New findings shed light on one of the cosmos’ most extreme environments

A group of physicists has developed a model that maps a star’s unexpected orbit around a supermassive black holethat reveals new insights into one of the cosmos’ most extreme environments.

Millions of light years away in a distant galaxy, a star is being torn apart by the immense gravity of a supermassive black hole. The destruction of the star results in a stream of debris that falls back onto the black hole and forms an accretion disk – a bright and hot disk of material swirling around the black hole.

The process of a star being destroyed by a supermassive black hole and fueling a luminous flare is known as a tidal disruption event (TDE). These events are thought to occur approximately once every 10,000 to 100,000 years in a given galaxy.

With luminosities exceeding entire galaxies (i.e., billions of times brighter than our Sun) for short periods (months to years), accretion events allow astrophysicists to study supermassive black holes (SMBHs) from cosmological distances, providing a window into the central regions of otherwise quiescent – ​​or sleeping – galaxies. By probing these “strong gravity” events, where Einstein’s theory of general relativity is crucial in determining how matter behaves, TDEs provide information about one of the most extreme environments in the universe: the event horizon – the point of no return – of a black hole.

TDEs are usually “once-and-done” because the extreme gravitational field of the SMBH destroys the star, meaning the SMBH fades back into obscurity after the accretion flare. In some cases, however, the high-density core of the star can survive the gravitational interaction with the SMBH, allowing it to orbit the black hole more than once. Researchers call this a recurrent partial TDE.

A team of physicists, including lead author Thomas Wevers, Fellow of the European Southern Observatory, and co-authors Eric Coughlin, assistant professor of physics at Syracuse University, and Dheeraj R. “DJ” Pasham, a researcher at MIT’s Kavli Institute for Astrophysics and Space Research have proposed a model for a recurrent partial TDE.

Their results, published in Astrophysical Journal Letters, describe the capture of the star by an SMBH, the stripping of the material each time the star comes close to the black hole, and the delay between when the material is stripped and when it re-feeds the black hole. The team’s work is the first to develop and use a detailed model of a repeating partial TDE to explain the observations, make predictions about the orbital properties of a star in a distant galaxy, and understand the partial tidal disruption process.

The team is studying a TDE known as AT2018fyk (AT stands for “Astrophysical Transient”). The star was captured by an SMBH through an exchange process known as “Hills capture”, where the star was originally part of a binary system (two stars orbiting each other under their mutual gravitational attraction) which was torn apart by the gravitational field of the black hole. The second (non-captured) star was ejected from the center of the galaxy at velocities equal to ~1000 km/s, which is known as a hypervelocity star.

Once bound to the SMBH, the star driving the emission from AT2018fyk has been repeatedly stripped from its outer envelope each time it passes through its closest approach with the black hole. The stripped outer layers of the star form the bright accretion disk, which scientists can study using X-ray and ultraviolet/optical telescopes that observe light from distant galaxies.

Animation describing a partial tidal disruption – where a black hole repeatedly destroys a star. Credit: Syracuse University, Wevers, Coughlin, Pasham et al. (2022)

According to Wevers, the possibility of studying a partial TDE provides unprecedented insight into the existence of supermassive black holes and the orbital dynamics of stars in the centers of galaxies.

“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 to the star, that is, the star is completely destroyed,” he says. “However, unlike all other TDEs we know of, when we pointed our telescopes at the same spot again several years later, we found that it had brightened again. This led us to suggest that instead of being mortal, part of the star survived the first encounter and returned to the same place to be stripped of material once more, which explains the re-enlightening phase.”

First discovered in 2018, AT2018fyk was initially thought to be a regular TDE. For approximately 600 days, the source remained bright in the X-ray beam, but then suddenly became dark and was undetectable – a result of the stellar remnant core returning to a black hole, explains WITH Physicist Dheeraj R. Pasham.

“When the nucleus returns to the black hole, it essentially steals all the gas away from the black hole via gravity, and as a result there’s nothing to accumulate, and so the system goes dark,” says Pasham.

It was not immediately clear what caused the sharp drop in the brightness of AT2018fyk because TDEs usually decay smoothly and gradually—not abruptly—in their emission. But about 600 days after the fall, the source was again found to be X-rays. This led the researchers to suggest that the star survived its close encounter with the SMBH the first time and was in orbit around the black hole.

Using detailed modeling, the team’s results suggest that the star’s orbital period around the black hole is about 1,200 days, and it takes about 600 days for the material ejected from the star to return to the black hole and start accreting. Their model also constrained the size of the captured star, which they believe was roughly the size of the Sun. As for the original binary, the team believes the two stars were extremely close to each other before they were torn apart by the black hole, which likely orbited each other every few days.

So how could a star survive its brush with death? It all comes down to a matter of proximity and trajectory. If the star collided head-on with the black hole and passed the event horizon—the threshold at which the speed required to escape the black hole exceeds the speed of light—the star would be consumed by the black hole. If the star passed very close to the black hole and crossed the so-called “tidal radius” — where the tidal force of the hole is stronger than the gravitational force holding the star together — it would be destroyed. In the model they have proposed, the star’s orbit reaches a point of closest approach that is just outside the tidal radius, but does not completely cross it: some of the material at the star’s surface is stripped by the black hole, but material at its center remains intact .

How, or whether, the process of the star orbiting the SMBH can occur over many repeated passages is a theoretical question that the team plans to investigate with future simulations. Syracuse physicist Eric Coughlin explains that they estimate that between 1 and 10% of the star’s mass is lost each time it passes the black hole, with the large range due to uncertainties in the modeling of the emission from the TDE.

“If the mass loss is only at the 1% level, then we expect the star to survive for many more encounters, while if it’s closer to 10%, the star may have already been destroyed,” notes Coughlin.

The team will keep their eyes skyward in the coming years to test their predictions. Based on their model, they predict that the source will abruptly disappear around August 2023 and shine again when the newly stripped material accumulates on the black hole in 2025.

The team says their study offers a new way forward for tracking and monitoring follow-on sources that have been discovered in the past. The work also suggests a new paradigm for the origin of repeated bursts from the centers of external galaxies.

“In the future, it is likely that more systems will be checked for late bursts, especially now that this project provides a theoretical picture of stellar capture through a dynamical exchange process and the subsequent repeated partial tidal disruption.” says Coughlin. “We hope this model can be used to infer the properties of distant supermassive black holes and gain an understanding of their ‘demography’, which is the number of black holes within a given mass range, which is otherwise difficult to obtain directly.”

The team says the model also makes several testable predictions about the tidal disruption process, and with more observations of systems like AT2018fyk, it should provide insight into the physics of partial tidal disruption events and the extreme environments around supermassive black holes.

“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’s pretty remarkable that on Earth we can align our telescopes to black holes millions of light years away to understand how they feed and grow.”

Reference: “Live to Die Another Day: The Rebrightening of AT 2018fyk as a Repeating Partial Tidal Disruption Event” by T. Wevers, ER Coughlin, DR Pasham, M. Guolo, Y. Sun, S. Wen, PG Jonker, A. Zabludoff, A. Malyali, R. Arcodia, Z. Liu, A. Merloni, A. Rau, I. Grotova, P. Short, and Z. Cao, 12 January 2023, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ac9f36