Physicists observe how a star survives a black hole

Jan. 20 () –

A team of physicists has been able to observe the surprising orbit of a star around a supermassive black hole, surviving in the first instance the interaction with it.

Hundreds of millions of light-years away, in a distant galaxy, a star orbiting a supermassive black hole is being violently torn apart by the black hole’s immense gravitational pull. As the star is torn apart, its remains are transformed into a stream of debris that rains back down on the black hole to form a disk of very hot and very bright material that rotates around the black hole, called an accretion disk.

This phenomenon, in which a star is destroyed by a supermassive black hole and generates a luminous accretionary flare, is known as a tidal disruption event (TDE). and TDEs are predicted to occur about once every 10,000 to 100,000 years in any given galaxy.

With luminosities exceeding those of 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 ( SMBH) from cosmological distances, providing a window into the central regions of galaxies that would otherwise lie dormant.

By probing these events of “heavy gravity”, In which Einstein’s general theory of relativity is essential to determine 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.

Typically, TDEs occur “once and that’s it” because the extreme gravitational field of the SMBH destroys the star, meaning that the SMBH fades into darkness after the accretion flare. In some cases, however, 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 repetitive partial TDE.

A team of physicists, including lead author Thomas Wevers, a fellow at 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 the Kavli Institute for MIT Astrophysics and Space Research, have proposed a repetitive partial TDE model based on observation.

Their conclusions, published in The Astrophysical Journal Letters, they describe the capture of the star by an SMBH, the extraction of the material each time the star approaches the black hole, and the delay between the moment the material is extracted and the moment it feeds the black hole again.

Team work is 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 process of partial tidalism.

The team is studying a TDE known as AT2018fyk (AT stands for “Astrophysical Transient”). The star was captured by an SMBH using an exchange process known as “Hills capture”, in which the star was originally part of a binary system (two stars orbiting around each other under their mutual gravitational attraction) that was torn apart by the black hole’s gravitational field. The other star (not captured) was ejected from the center of the galaxy at speeds comparable to about 1,000 km/s, what is known as a hypervelocity star.

Once attached to the SMBH, the star feeding AT2018fyk’s emission has been repeatedly stripped of its outer envelope each time it passes its point of closest approach to the black hole. The stripped outer layers of the star form the bright accretion disk, which researchers can study using optical X-ray and ultraviolet telescopes. who observe the light of distant galaxies.

According to Wevers, having the opportunity to study a repeated 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 was 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 completely destroyed,” he says. it’s a statement. “But unlike all the other TDEs we know of, when we repointed our telescopes at the same spot several years later, we found that it had 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 spot to be stripped of material once more, explaining the rebrightness phase.”