Stars interacting with supermassive black holes (SMBHs) can be partially or completely destroyed by tides. While some of the tidal disturbances (TDEs) identified so far have exhibited such long-lasting behavior, a significant proportion show a different evolution of the light curve, which in some cases is completely decoupled from the mass fallback rate.
In a new study, a team of physicists from Syracuse University describes how the star is captured by an SMBH, how the material is removed each time the star approaches the black hole, how long it takes for the material to be removed and when it will be removed. feeds the black hole again.
Physicists have developed and used a detailed model of a repeating partial TDE. The model maps a star’s surprising orbit around a supermassive black hole, revealing new information about one of the most extreme environments in the cosmos.
This model allowed scientists to explain TDE observations and make predictions about the orbital properties of a star in a distant galaxy. It also helps them understand the process of partial tidal disturbance.
In particular, physicists studied a TDE known as AT2018fyk. The TDE AT 2018fyk showed anomalous brightening in both the UV and X-ray bands to luminosities within a factor of 10 of their peak values, which is unprecedented in observations of TDEs.
The model showed that this first burst behavior was caused by the partial perturbation of a star that was part of a binary system. The partially disrupted 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 pull) that was torn apart by gravity . field of the black hole. The other (uncaptured) star was ejected from the center of the galaxy at speeds comparable to ~1000 km/s, known as a hypervelocity star.
The star powering AT2018fyk’s emission was formerly associated with the SMBH, but each time it passes the point of closest approach to the black hole, it is repeatedly stripped of its outer envelope. Researchers can examine the brilliant accretion disk, which is made up of the star’s stripped outer layers, using X-ray, ultraviolet and optical telescopes that look at light from distant galaxies.
Lead author Thomas Wevers, Fellow of the European Southern Observatory, said: “Having the opportunity to study a repeating 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 outcome will be fatal for the star, that is, the star will be destroyed. But unlike all the other TDEs we know of, when we aimed our telescopes at the same location a few years later, we found that it had brightened again. This led us to hypothesize that instead of being fatal, part of the star survived the initial encounter and returned to the same location to be stripped of material again, explaining the brightening phase.”
MIT physicist Dheeraj R. Pasham. said, “It wasn’t immediately clear what caused AT2018fyk’s sharp decrease in brightness, because TDEs normally decrease smoothly and gradually – not abruptly – in their emission. But about 600 days after the fall, the source was found to be X-ray clear again. This led the researchers to hypothesize that the star survived the first encounter with the SMBH and orbited the black hole.”
The results of the team’s thorough modeling imply that the star’s orbit around the black hole has a period of about 1,200 days, and that it takes almost 600 days for the material ejected from the star to return to the black hole and starts to grow. The size of the captured star, which was about the size of the sun according to their model, was also limited. In terms of the original binary, the team thinks the two stars probably orbited each other every few days before being ripped apart by the black hole.
So, how can a star survive its brush with death?
It’s because of the proximity and the trajectory. The star would be sucked into the black hole if it collided head-on with it and reached the event horizon, the point above which it would be impossible to escape at the speed of light. The star would be destroyed if it got very close to the black hole and exceeded its “tidal radius,” which is the distance beyond which the hole’s gravity overwhelms that of the star.
In their proposed model, the star’s orbit partially crosses the tidal radius at its point of closest approach, but not completely. This is because some of the material on the surface of the star is stripped by the black hole, but the material in the center remains intact.
Scientists noted, “The study provides a new way forward for tracking and monitoring follow-up sources discovered in the past. The work also suggests a new paradigm for the origin of repeating flares from the centers of remote galaxies.”
Syracuse physicist Eric Coughlin explains: “More systems are likely to be monitored for late flares in the future, especially now that this project provides a theoretical view of the capture of the star through a dynamic exchange process and the resulting repeated partial tidal disturbance. We hope that this model can be used to infer the properties of distant supermassive black holes and understand their ‘demography’, which is the number of black holes within a certain mass range, which is otherwise difficult to reach directly.
The model also makes several testable predictions about the tidal disturbance process. With more observations from systems like AT2018fyk, it should provide insight into the physics of partial tidal disturbances and the extreme environments around supermassive black holes.
- T. Wevers, ER Coughlin, et al. Live to die another day: the rebrightening of AT 2018fyk as a repeating partial tidal disturbance. The Astrophysical Journal Letters. DOI 10.3847/2041-8213/ac9f36