Black holes don't glow - in fact, they're famous for doing the opposite. But if they're actively devouring material from the space around them, that material can blaze like a billion X-ray Suns.
And for the first time, astronomers have now seen that blaze mysteriously snuffed out, before gradually returning to brightness.
The supermassive black hole is a beast clocking in at 19 million solar masses, powering a galactic nucleus 275 million light-years away, in a galaxy called 1ES 1927+654.
Over a period of just 40 days, astronomers watched as its corona absolutely plummeted in brightness, before rising again to shine brighter than before.
"We expect that luminosity changes this big should vary on timescales of many thousands to millions of years," said physicist Erin Kara of the Massachusetts Institute of Technology (MIT).
"But in this object, we saw it change by [a factor of] 10,000 over a year, and it even changed by a factor of 100 in eight hours, which is just totally unheard of and really mind-boggling."
There are several components to the area immediately around a black hole. There's the event horizon; that's the famous "point of no return", at which even light speed is not sufficient to attain escape velocity.An active black hole also has an accretion disc, a huge disc of material swirling into the object, like water circling a drain.
And just outside the event horizon of an active black hole, around the inner edge of the accretion disc, is the corona.
This is a region of scorchingly hot electrons thought to be powered by the black hole's magnetic field, acting like a synchrotron to accelerate the electrons to such high energies that they shine brightly in X-ray wavelengths.
Astronomers first noticed something strange occurring in 1ES 1927+654 in 2018, when the All-Sky Automated Survey for Super-Novae (ASASSN) - an automated survey looking for bright flashes of light across the entire sky - caught an incredibly bright flare from the galaxy, 40 times its normal brightness.
This caught astronomers' attention, and they pointed a bunch of telescopes in the galaxy's direction to find out more. For a while, everything was pretty normal - but then, around 160 days after the flare, 1ES 1927+654's nucleus started to dim. Over a period of 40 days, the X-ray glow was totally snuffed out.
"After ASSASN saw it go through this huge crazy outburst, we watched as the corona disappeared," Kara said. "It became undetectable, which we have never seen before."
But then the brightness started to steadily climb again. By 300 days after the initial flare, the galaxy's nucleus was shining almost 20 times more brightly than it had been prior to the initial event.
"We just don't normally see variations like this in accreting black holes," said astrophysicist Claudio Ricci of Diego Portales University in Chile, and lead author of the study.
"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."
Astronomers are not entirely sure how black hole coronae are generated and powered. But if, as theorised, it has something to do with the black hole's magnetic fields, then the dramatic changes observed in 1ES 1927+654's black hole could have been caused by something disrupting those magnetic fields.
We know that black holes can change pretty rapidly when they capture and devour a star that ventures just a little too close. The star is torn apart in a process called tidal disruption, screaming a flare of bright light, before getting slurped beyond the event horizon to meet its mysterious fate.
And if a runaway star so happened to encounter 1ES 1927+654's black hole, the events could fit the observed changes in the X-ray radiation. First, the star is tidally disrupted, causing the initial flare. Then debris from the star could have temporarily disrupted the black hole's magnetic field, after which it rebuilt itself as the space around the black hole settled back into a more normal state.
And that could be an important clue for understanding the radius within which a magnetic field controls a black hole corona.
"What that tells us is that, if all the action is happening within that tidal disruption radius, that means the magnetic field configuration that's supporting the corona must be within that radius," Kara said.
"Which means that, for any normal corona, the magnetic fields within that radius are what's responsible for creating a corona."
If a star was responsible for the event, the team calculated that the tidal disruption had to have occurred within four light minutes of the black hole's centre, around half the distance between the Earth and the Sun.
But there could be some other reason for the light show.
We know that black hole coronae can vary in brightness, although generally that occurs on much longer timescales. It's possible that the extreme behaviour observed in 1ES 1927+654 is actually also pretty normal behaviour - we just haven't spotted it until now.
"This dataset has a lot of puzzles in it," Kara said. "But that's exciting, because it means we're learning something new about the universe. We think the star hypothesis is a good one, but I also think we're going to be analyzing this event for a long time."
The research has been published in The Astrophysical Journal Letters.
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