Supermassive black holes are surrounded by a corona, a region of high-energy particles trapped in a magnetic field near the black hole. Changes to the corona usually take hundreds of thousands if not millions of years, but now astronomers have observed a black hole corona become 10,000 times dimmer in just 40 days.
"We just don't normally see variations like this in accreting black holes," lead author Claudio Ricci, an assistant professor at Diego Portales University in Santiago, Chile, said in a statement. "It was so strange that at first we thought maybe there was something wrong with the data. When we saw it was real, it was very exciting. But we also had no idea what we were dealing with; no one we talked to had seen anything like this."
The incredible event is reported in The Astrophysical Journal Letters. The All-Sky Automated Survey for Super-Nova spotted an increase in the brightness around the black hole, which prompted researchers to keep an eye on the object. After 160 days, the black hole suddenly and dramatically dimmed.
The increase in brightness happened in December 2017, and between June and August 2018, the corona was gone. Over the following year or so, the researchers continued to follow the object's dramatic variation, becoming 100 times more or less bright in just 8 hours. Despite these changes, over time, the corona formed again, going back to normal and becoming even brighter.
"This seems to be the first time we've ever seen a corona first of all disappear, but then also rebuild itself, and we're watching this in real-time," co-author Erin Kara, an assistant professor of physics at MIT, said in a statement. "This will be really important to understanding how a black hole's corona is heated and powered in the first place."
The cause of this never-before-seen event is believed to be a star. The researchers think it's likely that a star moved too close to the black hole and the gravity of the black hole ripped it apart, turning it into plasma. Such a phenomenon is known as a tidal disruption event.
The hot, electrically charged plasma disrupted the magnetic fields around the black hole and led to the corona and other material surrounding the black hole to fall into the gargantuan object. If this picture is correct, it means the corona is generated close to the black hole at about half the distance the Earth is from the Sun.
The team continues to monitor the galaxy in case another extraordinary event happens. They are particularly interested to see if the corona disappears when another tidal disruption event happens around another black hole.
“We want to keep an eye on it,” Kara says. “It’s still in this unusual high-flux state, and maybe it’ll do something crazy again, so we don’t want to miss that.”