Star burst: supernova ASASSN-15lh is defying explanation Beijing Planetarium/Jin Ma

What we thought was the largest supernova ever observed might in fact be the death gasp of a star being swallowed by a black hole.

The flash, called ASASSN-15lh, was spotted in June 2015 in the All Sky Automated Survey for SuperNovae (ASAS-SN). Later analysis showed it was 200 times more powerful than most supernovae, shining 20 times brighter than the combined output of the entire Milky Way.

Supernovae mark the violent deaths of stars that collapse on themselves and explode, and are some of the brightest and most energetic objects in the universe. At first we thought the flash was a superluminous supernova – brighter than most, but nothing too exotic.


But shortly after its discovery, both the Hubble Space Telescope and the Swift gamma-ray space telescope scrutinised it in more detail. Initially, it started to dim, as supernovae typically do. But about three months later, its ultraviolet radiation grew stronger, increasing by a factor of five before levelling out for four months.

Meanwhile, it continued to fade away in the visible part of the light spectrum. This was unusual, says Krzysztof Stanek at the Ohio State University, who found the object. Although supernovae sometimes re-brighten through interactions with gas the star blew away, the particulars of gas emission suggest that is not what happened here, Stanek says.

Peter Brown at Texas A&M University in College Station, examined Swift and Hubble data and also said ASASSN-15lh defied explanation. “It doesn’t look like any supernova we’ve ever seen, and it doesn’t look like any tidal disruption event we’ve seen before,” he says. “It’s an extreme object either way.”

A new hope

Now, a second team of researchers has a new explanation. Giorgos Leloudas at the Weizmann Institute of Science in Israel argues the bright flare was not a supernova at all, but a star being torn apart by a spinning black hole.

Several factors point to a rapidly spinning black hole, including the object’s temperature changes, the charged gas clouds surrounding it and its central location at the heart of a passive, older galaxy. Leloudas’s team argues they have evidence for hydrogen and helium emission, a hallmark of a black hole’s lunch.

“The rapid spin and high black hole mass can explain the high luminosity of this event,” the authors say.

Brown says as a supernova researcher, he had hoped for an exploding star, but he is convinced by Leloudas’s explanation.

“Previously, a black hole was ruled out because the black hole would have been too big to have a tidal disruption event. It should have just swallowed the star whole,” he says. “But as Giorgos explains in his paper, if it is spinning fast enough, then a certain star with a certain velocity can be torn apart before getting pulled in.”

Stanek says he is not convinced, although he admits the team did a good job pointing out why it might not be a supernova.

“But I still think it smells much more like a supernova,” he says.

Stanek acknowledges neither the black hole scenario nor the supernova scenario offers a truly satisfying explanation – but ultimately, that may be a good thing.

“The message is that we need to keep our eyes open for really unusual things,” he says. “Now and then something unusual happens, and that’s where we push the understanding of astrophysics.”

Journal reference: arXiv.org/1609.02927