It seems that the universe has a strange sense of humor. While a crown-encrusted virus has rushed around the world, a completely different corona about 100 million light-years from Earth has mysteriously disappeared.
For the first time, astronomers at MIT and elsewhere have seen how the corona of a supermassive black hole, the billion-degree ultrabright ring of high-energy particles surrounding the event horizon of a black hole, was abruptly destroyed .
The cause of this dramatic transformation is unclear, though researchers assume the source of the calamity may have been a star caught in the black hole’s gravitational pull. Like a stone thrown into a gearbox, the star may have bounced through the black hole’s swirling disk, causing everything in the vicinity, including the high-energy particles in the corona, to suddenly fall into the black hole. .
The result, as astronomers observed, was a dramatic and precipitous drop in the brightness of the black hole, by a factor of 10,000, in less than a year.
“We expect brightness changes of this size to vary over time scales of many thousands to millions of years,” says Erin Kara, an assistant professor of physics at MIT. “But on this object, we saw it change by 10,000 over a year, and it even changed by a factor of 100 in eight hours, which is totally unknown and really mind-blowing.”
After the corona’s disappearance, astronomers continued to watch as the black hole slowly began to gather material from its outer edges to reform its spinning accretion disk, which in turn began to spin high-energy X-rays near the horizon of black hole events. . In this way, in just a few months, the black hole was able to generate a new corona, almost back to its original luminosity.
“This seems to be the first time we’ve seen a crown disappear in the first place, but then also rebuilt, and we’re seeing this in real time,” says Kara. “This will be really important in understanding how a black hole corona is heated and fed in the first place.”
Kara and her co-authors, including lead author Claudio Ricci of the Diego Portales University in Santiago, Chile, have published their findings today in Astrophysical Journal Letters. MIT co-authors include Ron Remillard and Dheeraj Pasham.
An agile washing machine
In March 2018, an unexpected explosion lit up ASSASN’s view, the Automated All-Sky Survey for Super-Novae, which examines supernova activity across the entire night sky. The survey recorded a flash of 1ES 1927 + 654, an active galactic nucleus, or AGN, which is a type of supermassive black hole with higher-than-normal brightness at the center of a galaxy. ASSASN observed that the object’s brightness jumped to approximately 40 times its normal luminosity.
“This was an AGN that we knew about, but it wasn’t very special,” says Kara. “Then they realized that this current AGN suddenly became bright, which caught our attention, and we started targeting many other telescopes at many other wavelengths to look at it.”
The team used multiple telescopes to observe the black hole in the X-ray, optical, and ultraviolet wave bands. Most of these telescopes periodically aimed at the black hole, for example, recording observations for an entire day, every six months. The team also observed the black hole daily with NASA’s NICER, a much smaller X-ray telescope installed aboard the International Space Station, with detectors developed and built by MIT researchers.
“NICER is great because he is very agile,” says Kara. “It’s this little washer that bounces around the ISS, and it can collect a ton of X-ray photons. Every day, NICER could take a quick look at this AGN, and then go and do something else.”
With frequent observations, the researchers were able to catch the black hole when it abruptly dropped in brightness, in virtually all of the wavebands they measured, and especially in the high-energy X-ray band, an observation that noted that the black hole’s corona was it had vaporized completely and suddenly.
“After ASSASN saw him go through this huge crazy bang, we saw the crown disappear,” Kara recalls. “It became undetectable, which we’ve never seen before.”
A shaken flash
Physicists are not sure exactly what causes a corona to form, but they believe it has something to do with the configuration of the magnetic field lines running through the accretion disk of a black hole. In the outer regions of the rotating black hole material disk, the magnetic field lines are more or less straightforward. Closer, and especially closer to the event horizon, the circles of material with more energy, in a way that can cause the magnetic field lines to twist and break, then reconnect. This tangle of magnetic energy could spin swirling particles near the black hole, at the high-energy X-ray level, forming the corona-shaped corona surrounding the black hole.
Kara and her colleagues believe that if a stray star were actually to blame for the disappearance of the corona, it would first have been shattered by the gravitational pull of the black hole, scattering stellar debris on the accretion disk. This may have caused the temporary flash of brightness that ASSASN captured. This “tidal break”, as astronomers call such a shaking event, would have caused much of the material on the disk to suddenly fall into the black hole. It could also have thrown the disk’s magnetic field lines out of control in a way that could no longer generate and support a high-energy corona.
This last point is potentially important for understanding how the first crowns are formed. Depending on the mass of a black hole, there is a certain radius within which a star will surely be attracted by the gravity of a black hole.
“What it tells us is that, if all the action is taking place within that tidal interruption radius, that means that the configuration of the magnetic field that supports the corona must be within that radius,” says Kara. “Which means that, for any normal corona, the magnetic fields within that radius are responsible for creating a corona.”
The researchers calculated that if a star were to cause the black hole’s missing corona, and if a corona formed in a supermassive black hole of similar size, it would do so within a radius of about 4 light minutes, a distance that translates approximately 75 million km from the center of the black hole.
“With the caveat that this event occurred due to a disruption of the stellar tide, these would be some of the strictest restrictions we have on where the corona should exist,” says Kara.
Since then, the crown has been reshaped, lighting up with high-energy X-rays that the team was also able to observe. It’s not as bright as it was before, but researchers continue to monitor it, albeit less frequently, to see what else this system has in store.
“We want to keep an eye on him,” says Kara. “He’s still in this unusual high-flow state, and maybe he’s going to do something crazy again, so we don’t want to miss that.”
This research was funded, in part, by NASA.
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