Black Holes

A Black Hole is Savoring its Meal, Feeding on the Same Star Over and Over Again

Something extraordinary happens about every 10,000 to 100,000 years in galaxies like the Milky Way. An unwary star approaches the supermassive black hole (SMBH) at the galaxy’s center and is torn apart by the SMBH’s overpowering gravity. Astronomers call the phenomenon a tidal disruption event (TDE.)

Usually, a TDE spells doom for the star as its gas is torn away into the black hole’s accretion ring, causing a bright flaring visible for hundreds of millions of light years. But researchers have found one black hole that’s playing with its food.

It’s difficult to fathom the powerful forces at work when an SMBH eats a star.

Our own Sun is massive and incorporates 99.86% of the mass in the Solar System. That gives it enormous gravitational power. Its reach extends from tiny, speedy Mercury, its nearest neighbour, all the way out to the Oort Cloud, the hypothesized home of icy long-period comets up to 200,000 astronomical units, or 3.2 light-years, away.

But SMBHs are so massive that the Sun barely registers in comparison. An SMBH’s gravitational force is so mighty that it seems to hold their entire galaxy together.

In a new study, a team of astronomers observed an SMBH in a galaxy hundreds of millions of light-years away as it ate a star. The SMBH has an inferred black hole mass greater than 50 million solar masses. That an object that massive can completely destroy a star, puny in comparison, is axiomatic.

This illustration depicts a star (in the foreground) experiencing spaghettification as it’s sucked in by a supermassive black hole (in the background) during a ‘tidal disruption event’. Credit: ESO/M. Kornmesser

But this TDE isn’t like most other TDEs. The light coming from the event shows that the star wasn’t completely destroyed, and that the black hole is taking repeated bites from it.

“…the assumption has been that when we see the aftermath of a close encounter between a star and a supermassive black hole, the outcome will be fatal for the star,”

Thomas Wevers, ESO

The paper is “Live to Die Another Day: The Rebrightening of AT 2018fyk as a Repeating Partial Tidal Disruption Event.” It’s published in The Astrophysical Journal Letters. The lead author is Thomas Wevers from the European Southern Observatory.

“Until now, the assumption has been that when we see the aftermath of a close encounter between a star and a supermassive black hole, the outcome will be fatal for the star, that is, the star is completely destroyed,” Wevers said in a press release. “But contrary to all other TDEs we know of, when we pointed our telescopes to the same location again several years later, we found that it had re-brightened again. This led us to propose that rather than being fatal, part of the star survived the initial encounter and returned to the same location to be stripped of material once more, explaining the re-brightening phase.”

The TDE is called AT 2018fyk (AT: Astrophysical Transient), and it’s a transient first discovered in 2018 by the All-Sky Automated Survey for Supernovae. Different teams of scientists have been observing the event since its discovery, and this new research is a continuation of the attempt to understand it.

During a TDE, gas from a star falls toward a black hole. But it doesn’t fall directly into the hole. It accumulates on the accretion disk swirling around the SMBH. This heats the gas up, and it emits brilliant light. This event gives astronomers an opportunity to study the fascinating region around black holes as they’re briefly lit up.

But AT 2018fyk is different. Usually, a TDE consists of one jump in brightness as the star is destroyed. But not this one. Instead, it looks like the SMBH is stripping away the less dense outer layers of the star on each of its closest approaches, leaving a denser core unaffected.

This TDE is a result of what’s called a Hills Capture. That’s when a binary star approaches a black hole, and gravity separates the once-bound pair of stars. One star becomes a hypervelocity star and is ejected from the galaxy’s central regions at a speed of about 1000 km/second.

But there’s no escape for the other star. It enters into an orbit around the black hole, and its fate is foretold. The black hole will eventually destroy it, even though that doesn’t happen instantly.

The fact that the star was once part of a binary pair explains how it keeps passing close enough to the black hole to be partially stripped without being destroyed. “Typically, tidally disrupted stars are on approximately parabolic orbits, which begs the question of how a partial TDE could yield a brightening,” the authors write in their paper. For an SMBH this size, and a star similar in mass to the Sun, the orbit should last about 1,000 years, much longer than the star at AT 2018fyk. “One can bind the partially disrupted star more tightly if the star was initially part of a binary system that was destroyed through Hills capture.”

