Almost a year ago, astronomers announced the discovery of ASASSN-15lh, the brightest supernova ever observed. But now a new study describes it not as a supernova but as a tidal disruption event (TDE), the light emitted when a star is ripped apart and eaten by a supermassive black hole.
In a paper, published in Nature Astronomy, an international team of astronomers looked at the mysterious object for 10 months and noted the way its brightness kept changing. Its location in the core of a passive galaxy also made it more likely to be a TDE than a huge star going supernova.
“We've only been studying the optical flares of tidal disruptions for the last few years,” said study author Iair Arcavi, principal investigator of the program used to observe ASASSN-15lh on Las Cumbres Observatory. "ASASSN-15lh is similar in some ways to the other events we've been seeing, but is different in ways we didn't expect. It turns out that these events, and the black holes that make them, are more diverse than we had previously imagined."
The event was first discovered by the All-Sky Automated Survey for Supernovae (ASASSN). The possibility that ASASSN-15lh was a TDE was thought unlikely because the black hole in that galaxy is so massive that it should swallow the star whole.
Some astronomers didn’t agree with that explanation. Observations from the Hubble telescope located the event near the galaxy center, and stars that are massive enough to produce such an explosion would not be found there.
The team also used the Las Cumbres Observatory (LCO), an automated series of telescopes around the world, to observe the object every few days. ASASSN-15lh first became dim, then rebrightened in ultraviolet radiation, and then became dim again. Such chemical signals were also unlikely to have come from such a powerful supernova.
All these clues pointed towards a TDE rather than a superluminous supernova.
“This is like discovering a new kind of dinosaur,” said study author Andy Howell, the leader of the supernova group at LCO. “Now that we have the right tools and know what to look for, we’re going to find more and get a better sense of the population. It is so exciting to have new ways of learning about black holes and stellar death!”