ASASSN-15lh, an extremely bright explosion in a massive galaxy 3.82 billion light-years away, was thought to be the most luminous supernova ever seen. But observations from several space and ground-based telescopes have now cast doubt on this classification.
This artist’s impression depicts a Kerr (rapidly spinning) black hole, surrounded by an accretion disc: this thin disc of rotating material consists of the leftovers of a Sun-like star which was ripped apart by the tidal forces of the black hole; shocks in the colliding debris as well as heat generated in accretion led to a burst of light, resembling a supernova explosion. Image credit: ESO / ESA / Hubble / M. Kornmesser.
ASASSN-15lh was detected in 2015 by astronomers with the All Sky Automated Survey for SuperNovae (ASAS-SN) and was classified as a superluminous supernova.
It was twice as bright as the previous record holder, iPTF 13ajg, and at its peak was approximately 200 times more powerful than a typical Type Ia supernova, 570 billion times brighter than the Sun, and 20 times brighter than all the stars in the Milky Way Galaxy combined.
Instead, a large group of astronomers proposes that the source was an even more extreme and very rare event.
“We observed the source for 10 months following the event and have concluded that the explanation is unlikely to lie with an extraordinarily bright supernova,” said Dr. Giorgos Leloudas, an astronomer at the Weizmann Institute of Science in Israel and the Dark Cosmology Centre in Denmark and lead author of a paper on this research released today in the journal Nature Astronomy (arXiv.org preprint).
“Our results indicate that the event was probably caused by a rapidly spinning supermassive black hole as it destroyed a low-mass star.”
In this scenario, the extreme gravitational forces of a supermassive black hole, located in the center of the host galaxy, ripped apart a Sun-like star that wandered too close — a so-called tidal disruption event.
In the process, the star was ‘spaghettified’ and shocks in the colliding debris as well as heat generated in accretion led to a burst of light.
This gave ASASSN-15lh the appearance of a luminous supernova explosion, even though the star would not have become a supernova on its own as it did not have enough mass.
The data revealed that the event went through three distinct phases over the 10 months of follow-up observations.
An observed re-brightening in UV light and a temperature increase reduce the likelihood of a supernova event.
Furthermore, the location of ASASSN-15lh — a red, massive and passive galaxy — is not the usual home for a superluminous supernova, which normally occur in blue, star-forming dwarf galaxies.
A supernova source is therefore very unlikely, we accept that a classical tidal disruption event would not be an adequate explanation for the event either,” the authors said.
“The tidal disruption event we propose cannot be explained with a non-spinning supermassive black hole,” added co-author Dr. Nicholas Stone, from Columbia University.
“We argue that ASASSN-15lh was a tidal disruption event arising from a very particular kind of black hole.”
The mass of the host galaxy implies that the supermassive black hole at its center has a mass of at least 100 million solar masses.
A black hole of this mass would normally be unable to disrupt stars outside of its event horizon — the boundary within which nothing is able to escape its gravitational pull.
However, if the black hole is a particular kind that happens to be rapidly spinning — a so-called Kerr black hole — the situation changes and this limit no longer applies.
“Even with all the collected data we cannot say with 100% certainty that the ASASSN-15lh event was a tidal disruption event. But it is by far the most likely explanation,” Dr. Leloudas said.
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G. Leloudas et al. 2016. The superluminous transient ASASSN-15lh as a tidal disruption event from a Kerr black hole. Nature Astronomy 1, article number: 0002; doi: 10.1038/s41550-016-0002
This article is based on a press-release from the European Southern Observatory.
