A Star’s Unexpected Survival – Zoo House News

A Star’s Unexpected Survival – Zoo House News

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  • January 14, 2023
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Hundreds of millions of light-years away in a distant galaxy, a star orbiting a supermassive black hole is being violently torn apart under the black hole’s immense gravitational pull. As the star is shredded, its remains turn into a debris stream that rains back onto the black hole to form a very hot, very bright disk of material that swirls around the black hole, called an accretion disk. This phenomenon – in which a star is destroyed by a supermassive black hole, triggering a luminous accretion eruption – is known as the Tidal Disruption Event (TDE), and TDEs are predicted to occur approximately every 10,000 to 100,000 years in a given galaxy.

With a luminosity that surpasses entire galaxies (i.e., billions of times brighter than our Sun) for short periods of time (months to years), accretion events allow astrophysicists to study supermassive black holes (SMBHs) from cosmological distances, providing a window into the central regions from otherwise quiet – or dormant – galaxies. By studying these “strong-gravity” events, where Einstein’s general theory of relativity is crucial for 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 “one off” because the SMBH’s extreme gravitational field is destroying the star, meaning the SMBH will revert to darkness after the accretion eruption. In some cases, however, the star’s high-density core can survive gravitational interaction with the SMBH, allowing it to orbit the black hole more than once. Researchers call this a repeating 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, research scientist at MIT’s Kavli Institute for Astrophysics and Space Research have proposed a model for a repetitive partial TDE. Their results, published in Astrophysical Journal Letters, describe the capture of the star by an SMBH, the shedding of material each time the star approaches the black hole, and the delay between the material being stripped and the black hole being fed again. The team’s work is the first to develop and use a detailed model of a repetitive partial TDE to explain the observations, make predictions about the orbital properties of a star in a distant galaxy, and understand the partial tidal disturbance process.

The team is investigating a TDE called AT2018fyk (AT stands for Astrophysical Transient). The star was captured by an SMBH through an exchange process known as “hills capture,” in which the star was originally part of a binary system (two stars orbiting each other under their mutual gravitational pull) that was torn apart by the gravitational field of the black hole. The other (uncaptured) star was ejected from the center of the galaxy at a speed of about 1000 km/s, known as a hypervelocity star.

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

According to Wevers, the ability to study a partial TDE gives unprecedented insight into the existence of supermassive black holes and the orbital dynamics of stars at the centers of galaxies.

“Previously, it was assumed that if we see the aftermath of a close encounter between a star and a supermassive black hole, the outcome will be deadly for the star, that is, the star will be completely destroyed,” he says. “But unlike all other TDEs we know of, when we pointed our telescopes back to the same spot a few years later, we found that it brightened again, survived the initial encounter, and returned to the same spot to see it again.” being rid of material, which explains the re-enlightenment phase.”

First discovered in 2018, AT2018fyk was initially perceived as an ordinary TDE. The source remained bright in the X-ray image for about 600 days, but then suddenly dimmed and became undetectable — a result of the star’s remnant core returning to a black hole, explains MIT physicist Dheeraj R. Pasham.

“When the core returns to the black hole, it essentially steals all the gas out of the black hole by gravity, and as a result there’s no matter to accumulate, and therefore the system goes dark,” says Pasham.

It wasn’t immediately clear what was causing AT2018fyk’s precipitous fall in luminosity, since TDEs typically fall off gently and gradually — not abruptly — in their emission. But around 600 days after the collapse, it turned out that the source was again X-ray bright. This led researchers to believe that the star survived its first close encounter with the SMBH and was in orbit around the black hole.

Using detailed modelling, 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 material expelled from the star to return to the black hole and with the accretion begins. Their model also limited 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 together before being torn apart by the black hole that likely orbits each other every few days.

So how could a star survive its death? It all boils 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 needed to escape the black hole exceeds the speed of light — the star would be engulfed by the black hole. If the star flies very close to the black hole and traverses the so-called “tidal radius” — where the hole’s tidal force is stronger than the gravitational force holding the star together — it would be destroyed. In the model they propose, the star’s orbit reaches a point of closest approach, just outside the tidal radius but not completely crossing it: some of the material on the star’s surface is stripped off by the black hole, but the material at its center remains intact.

How, or if, the process of the star orbiting the SMBH can occur over many repeated passages is a theoretical question the team hopes to explore 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 through the black hole, with the long range being due to the uncertainty in modeling the TDE’s emission .

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

The team will be looking to the sky for years to come to test their predictions. Based on their model, they predict the source will abruptly disappear around March 2023, brightening again as the freshly sloughed material deposits on the black hole in 2025.

The team say their study offers a new avenue for tracking and monitoring follow-on sources discovered in the past. The work also proposes a new paradigm for the formation of repetitive flares from the centers of external galaxies.

“More systems are likely to be checked for late-period flares in the future, especially now that this project provides a theoretical picture of star capture by a dynamic exchange process and the resulting repeated partial tidal disturbance,” says Coughlin. “We hope that this model can be used to infer the properties of distant supermassive black holes and gain an understanding of their ‘demographics’, that is, the number of black holes within a given mass range, which is otherwise difficult to achieve directly.”

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

“This study outlines methods to potentially predict the next snack times of supermassive black holes in external galaxies,” says Pasham. “If you think about it, it’s quite remarkable that on Earth we can point our telescopes at black holes millions of light years away to understand how they feed and grow.”

Additional co-authors are: M. Guolo, Department of Physics and Astronomy, Johns Hopkins University; Y. Sun, University of Arizona; S. Wen, Institute of Astrophysics/IMAPP, Radboud University; PG Jonker, Institute for Astrophysics/IMAPP, Radboud University and SRON, 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 Institute for extraterrestrial Physics, Germany; P. Short, Institute of Astronomy, University of Edinburgh; and Z. Cao, Institute of Astrophysics/IMAPP, Radboud University


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