This illustration depicts a star (in the foreground) surviving spaghettification as it’s sucked in by a supermassive black hole (in the background) during a tidal disruption event. © ESOM Kornmesser.
A distant star appears to be surviving close encounters with a supermassive black hole, suggesting it is only losing part of its mass each time it orbits too closely
Hundreds of millions of light-years away in a distant galaxy, a star orbiting a supermassive black hole is being violently ripped apart under the black hole’s immense gravitational pull.
As the star is shredded, its remnants are transformed into a stream of debris that rains back down onto the black hole to form a very hot, very bright disc of material swirling around the black hole, called an accretion disc.
This phenomenon – where a star is destroyed by a supermassive black hole and fuels a luminous accretion flare – is known as a tidal disruption event, and it is predicted that TDEs occur roughly once every 10,000 to 100,000 years in a given galaxy.
With luminosities exceeding entire galaxies (i.e., billions of times brighter than our sun) for brief periods of time (months to years), accretion events enable astrophysicists to study supermassive black holes from cosmological distances, providing a window into the central regions of otherwise-quiescent – or dormant – galaxies.
By probing these ‘strong-gravity’ events, where Einstein’s general theory of relativity is critical for 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 – of a black hole.
This illustration shows a glowing stream of material from a star as it is being devoured by a supermassive black hole in a tidal disruption flare. When a star passes within a certain distance of a black hole – close enough to be gravitationally disrupted – the stellar material gets stretched and compressed as it falls into the black hole. © NASAJPL-Caltech.
capture of a star
TDEs are usually ‘once-and-done’ because the extreme gravitational field of the supermassive black hole destroys the star, meaning that the black hole fades back into darkness following the accretion flare. In some instances, however, the high-density core of the star can survive the gravitational interaction with the black hole, allowing it to orbit 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 repeating partial TDE.
Their findings describe the capture of the star by a supermassive black hole, 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.
The team’s work is the first to develop and use a detailed model of a repeating partial TDE to explain the observations, make predictions about the orbital properties of a star in a distant galaxy, and understand the partial tidal disruption process.
The team is studying a TDE known as AT2018fyk. The star was captured by a supermassive black hole through an exchange process known as ‘Hills capture’, where the star was originally part of a binary system that was ripped apart by the gravitational field of the black hole. The second star in the binary system was ejected from the centre of the galaxy at speeds comparable to ~ 1,000km/s, which is known as a ‘hypervelocity star’.
Once bound to the supermassive black hole, the star powering the emission from AT2018fyk has been repeatedly stripped of its outer envelope each time it passes through its point of closest approach with the black hole.
The stripped outer layers of the star form the bright accretion disc, which researchers can study using x-ray and ultraviolet/ optical telescopes that observe light from distant galaxies.
Wevers said: “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 completely destroyed.
“But contrary to all other TDEs we know of, when we pointed our telescopes to the same location again 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 once more, explaining the re-brightening phase.”
Pasham said: “When the core returns to the black hole it essentially steals all the gas away from the black hole via gravity and as a result there is no matter to accrete and hence the system goes dark.”
It wasn’t immediately clear what caused the precipitous 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 supermassive black hole the first time and was in orbit around it.
Using detailed modelling, the team’s findings suggest that the orbital period of the star about the black hole is roughly 1,200 days, and it takes approximately 600 days for the material that is shed from the star to return to the black hole and start accreting.
Their model also constrained 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 to one another before being ripped apart by the black hole, likely orbiting each other every few days.
The findings are published in Astrophysical Journal Letters.