Researchers have discovered a supermassive black hole spinning at 50 percent the speed of light. This incredible find is reported in the latest issue of Science and is being presented at the 233rd meeting of the American Astronomical Society.
The story that led to this discovery started on November 22, 2014. On that night, astronomers discovered a supermassive black hole ripping apart a star, producing a bright flash of light. This phenomenon is known as a "tidal disruption event" (TDE), and this particular one – ASASSN-14li – has fascinated scientists ever since.
There are two reasons for this fascination. ASASSN-14li is a perfect specimen of tidal disruption flares, agreeing with many theoretical predictions. Also, the repeating X-ray emission brightens and fades every 131 seconds. This emission has been visible for at least 450 days, which is how the researchers were able to work out the black hole spins on itself at around 150,000 kilometers (93,000 miles) per second.
"That's not super fast – there are other black holes with spins estimated to be near 99 percent the speed of light," lead author Dr Dheeraj Pasham from MIT said in a statement. "But this is the first time we're able to use tidal disruption flares to constrain the spins of supermassive black holes."
The cause of the periodic emission is a bit of a puzzle. The team modeled different scenarios to explain it, with the most likely one requiring a third partner to work. It's possible that the supermassive black hole is orbited by a white dwarf at the “innermost stable circular orbit”, the last safe path for material before plunging into the black hole. If so, this white dwarf must have been there before the 2014 TDE event. When the supermassive black hole ripped apart the star, some of the material was stolen by the white dwarf. This compact star is now shrouded by a luminous cape that shines in our direction every 131 seconds.
"The problem with this scenario is that, if you have a black hole with a mass that's 1 million times that of the Sun, and a white dwarf is circling it, then at some point over just a few hundred years, the white dwarf will plunge into the black hole," Pasham says. "We would've been extremely lucky to find such a system. But at least in terms of the properties of the system, this scenario seems to work."
Understanding TDEs and black holes helps astronomers work out many of the effects these incredible objects have on their host galaxies.