Colliding Star Destroys Supermassive Black Hole’s Corona in Distant Galaxy

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An international team of astronomers has watched as an X-ray corona of a supermassive black hole in the active galactic nucleus 1ES 1927+654 was abruptly destroyed. The researchers think that their observations could be explained by the interaction between the accretion flow around the black hole and debris from a tidally disrupted star.

This two-panel illustration shows a black hole surrounded by a disk of gas, before and after the disk is partially dispersed. In the left panel, the ball of white light above the black hole is the black hole corona, a collection of ultra-hot gas particles that forms as gas from the disk falls into the black hole. The streak of debris falling toward the disk is what remains of a star that was torn apart by the black hole’s gravity. The right panel shows the black hole after the debris from the star has dispersed some of the gas in the disk, causing the corona to disappear. Image credit: NASA / JPL-Caltech.

This two-panel illustration shows a black hole surrounded by a disk of gas, before and after the disk is partially dispersed. In the left panel, the ball of white light above the black hole is the black hole corona, a collection of ultra-hot gas particles that forms as gas from the disk falls into the black hole. The streak of debris falling toward the disk is what remains of a star that was torn apart by the black hole’s gravity. The right panel shows the black hole after the debris from the star has dispersed some of the gas in the disk, causing the corona to disappear. Image credit: NASA / JPL-Caltech.

In March 2018, an unexpected burst lit up the view of the All-Sky Automated Survey for Super-Novae (ASSASN), which surveys the entire night sky for supernova activity.

The survey observed that the brightness of 1ES 1927+654 jumped to about 40 times its normal luminosity.

“This was an active galactic nucleus (AGN) that we sort of knew about, but it wasn’t very special,” said co-author Dr. Erin Kara, an astrophysicist at the MIT Kavli Institute for Astrophysics and Space Research.

“Then ASSASN noticed that this run-of-the-mill AGN became suddenly bright, which got our attention, and we started pointing lots of other telescopes in lots of other wavelengths to look at it.”

“We expect that luminosity changes this big should vary on timescales of many thousands to millions of years,” she added.

“But in 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 just totally unheard of and really mind-boggling.”

“We just don’t normally see variations like this in accreting black holes,” said lead author Professor Claudio Ricci, a researcher at Diego Portales University.

“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.”

Dr. Kara, Dr. Ricci and their colleagues used multiple telescopes to observe 1ES 1927+654’s supermassive black hole in the X-ray, optical, and ultraviolet wave bands.

Most of these telescopes were pointed at the black hole periodically, for example recording observations for an entire day, every six months.

The researchers also watched the black hole daily with NASA’s Neutron Star Interior Composition Explorer (NICER).

With frequent observations, they were able to catch the black hole as it precipitously dropped in brightness, in virtually all the wave bands they measured, and especially in the high-energy X-ray band — an observation that signaled that the black hole’s corona had completely and suddenly vaporized.

“After ASSASN saw it go through this huge crazy outburst, we watched as the corona disappeared,” Dr. Kara said.

“It became undetectable, which we have never seen before.”

Astrophysicists are unsure exactly what causes a corona to form, but they believe it has something to do with the configuration of magnetic field lines that run through a black hole’s accretion disk.

Closer in, and especially near the event horizon, material circles with more energy, in a way that may cause magnetic field lines to twist and break, then reconnect. This tangle of magnetic energy could spin up particles swirling close to the black hole, to the level of high-energy X-rays, forming the crown-like corona that encircles the black hole.

Dr. Kara and co-authors believe that if a wayward star was indeed the culprit in the corona’s disappearance, it would have first been shredded apart by the black hole’s gravitational pull, scattering stellar debris across the accretion disk.

This may have caused the temporary flash in brightness that ASSASN captured.

This tidal disruption would have triggered much of the material in the disk to suddenly fall into the black hole. It also might have thrown the disk’s magnetic field lines out of whack in a way that it could no longer generate and support a high-energy corona.

This last point is a potentially important one for understanding how coronas first form. Depending on the mass of a black hole, there is a certain radius within which a star will most certainly be pulled in by a black hole’s gravity.

The scientists calculated that if a star indeed was the cause of the black hole’s missing corona, and if a corona were to form in a supermassive black hole of similar size, it would do so within a radius of about 4 light minutes — a distance that roughly translates to about 75 million km from the black hole’s center.

“With the caveat that this event happened from a stellar tidal disruption, this would be some of the strictest constraints we have on where the corona must exist,” Dr. Kara said.

The corona has since reformed, lighting up in high-energy X-rays which the team was also able to observe.

It’s not as bright as it once was, but the authors are continuing to monitor it, though less frequently, to see what more this system has in store.

“We want to keep an eye on it. It’s still in this unusual high-flux state, and maybe it will do something crazy again, so we don’t want to miss that,” Dr. Kara said.

The team’s paper was published in the Astrophysical Journal Letters.

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C. Ricci et al. 2020. The Destruction and Recreation of the X-Ray Corona in a Changing-look Active Galactic Nucleus. ApJL 898, L1; doi: 10.3847/2041-8213/ab91a1

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