In a distant galaxy hundreds of millions of light-years away, a star orbiting a supermassive black hole is being violently torn apart by the black hole’s enormous gravity. When a star is shredded, its remnants become a stream of debris that falls back into the black hole, forming a disk of very hot, very bright material that swirls around the black hole, called an accretion disk. This phenomenon—a star being destroyed by a supermassive black hole and triggering a glowing accretion flare—is known as a tidal disintegration event (TDE), and TDEs are predicted to occur roughly every 10,000 to 100,000 years for a given period of time Galaxy at once.
Accretion events allow astrophysicists to study supermassive black holes (SMBHs) from cosmological distances due to their luminosity exceeding that of entire galaxies (i.e., billions of times brighter than our sun) for short periods of time (months to years), Thereby providing a window into the central regions of otherwise quiescent – or dormant – galaxies. By detecting these “strong gravitational” events, where Einstein’s general theory of relativity is central to determining how matter behaves, TDEs yield information about one of the most extreme environments in the universe: the event horizon — the point of no return — and the black hole.
TDEs are usually “set and forget” because the SMBH’s extreme gravitational field destroys the star, meaning the SMBH disappears back into darkness after an accretion flare. In some cases, however, a star’s dense core can survive gravitational interactions with the SMBH, causing it to orbit the black hole more than once. The researchers call this repeated partial TDE.
A team of physicists, including lead author Thomas Wevers, a researcher at the European Southern Observatory, and co-author Eric Coughlin, an assistant professor of physics at Syracuse University, and Dheeraj R. “DJ” Pasham, a research scientist at MIT’s Kavli Institute for Astrophysics, and For space studies, a repetitive partial TDE model is proposed.Their findings were published in The Astrophysical Journal Letters, describes the capture of the star by the SMBH, the stripping of matter each time the star approaches the black hole, and the delay between the material being stripped and re-entering the black hole. The team’s work is the first to develop and use detailed models of repeating partial TDEs to explain observations, predict orbital properties of stars in distant galaxies, and understand partial tidal disruption processes.
The team is studying a TDE called AT2018fyk (AT stands for “astrophysical transient”). The star was captured by the SMBH through an exchange process called “Shiels capture,” in which the star was originally part of a binary system (two stars orbiting each other under mutual gravitational pull) ), the black hole is torn apart by the gravitational field of the binary star. Another (uncaptured) star is ejected from the center of the Milky Way at a speed equivalent to about 1000 km/s, which is called a hypervelocity star.
Once bound to the SMBH, the star powering AT2018fyk repeatedly sheds its outer shell each time it passes its closest point to the black hole. The outer layers stripped off by stars form bright accretion disks, which can be studied by researchers using X-ray and ultraviolet/optical telescopes to observe light from distant galaxies.
According to Wevers, having the opportunity to study partial TDEs provides unprecedented insight into the existence of supermassive black holes and the orbital dynamics of stars at the centers of galaxies.
“Until now, it has been assumed that when we see the consequences of a star’s close encounter with a supermassive black hole, the outcome will be fatal for the star, that is, the star is completely destroyed,” he said. “But contrary to all the other TDEs we know of, when we pointed the telescope at the same location again a few years later, we found it brightened again. This led us to propose that part of the star was not deadly, but rather in the Surviving the initial encounter and returning to the same location to strip the material again explains the re-brightening phase.”
AT2018fyk was first spotted in 2018 and was initially seen as a common TDE. For about 600 days, the source remained bright in X-rays, but then suddenly dimmed and became undetectable—the result of the remnant core of the star returning to the black hole, explains MIT physicist Dheeraj R. Pasham. .
“When the core returned to the black hole, it basically stole all the gas from the black hole via gravity, so there was nothing to accrete, so the system went black,” Pasham said.
It’s not clear what caused AT2018fyk’s sharp drop in luminosity, as TDE typically decays smoothly and gradually — rather than suddenly — over the course of its launch. But about 600 days after the fall, the source was found to be X-ray luminous again. This led the researchers to propose that the star survived its first close encounter with the SMBH and was in orbit around the black hole.
Through detailed modeling, the team’s findings suggest that the orbital period of a star around a black hole is about 1,200 days, and that it takes about 600 days for material shed from the star to return to the black hole and begin accreting. Their model also constrains the size of the captured star, which they believe is about the size of our sun. As for the original binary system, the team believes that the two stars were very close to each other before being torn apart by the black hole, most likely orbiting each other every few days.
So how does a star survive its brush with death? It all comes down to a matter of distance and trajectory. If a star collides head-on with a black hole and passes the event horizon—the threshold at which the velocity required to escape a black hole exceeds the speed of light—the star will be swallowed by the black hole. If a star comes very close to a black hole and crosses what is known as its “tidal radius” — where the black hole’s tidal forces are stronger than the gravitational pull that holds the star together — it is destroyed. In their proposed model, the star’s orbit reaches a point of closest approach just outside the tidal radius, but doesn’t quite pass through it: Some material on the star’s surface is stripped by the black hole, but material at its center remains intact.
How or whether a star’s orbit around an SMBH can occur in many repeating passages is a theoretical question that the team plans to investigate in future simulations. Syracuse physicist Eric Coughlin explained that they estimate that stars lose between 1% and 10% of their mass each time they pass by a black hole, a large range due to uncertainties in TDE emission modeling.
“If the mass loss is only at the 1 percent level, then we expect the star to survive many more encounters, whereas if the mass loss is closer to 10 percent, the star may have been destroyed,” Coughlin noted.
The team will be watching the sky closely over the next few years to test their predictions. Based on their model, they predict that the source will suddenly disappear around March 2023 and brighten again in 2025 as newly stripped material accumulates on the black hole.
The team says their study provides a new way to track and monitor follow-up sources detected in the past. The work also suggests a new paradigm for the origin of recurring flares in the centers of outer galaxies.
“In the future, it is likely that more systems will be examined for late flares, especially now that this project presents a theoretical picture of star capture through dynamic exchange processes and subsequent iterative partial tidal disruption,” Coughlin said. “We hope this model can be used to infer the properties of distant supermassive black holes and understand their ‘demographics’, the number of black holes in a given mass range, which is otherwise difficult to achieve directly.”
The model also makes several testable predictions about tidal disruption processes, and with more observations of systems such as AT2018fyk, it should provide insight into the physics of some tidal disruption events and the dynamics around supermassive black holes, the team said. extreme environment.
“This study outlines ways to possibly predict the next snacking time of supermassive black holes in outer galaxies,” Pasham said. “If you think about it, it’s pretty remarkable that we Earthlings can point our telescopes at black holes millions of light-years away to see how they eat and grow.”
Additional co-authors include: M. Guolo, Department of Physics and Astronomy, Johns Hopkins University; Y. Sun, University of Arizona; S. Wen, Department of Astrophysics/IMAPP, Radboud University; PG Jonker, Radboud University and SRON Department of Astrophysics/IMAPP, Netherlands Institute for Space Research; A. Zabludoff, University of Arizona; A. Malyali, R. Arcodia, Z. Liu, A. Merloni, A. Rau and I. Grotova, Max-Planck-Institut fu Ör extraterrestrische Physik, Germany; P. Short, Institute for Astronomy, University of Edinburgh; and Z. Cao, Department of Astrophysics, Radboud University/IMAPP
video: https://youtu.be/_TRtPDbaQ2k
