmann Auditorium of the museum. For more information, please visit the LCO website and the SBMNH calendar.