This illustration shows how the Hills Mechanism works. A binary star approaches an SMBH, and the powerful gravity splits the binary pair apart. One star is ejected from the galaxy as a hyper-velocity star. The other takes up a tight orbit around the SMBH. Image Credit: Warren R. Brown 2015/J. Guillochon.

In AT 2018fyk’s case, the black hole can’t quite destroy it all at once because even though the star is closely-bound to the SMBH, it doesn’t cross the tidal radius. The tidal radius is where the black hole’s gravity is stronger than the gravity holding the star together. Instead, about every 1200 days, the black hole gets another chance as the star makes its closest approach. It strips more of the star’s envelope away each time, forming the bright accretion disk around the SMBH. The evidence is in the light coming from the region. Our X-Ray and Ultraviolet /Optical telescopes can observe the light even though it’s hundreds of millions of light-years away.

A star that comes within the tidal-disruption radius RT of the supermassive black hole at the heart of a galaxy will be torn apart by the black hole’s gravity. Note that this image represents bound and unbound debris from common TDEs, not the exceptional AT 2018fyk in this study. Image Credit: NASA’s Goddard Space Flight Center/Suvi Gezari.

When astronomers first spotted AT 2018fyk, they thought it was like any other TDE. But subsequent observations showed how complex the TDE is. It was bright in x-ray emissions for about 600 days, then darkened and became undetectable. The decline in luminosity was quick, whereas other TDEs show a gradual decline. The abrupt decline was when the stellar core returned to its closest approach.

“When the core returns to the black hole, it essentially steals all the gas away from the black hole via gravity, and as a result, there is no matter to accrete, and hence the system goes dark,” said study co-author Dheeraj Pasham, an astrophysicist at MIT.

But then, 600 days after the drop in x-ray emissions, the region was bright again. The researchers think this is when material shed from the star starts accreting again.

This figure from the study illustrates what’s happening at AT 2018fyk. (a) shows the disruption of the binary star by the Hills Mechanism at -600 days. (b) shows how unbound debris could explain the extended dark period at -300 days. (c) shows how streams of material trigger the accretion disk at day zero. (d) shows how a quasi-spherical flow of material onto the disk transitions between emission states at +300 days. (e) shows how the stellar remnant makes its next closest approach and cuts off the flow of mass to the accretion disk at +600 days. (f) shows how the stream accretes onto the disk again, triggering more luminosity at +1200 days. Image Credit: T. Wevers et al 2023 ApJL 942 L33

It’s difficult to determine how long this can go on and how many passes the star can make. The researchers estimate that up to 10% of the star’s mass is stripped away on each approach, but it could also be as low as 1%. The TDE is hundreds of millions of light-years away, so understanding and modelling the light is challenging. But if the 10% figure is correct, the star is already gone.

“If the mass loss is only at the 1% level, then we expect the star to survive for many more encounters, whereas if it is closer to 10%, the star may have already been destroyed,” said co-author Eric Coughlin, a physicist at Syracuse University.

Now that astronomers have spotted the unusual AT 2018fyk and come up with a theoretical explanation, it could explain other repeating flares in the heart of other galaxies. In fact, this research might entice researchers to revisit other flaring black holes for additional activity. Will repeated TDEs be more common than thought? Can they only result from binary stars and the Hills Mechanism?

An artist’s illustration of an eclipsing binary star as seen from the surface of an exoplanet. Binary stars are common; will we find more repeating tidal disruption events? Image Credit: NASA

“In the future, it is likely that more systems will be checked for late-time flares, especially now that this project puts forth a theoretical picture of the capture of the star through a dynamical exchange process and the ensuing repeated partial tidal disruption,” says Coughlin. “We’re hopeful this model can be used to infer the properties of distant supermassive black holes and gain an understanding of their ‘demographics,’ being the number of black holes within a given mass range, which is otherwise difficult to achieve directly.”

Some of what the team has come up with to explain the TDE is testable. That fact, combined with more observations of phenomenon like AT 2018fyk, could teach us a lot about the extreme environments around black holes and the physics of partial TDEs.

“This study outlines methodology to potentially predict the next snack times of supermassive black holes in external galaxies,” says Pasham. “If you think about it, it is pretty remarkable that we on Earth can align our telescopes to black holes millions of light years away to understand how they feed and grow.”

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Evan Gough

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