Astronomers detect magnetar forming inside supernova via light curve wobbles

Astronomers have caught what appears to be a magnetar being born inside a stellar explosion, an event that has been theorised for decades but never directly observed until now. The evidence comes not from the magnetar itself, which is far too small and newly formed to detect directly, but from strange periodic wobbles in the brightness of an exceptionally luminous supernova. The findings were reported in Nature.

A light curve is simply a graph of how bright an astronomical object appears over time. Most supernovae produce light curves that follow a predictable arc: a rapid rise to peak brightness followed by a gradual decline as the explosion expands and cools. The supernova in this study deviated from that pattern in a way that has no straightforward explanation other than a compact, rapidly spinning object inside the explosion pumping additional energy into the surrounding material at regular intervals.

What a magnetar is and why it would cause wobbles

A magnetar is a type of neutron star with an extraordinarily strong magnetic field, typically in the range of 10 to the power of 14 to 10 to the power of 15 gauss. For comparison, a refrigerator magnet produces about 100 gauss. The Earth's magnetic field is less than 1 gauss. A magnetar's field is strong enough to distort atoms and affect the behaviour of light in its immediate vicinity.

When a massive star collapses and forms a neutron star, that neutron star can spin dozens or hundreds of times per second. If the neutron star has an exceptionally strong magnetic field, the spinning creates a pulsar wind, a continuous outflow of highly energised particles. When that wind slams into the expanding supernova shell from the inside, it deposits energy into the shell and makes the supernova brighter than it would otherwise be. If the magnetar's spin axis is slightly offset from its magnetic axis, the energy injection is not perfectly uniform over each rotation cycle, producing the periodic brightness variations that the research team detected.

Why this supernova was unusually bright to begin with

The supernova in question belongs to a class called superluminous supernovae, which are roughly 10 to 100 times brighter than ordinary core-collapse supernovae. An ordinary core-collapse supernova releases energy equivalent to about 10 to the power of 44 joules of light over its entire lifetime. Superluminous supernovae can exceed that by two orders of magnitude, and radioactive nickel decay, the energy source that powers most supernovae, cannot account for that much brightness.

The magnetar engine hypothesis has been the leading explanation for superluminous supernovae since it was proposed by researchers including Kasen and Bildsten in a 2010 paper in the Astrophysical Journal. The idea is that a newly formed magnetar spinning down over days to weeks transfers rotational energy into the surrounding ejecta, powering continued emission long after the initial explosion energy would have faded. The wobbles detected in this new study are the most direct observational evidence yet that this mechanism is real and operating in at least one known superluminous supernova.

How the wobble signature was identified

The research team used data from multiple telescope facilities to construct a high-resolution light curve of the supernova, designated SN 2022xxf, over approximately 200 days of observation. Within that light curve, they identified quasi-periodic oscillations with a timescale of roughly 12 to 14 days. These oscillations were too regular to be random noise and too slow to be explained by the rotation period of the magnetar itself, which would spin thousands of times faster.

The team's interpretation is that the 12 to 14 day period corresponds to a precession cycle in the magnetar, meaning the spin axis itself is slowly wobbling around a fixed orientation. This precession modulates how efficiently the pulsar wind couples to the supernova ejecta, producing a brightness variation on the precession timescale rather than the spin timescale. The lead author, Dr. Aleksandar Cikota of the European Southern Observatory, described the detection as the closest thing to a direct fingerprint of a magnetar engine that current observational technology can produce.

What this means for understanding superluminous supernovae

Superluminous supernovae are used as distance indicators in cosmology in much the same way as Type Ia supernovae, but their physical mechanism has remained poorly constrained. If a magnetar engine is confirmed as the dominant power source for a significant fraction of superluminous supernovae, it changes the calibration models astronomers use when estimating the distances and therefore the cosmological implications of these events.

The research team plans to search archival light curves of previously observed superluminous supernovae for similar oscillation signatures that may have been overlooked. There are currently around 150 confirmed superluminous supernovae with sufficient photometric data to conduct that kind of retrospective analysis. If wobble signatures appear in a statistically meaningful fraction of them, it would move the magnetar engine from a plausible hypothesis to the established explanation for this entire class of stellar explosion.