A flash in the sky that earlier this year was called the brightest supernova ever detected—tens of times brighter than our entire Milky Way galaxy—may be something much more exotic: a supermassive black hole tearing apart—and consuming—a star that strayed too close, according to a new study.
The flash was first spotted in 2015 by the All-Sky Automated Survey for SuperNovae (ASAS-SN), a network of small telescopes in Chile and Hawaii that monitors the sky for fast-changing objects. Astronomers assumed it was a superluminous supernova (SLSN), which occurs when a massive star collapses under its own gravity at the end of its life, spewing out a fireball of hot dust and gas that glows brightly for a short time before gradually fading. At the time, the event was twice as bright as the previous record holder.
But ASASSN-15lh, as the event was named, was in the wrong sort of galaxy for an SLSN. The right sort is generally a young dwarf galaxy full of gas and dust where huge stars can form rapidly, burn brightly, and explode in a blaze of supernova glory. ASASSN-15lh, however, was in an old, burned-out galaxy with little evidence of star formation. “The minute they told me about this event, I was suspicious. It just didn’t seem right,” says Giorgos Leloudas, an astronomer at the Weizmann Institute of Science in Rehovot, Israel, who was not a member of the original team.
Leloudas and his colleagues began gathering more data from a variety of sources, including the Swift gamma-ray satellite, the Las Cumbres Observatory global telescope network, the Hubble Space Telescope, and the European Southern Observatory’s Very Large Telescope and the New Technology Telescope, both based in Chile. Hubble data showed that the source of the flash was close to the center of its galaxy, whereas the rapid star formation that produces SLSNs typically happens farther out. Also, unlike a normal SLSN, ASASSN-15lh seemed to fade before getting brighter again weeks later, indicating a rise in temperature maintained for about 100 days, Leloudas says. A spectrum of the ultraviolet light from the event, recorded by Hubble, suggested a low-mass star in the prime of life, not on its deathbed.
As the team reports today in Nature Astronomy, all these signs pointed to the idea that ASASSN-15lh might, in fact, be the dying gasp of a star that strayed too close to the supermassive black hole at the center of its galaxy and was ripped apart by the extreme gravitational field, a so-called tidal disruption event (TDE). TDEs are very rare—there are only about 10 such events currently suspected by astronomers. But, Leloudas says, the changes of output from ASASSN-15lh suggested a TDE: the initial flash from gravity tearing the star apart and heating its remains to high temperatures; the later burst from those remains being heated again as they were accreted onto the surface of the black hole.
The one flaw in this argument is that the galaxy in question is thought to have a very massive black hole at its heart: more than 100 million times the mass of our sun. Theorists predict that such a leviathan would more likely swallow a star whole and only tear it up once it’s below the event horizon, where it can’t be seen. But the team realized there was a scenario in which the black hole would chew first and swallow later—if it were spinning. The gravitational field around a rotating black hole is different from a nonrotating one and would allow a visible TDE to occur.
If it is confirmed that this was the fate of ASASSN-15lh, it will be the first verified rotating black hole at the center of a quiescent galaxy. The team will continue to observe ASASSN-15lh, hoping to learn more as its dazzle ceases to illuminate the rest of the galaxy. And because other candidate TDEs all occur around smaller black holes, ASASSN-15lh broadens the range of places TDEs may occur. “By adding to the diversity, we will learn more about the physics that happens during a disruption,” Leloudas says.
“This whole new phenomenon of tidal disruption events gives us a unique opportunity to learn about supermassive black holes during their quiescent phase,” says astronomer Benny Trakhtenbrot of the Swiss Federal Institute of Technology in Zurich, who was not involved in the study. If you can determine how close the disrupted star passed to the black hole, he says, “that can directly tell us how fast the black hole is spinning.” And spin can reveal something of the formation history of otherwise inscrutable black holes, he says.