New Findings Sheds Light on One of the Cosmos’ Most Extreme Environments

This illustration shows a glowing stream of material from a star as it is being swallowed by a supermassive black hole in a tidal disruption explosion. When a star passes within a certain distance of a black hole—close enough to be gravitationally disrupted—the stellar material stretches and compresses as it falls into the black hole. Credit: NASAJPL-Caltech

A group of physicists has developed a model that describes the sudden orbit of a star around a supermassive black hole, revealing new insights into one of the most extreme environments of the cosmos.

Millions of light years away in a distant galaxy, a star is being torn apart by the immense gravitational pull of a supermassive black hole. The destruction of the star results in a stream of debris that falls back into the black hole, forming an accretion disk—a bright, hot disk of material that orbits the black hole.

The process of a star being destroyed by a supermassive black hole and triggering a bright accretion burst is known as a tidal disruption event (TDE). These events are believed to occur approximately once every 10,000 to 100,000 years in any given galaxy.

With luminosities that outshine entire galaxies (ie, billions of times brighter than our Sun) for short periods of time (months to years), accretion events enable astrophysicists to study supermassive black holes (SMBHs) from cosmological distances, providing a window into the central regions. of other quiescent – ​​or dormant – galaxies. By probing these “strong gravity” events, where Einstein’s general theory of relativity is critical to 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-done” because the extreme gravitational field of the SMBH destroys the star, meaning that the SMBH fades back into obscurity after the accretion burst. However, in some cases, 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 partial recurrent TDE.

This illustration depicts a star (foreground) experiencing spaghettification as it is absorbed by a supermassive black hole (background) during a ‘tidal disruption event’. Credit: ESOM Kornmesser

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

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

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 “Hill Capture”, where the star was originally part of a binary system (two stars orbiting each other under their mutual gravitational pull) that was separated by the gravitational field the black hole. The other (uncaptured) star was ejected from the galactic center at speeds comparable to ~1000 km/s, known as a hypervelocity star.

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

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

According to Wevers, being able to study a partial TDE provides unprecedented insight into the existence of supermassive black holes and the orbital dynamics of stars at 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 result will be fatal for the star, that is, the star is completely destroyed,” he says. “But unlike all the other TDEs we know of, when we pointed our telescopes at the same spot again a few years later, we found that it had risen 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-enlightenment phase.

First revealed in 2018, AT2018fyk was initially perceived as an ordinary TDE. For roughly 600 days, the source stayed bright in X-rays, but then suddenly dimmed and went undetected—as a result of the remnant stellar core collapsing into a black hole, explains MIT physicist Dheeraj R. Pasham.

“When the nucleus returns to the black hole, it essentially steals all the gas from the black hole via gravity and as a result, there is no matter to accretate and so the system darkens,” says Pasham.

It wasn’t immediately clear what caused AT2018fyk’s sharp drop in brightness, because TDEs normally decay smoothly and gradually—not suddenly—in their emission. But about 600 days after the collapse, the source was again found to be X-ray bright. This led the researchers to propose that the star survived the close encounter with the SMBH the first time and was in orbit around the black hole.

Using detailed modeling, the team’s findings suggest that the star’s orbital period around the black hole is approximately 1,200 days, and it takes approximately 600 days for material ejected from the star to return to the black hole and begin accretion. Their model also constrained the size of the captured star, which they believe was about 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, likely orbiting each other every few days.

So how can a star survive after death? It all depends on proximity and trajectory. If the star collided head-on with the black hole and crossed the event horizon – the threshold where the speed required to escape from the black hole exceeds the speed of light – the star would be consumed by the black hole. If the star passed too 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 collapse. 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 pass it completely: some of the material on the star’s surface is stripped from the black hole, but the material at its center remains intact.

How, or whether, the process of the star orbiting the SMBH can occur in 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 1 to 10% of the star’s mass is lost each time it passes the black hole, with the large range due to uncertainty in the modeling of the emission from TDE.

“If the mass loss is only on the 1% level, then we expect the star to survive for many more encounters, whereas if it’s closer to 10%, the star may already be 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 suddenly disappear around August 2023 and will shine again when the newly stripped material accumulates in the black hole in 2025.

The team says their study provides a new way forward for tracking and monitoring trace sources that have been discovered in the past. The work also suggests a new paradigm for the origin of recurrent flares from the centers of outer galaxies.

“In the future, it is likely that more systems will be screened for recent outbursts, especially now that this project presents a theoretical picture of star capture through a process of dynamical exchange and partial tidal disruption.” says Coughlin. “We hope that this model can be used to tease out the properties of distant supermassive black holes and to understand their ‘demography,’ being the number of black holes within a given mass range, which is otherwise difficult to reach directly.”

The team says the model also makes some 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 holes. supermassive black.

“This study describes the methodology to potentially predict the future feeding times of supermassive black holes in outer galaxies,” says Pasham. “If you think about it, it’s pretty remarkable that we on Earth can line up our telescopes with black holes millions of light years away to understand how they feed and grow.”

Reference: “Live to Die Another Day: The 2018fyk AT Resurgence as a Repeated 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