Several months ago, a suspected supernova was discovered that had scientists scratching their heads. If confirmed as a supernova, ASASSN-15lh would be twice as luminous and 10 times brighter than the previous record holder, stretching our theories of peak supernova brightness. But is this the brightest a supernova can be?

Two astrophysicists have sought to answer this question, and found that it is very nearly – but not quite – the limit. Ph.D. candidate Tuguldur Sukhbold from the University of California, Santa Cruz, and his supervisor Stan Woosley, think that a brightness 6 trillion times that of the Sun – 10 times the measured brightness of ASASSN-15lh – is the limit for so-called super-luminous supernovae (SLSNs).

In their paper, submitted to The Astrophysical Journal Letters and available on Arxiv, the two scientists compared theoretical models for various types of supernova, and found there was one in particular that can be extremely energetic. This is an explosion from a compact core of a massive star that forms a magnetar, a rapidly spinning neutron star with an incredibly strong magnetic field. The parent star loses mass to a companion and explodes, leaving behind the rapidly spinning magnetar, which boosts the supernova's power.

“We find that the most powerful source of energy is rotation, and the so-called magnetar-powered supernova events are capable of producing the brightest and the most luminous explosions,” Sukhbold told IFLScience. “In practice the numbers associated with ASASSN-15lh are pretty much the limit.”

According to their calculations, a magnetar with a magnetic field 20 trillion times stronger than the Sun (which is quite weak for a magnetar), and an initial spin of 1,500 rotations per second, could create this hypothetical maximum supernova. The reason that there is a limit is down to the initial size of the neutron star. As Woosley explains to IFLScience, if a neutron star has a binding energy holding it together more than one-sixth the rest mass energy of the Sun, it will likely collapse into a black hole before a supernova can take place.

^{An artist's impression of what ASASSN-15lh would look like from a planet 10,000 light-years from the event. Beijing Planetarium/Jin Ma}

If ASASSN-15lh is a supernova, the only possible explanation for its brightness seems to be this magnetar-formation theory, suggesting that the hypothesized 6-trillion-sun supernova could occur. Explosions of this magnitude are thought to be extremely rare, with only one in a million supernovae expected to be anywhere near as bright, and finding them may rely on upcoming telescopes like the James Webb Space Telescope (JWST).

However, we’re not sure ASASSN-15lh is a supernova yet; Sukhbold says it could be a very different type of violent phenomenon called a tidal-disruption event (TDE). “In a TDE scenario a star gets too close to the supermassive black hole at the center of the galaxy, and gets shredded by the black hole's powerful tidal forces,” he explained. “The resulting bright flare light often looks similar to a supernova explosion for some period of time, and so it is not always easy to distinguish the two right away.”

The astronomers also note that it is possible there are other more energetic types of supernova, as they only considered supernovae bright in optical light created through known methods. “We have ignored some other truly extreme cases, like those from Gamma-ray burst events and supermassive stars many ten thousand times the mass of the Sun that collapse through a very different mechanism,” said Sukhbold.

For now though, 6 trillion Suns appears to be the limit. “I can't think of any other conventional way of making a supernova brighter than a magnetar can,” said Sukhbold. “But nature is rich and she is very good at surprising us, and I am anxious to see what crazy things we will observe in the future.”