New insights into one of the cosmos’ most extreme environments

A star’s unexpected survival.

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Stars that interact with supermassive black holes (SMBHs) can be either entirely or partially destroyed by tides. While some of the tidal disruption events (TDEs) identified so far have displayed such long-term behavior, a significant fraction shows different light curve evolution, which in some cases is completely decoupled from the mass fallback rate.

In a new study, a team of physicists at Syracuse University describes 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 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 about a supermassive black hole – revealing new information about one of the cosmos’ most extreme environments.

Through this model, scientists could explain TDE observations and make predictions about the orbital properties of a star in a distant galaxy. It also helps them understand the partial tidal disruption process.

In particular, physicists studied a TDE known as AT2018fyk. The TDE AT 2018fyk showed an anomalous rebrightening 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 behavior that the initial flare was caused by the partial disruption of a star that was part of a binary system. The partially disrupted star was captured by a SMBH through an exchange process known as “Hills capture,” where the star was originally part of a binary system (two stars that orbit one another under their mutual gravitational attraction) that was ripped apart by the gravitational field of the black hole. The other (non-captured) star was ejected from the galaxy’s center at speeds comparable to ~ 1000 km/s, known as a hypervelocity star.

The star driving the emission from AT2018fyk was formerly connected to the SMBH, but every time it passes through the point of closest approach with the black hole, it is repeatedly stripped of its outer envelope. Researchers may investigate the brilliant accretion disc, which is made up of the stripped outer layers of the star, using X-ray, ultraviolet, and optical telescopes that look at light from far-off galaxies.

Lead author Thomas Wevers, Fellow of the European Southern Observatory, said, “Having the opportunity to study a repeating partial TDE gives 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 for the star, that is, the star is destroyed. But contrary to all other TDEs we know of, when we pointed our telescopes to the same location 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 again, explaining the rebrightening phase.”

MIT physicist Dheeraj R. Pasham. said, “It wasn’t immediately clear what caused the steep decline in the luminosity of AT2018fyk, because TDEs normally decay smoothly and gradually – not abruptly – in their emission. But around 600 days after the drop, the source was again found to be X-ray bright. This led the researchers to propose that the star survived its close encounter with the SMBH the first time and was in orbit about 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 around 1,200 days and that it takes nearly 600 days for the material shed by the star to return to the black hole and begin accreting. The size of the caught star, which according to their model, was around the size of the sun, was similarly restricted. In terms of the original binary, the team thinks the two stars were orbiting one another probably every few days before they were torn apart by the black hole.

So, how could a star survive its brush with death?

It is because of proximity and trajectory. The star would be sucked into the black hole if it collided with it head-on and reached the event horizon, the point beyond which it would be impossible to escape at the speed of light. The star would be annihilated if it came very close to the black hole and crossed its “tidal radius,” which is the distance beyond which the hole’s gravitational pull overwhelms the star’s own. 

In their proposed model, the star’s orbit crosses the tidal radius partially at the point of closest approach but not entirely. This is because some of the material at the stellar surface is stripped by the black hole, but the material at its center remains intact.

Scientists noted, “The study offers a new way forward for tracking and monitoring follow-up sources that have been detected in the past. The work also suggests a new paradigm for the origin of repeating flares from the centers of external galaxies.”

Syracuse physicist Eric Coughlin explains“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. We’re hopeful this model can be used to infer the properties of distant supermassive black holes and understand their ‘demographics,’ being the number of black holes within a given mass range, which is otherwise difficult to achieve directly.”

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

Journal Reference:

  1. T. Wevers, E. R. Coughlin, et al. Live to Die Another Day: The Rebrightening of AT 2018fyk as a Repeating Partial Tidal Disruption Event. The Astrophysical Journal Letters. DOI 10.3847/2041-8213/ac9f36