The light did not fade the way it was supposed to.
After blazing into view about a billion light-years from Earth, the supernova known as SN 2024afav settled into something stranger. Its brightness rose, peaked, and then began to dip in a series of rhythmic bumps. Each pulse came a little faster than the one before it, like a chirp tightening in pitch.
That unusual pattern has now given astronomers what they say is their clearest evidence yet that some of the universe’s brightest stellar explosions are powered by newborn magnetars, dense neutron stars with extreme magnetic fields and rapid spin rates. The results, published in Nature, also point to a role for Einstein’s general theory of relativity in shaping the light from a supernova for the first time.
“The idea that magnetars are involved has seemed to have a magical aspect for theoretical astronomers,” stated Dan Kasen, a UC Berkeley physicist. “However, with this evidence, we can now conclude with certainty that there is, indeed, a relevant and substantial magnetar component. The supernova’s light display illustrates how the magnetar’s presence ultimately shapes the light produced by the supernova.”
Instead of two bumps in brightness, a total of four bumps were found.
Superluminous supernovae have an apparent brightness exceeding ten times that of typical supernovae. Astronomers have debated for years concerning what maintains the high brightness for the extended period of these events. In 2010, Kasen proposed a theory in which additional energy from a newly formed magnetar supplied the energy needed to create these events.
The previously referenced idea was developed with Lars Bildsten and was also independently suggested by Stan Woosley at UC Santa Cruz. Direct evidence for this idea was unavailable at the time. On December 12, 2024, however, a new supernova called SN 2024afav emerged as an extremely bright object through the Gravitational-wave Optical Transient Observer collaboration. It was classified as a Type I superluminous supernova.
Las Cumbres Observatory used its international network of telescopes, consisting of 27 instruments, to monitor SN 2024afav over the course of approximately 200 days after its discovery. The object lies at a distance of roughly 327 megaparsecs, or nearly one billion light-years from Earth.
Joseph Farah, a graduate student at UC Santa Barbara who works with Las Cumbres Observatory, observed that the shape of SN 2024afav’s light curve did not exhibit a typical exponential decay following the point of maximum brightness. Instead, it displayed four distinct modulations in brightness, and possibly a fifth. The intervals between successive modulations became progressively shorter.
Other previously documented examples of superluminous supernovae exhibited one or two bumps rather than four or more. Their intervals of modulation also did not appear to significantly decrease over time.
“We examined multiple theoretical models to explain the data, from purely Newtonian-based effects to precession driven by a magnetar’s magnetic fields. None of these matched the timing of SN 2024afav. Lense-Thirring precession had the best concurrence with the data,” Farah said. “This is the first time that general relativity has had to be used to explain the emergence of a supernova.”
Farah and colleagues propose that material expelled from the explosion of the supernova fell back and formed an accretion disk around the compact object, the neutron star that was left behind. The accretion disk would have formed at an angle to the spin axis of the magnetar. Because of this misalignment, the disk would have produced a wobbling motion around the supernova remnant as it formed around the neutron star. This phenomenon is referred to as Lense-Thirring precession.
As the accretion disk moved closer toward the compact object, the speed of the wobble increased. The disk therefore varied in position over time. It could block, reflect, or redirect energy from the magnetar. This process would have produced the observed chirps seen in visible light.
The authors built a combined magnetar plus Lense-Thirring precession model that fits well with the overall brightening and fading of the supernova, along with the oscillations observed after maximum luminosity. They also calculated the spin period and magnetic field strength of the neutron star using the light curve.
The neutron star was estimated to have a spin period of approximately 4.2 milliseconds and a magnetic field strength of about 1.6 × 10¹⁴ gauss. This field is roughly 300 trillion times more powerful than Earth’s magnetic field.
Andy Howell, a senior scientist at Las Cumbres Observatory and an adjunct professor of physics at UC Santa Barbara, said, “Joseph has found the smoking gun. He has connected the bumps from this model to the magnetar models, and all of this can be explained using the most well-tested theory in astrophysics, general relativity.” He added, “This is unbelievably stunning.”
Alex Filippenko, a distinguished UC Berkeley astronomy professor and coauthor of the paper, described their findings as “conclusive evidence of a magnetar being formed through a superluminous supernova core collapse.”
Not all superluminous events have the same energy source. The authors do not argue that all superluminous Type I supernovae are powered in the same way.
Filippenko stated that other types of events may also become superluminous due to the supernova shockwave running into surrounding material, known as circumstellar material. Other possibilities include black holes being formed from some core-collapse events that produce superluminous explosions. In those cases, an additional unexplained component may boost the brightness, particularly when systems are misaligned with the accretion disk.
The article outlines that, for this event, there are limits to circumstellar material interaction being the cause of SN 2024afav. Producing four or more shrinking sinusoidal patterns would require multiple finely tuned parameters. The timing of the oscillations was far more consistent with a central engine model.
However, the existence of lower levels of circumstellar material interaction has not been ruled out.
“We have no way of knowing the number of Type I superluminous supernovae that might be powered by circumstellar material. What we do know is that it is a much smaller number than what we previously thought because we now have evidence of this,” Filippenko added.
Farah anticipates that with the start of the Vera C. Rubin Observatory‘s full-sky survey, astronomers will continue to gather additional examples of this phenomenon.
“This represents the most exciting experience I’ve ever been involved with,” said Farah. “It indicates the universe is telling us very clearly that we do not yet fully understand it, and it is urging us to find explanations for all of its mysteries.”
The authors provide a new mechanism for identifying newborn magnetars in stellar explosions by analyzing subtle variations in brightness over time.
They suggest that future surveys should use these types of “chirping” supernovae as a potential means of measuring the properties of magnetars.
Such observations may also help scientists understand how general relativity affects newly formed neutron stars.
Research findings are available online in the journal Nature.
The original story “Newborn magnetar offers rare evidence of Einstein’s relativity in a stellar explosion” is published in The Brighter Side of News.
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