Super-luminous supernovae are ten to 100 times brighter than normal stellar explosions. However, why is unclear. Now astronomers may have found the reason. The decisive clue was provided by a supernova of this type SNSNe-1, discovered in 2024, which could be observed with a telescope network for over 200 days. This revealed four “dents” in the light curve, the distances of which progressively shortened over time. This pattern fits a scenario in which the progenitor star collapsed into a “tilted” magnetar – a rapidly rotating neutron star whose rotation axis is tilted obliquely to the axis of its strong magnetic field. As a result, the contracting disk of matter around the magnetar also coalesces – and this creates the repeated lighting.
In the last 20 years or so, astronomers have discovered several supernovae that do not fit the usual scheme. Because these super-luminous stellar explosions (SLSNe-I) are ten to a hundred times brighter than usual and their glow lasts significantly longer than normal stellar explosions. This suggests that this additional energy and radiation is due to a previously unknown process – but which one? According to one hypothesis, a magnetar could be behind the additional energy boost – a rapidly rotating neutron star with an extremely strong magnetic field. This magnetic field rotates with its originator and releases part of its energy in the form of accelerated particles. If these collide with the previously ejected material from the star explosion, this could increase the brightness of the supernova and at the same time make it shine longer – according to the hypothesis.
Hump in the light curve
However, one feature of the superluminous supernovae did not fit this scenario: “The magnetar model predicts a rapid increase in brightness, followed by a smooth, monotonous decrease,” explain Joseph Farah of the University of California at Santa Barbara and his colleagues. “Humps or modulations can be seen in most of the light curves of these superluminous supernovae.” However, because previous observations could only record a maximum of two of these temporary luminosity increases in the afterglow of the explosions, it remained unclear what was behind them. But Farah and his team have now succeeded for the first time in tracking the light curve of a superluminous supernova over more than 200 days. They observed the stellar explosion SN 2024afav, discovered on December 12, 2024, using the Las Cumbres Observatory robotic telescope network. This consists of optical telescopes on almost all continents that can aim at a common target from these widely separated locations.
Through these observations, astronomers were able for the first time to record four “humps” in the light curve of this superluminous supernova – and a possible fifth – and analyze them in more detail. This showed that the “humps” in the light curve become weaker over time, and at the same time the intervals between the brightness spurts shorten in a specific way: Each time interval is around 29 percent shorter than its predecessor, as Farah and his colleagues determined. They compare this shortening frequency of bursts of brightness to the rising tone of a bird’s chirping, which is why they use the term “chirp” for this temporal pattern. “Las Cumbres Observatory’s uniquely clear and dense data allowed us to predict future bursts of brightness and adjust our observing campaign accordingly,” says Farah. The sequence, clearly detected for the first time, enabled astronomers to use a model to search for the possible cause – and in the process also test the magnetar hypothesis.
Tilted axes and the Lense-Thirring precession
In their modeling, Farah and his team tried to adapt the common magnetar scenario in such a way that it explains not only the enormous brightness of these superluminous supernovae, but also the accelerating sequence of brightness spurts. “We tested several ideas, including purely Newtonian effects and also precession caused by the magnetar’s magnetic fields,” reports Farah. Such precession occurs when the neutron star’s rotation axis is tilted against the axis of its dipole magnetic field. But it was only when the astronomers included another effect that they were able to reconstruct the observations made during the supernova SN 2024afav in the model. “It was only the Lense-Thirring precession that was perfectly timed,” says Farah. This is a consequence of the general theory of relativity. It states that a rotating mass pulls the space-time it curves and can therefore produce precession.
In the case of supernova SN 2024afav, the rotating mass is an accretion disk of stellar material that was ejected during the star explosion but then fell back onto the newly formed magnetar. As the model showed, this accretion disk in the magnetar of the supernova SN 2024afav must be asymmetrical and also slightly tilted against its axis of rotation. Due to the Lense-Thirring precession, this disk runs out of round and begins to tumble – like a spinning top. “This allows the accretion disk to periodically obscure or reflect the magnetar’s emissions,” the astronomers write. This causes the brightness of the supernova’s magnetar to fluctuate. The highlight, however, is that this effect can also explain why the time between bursts of brightness shortens: the material of the accretion disk is attracted to the magnetar and gradually falls onto it. This causes the disc to shrink and tumble faster and faster. This creates the “chirp” pattern of brightness bursts observed by telescopes on Earth.
Mystery solved?
“Our results provide the first observational evidence for the Lense-Thirring effect in a magnetar and confirm the magnetar model as an explanation for the extreme luminosity of such supernovae,” write the astronomers. According to their complementary analyses, their model also fits the light curves of some previously observed superluminous supernovae. However, it is still unclear whether all star explosions of this type are due to this mechanism or whether there are some cases with other causes. Farah’s team hopes that upcoming observation data from the Vera C. Rubin Observatory can provide more information here. This telescope, which recently went into operation in Chile, is specifically designed to detect changing phenomena in the sky – including supernovae.
Source: Joseph Farah (University of California, Santa Barbara) et al., Nature, doi: 10.1038/s41586-026-10151-0