Astronomers recently caught a supermassive black hole gulp down a star. It flared in exactly the same way as its smaller cousins do when those black holes have a snack. It just took longer and was a million times brighter.
Astronomers have been watching the feeding habits of small, stellar-mass black holes for decades. These black holes often orbit other stars, and occasionally feed from them. When material nears the black hole, it compresses down to form a thin accretion disk. The heat from that accretion generated a “soft” form of radiation, usually ultraviolet. But once the material from the disk thins out, a white-hot corona takes over, emitting “hard” radiation in the form of X-rays.
The whole process is over and done with in a matter of days.
Supermassive black holes also feed on their surrounding material too, but astronomers had long thought that it would be impossible to observe this process play out in real-time, because it would take millions of years to build up to a flare and transition on to a “soft” then “hard” phase.
But then TDE AT2018fyk happened. That’s the name given to a particular flare seen by the All-Sky Automated Survey for Supernovae (ASASSN) in September of 2018. It was a tidal eruption event, which happens when a giant black hole rips an entire star to shred before eating it alive.
A team of astronomers led by Dheeraj “DJ” Pasham, a research scientist in MIT’s Kavli Institute for Astrophysics and Space Research, followed up with further observations of the event.
“In a tidal disruption event, everything is abrupt,” Pasham says. “You have a sudden chunk of gas being thrown at you, and the black hole is suddenly woken up, and it’s like, ‘whoa, there’s so much food — let me just eat, eat, eat until it’s gone.’ So, it experiences everything in a short timespan. That allows us to probe all these different accretion stages that people have known in stellar-mass black holes.”
Over the course of two years, the astronomers were able to witness the entire messy story play out: an initial flash, the formation of an accretion disk with is “soft” UV emission, the transition to “hard” X-ray emission, and a final disappearance.
“We’ve demonstrated that, if you’ve seen one black hole, you’ve seen them all, in a sense,” says Pasham. “When you throw a ball of gas at them, they all seem to do more or less the same thing. They’re the same beast in terms of their accretion.”
“People have known this cycle to happen in stellar-mass black holes, which are only about 10 solar masses. Now we are seeing this in something 5 million times bigger,” Pasham says.
Besides being really cool, these observations are only the second time that astronomers have caught the formation of a corona around a black hole.
“A corona is a very mysterious entity, and in the case of supermassive black holes, people have studied established coronas but don’t know when or how they formed,” Pasham says. “We’ve demonstrated you can use tidal disruption events to capture corona formation. I’m excited about using these events in the future to figure out what exactly is the corona.”