A star that almost had a lucky escape after an encounter with a black hole has become the cosmic behemoth’s second course.
What’s more, the team are using their observations to predict when the black hole will feast again.
The study concerns a supermassive black hole 50 million times more massive than the Sun at the centre of a galaxy 860 million lightyears away.
That means it’s so far away, it takes light from this galaxy 860 million years to reach us.
Find out about Chandra’s observations of the Death Star black hole
Supermassive black holes and tidal disruption
Astronomers now know most major galaxies have a supermassive black hole at their centre, including our own, and that they are key to a galaxy’s formation and evolution.
Understanding more about what goes on at galactic cores and how supermassive black holes behave can tell us more about the evolution of the Universe.
In 2018, the optical All Sky Automated Survey for SuperNovae noticed the black hole system in question had become brighter.
A team of scientists used NASA’s Chandra X-ray Observatory and Neil Gehrels Swift Observatory as well as the European Space Agency’s XMM-Newton to observe the black hole.
They determined the increase in brightness was a result of a ‘tidal disruption event’, meaning a star was torn apart by wandering too close to the black hole.
This tidal disruption event was catalogued AT2018fyk.
The brightness surge was caused by the star getting closer and closer to the black hole, the energy generated during this event causing the star to get hotter and glow brighter in x-ray and ultraviolet light.
That signal then faded, suggesting the star had been completely devoured.
Back for seconds
Two years after the AT2018fyk event, the x-ray and ultraviolet signals got brighter again.
The astronomers believe this is most likely due to the star having not been completely devoured the first time around.
The star must have survived its encounter with the black hole and entered into an elliptical (egg-shaped) orbit around it.
Then, during a second close approach, more material was ingested by the black hole, causing the repeated brightness surge.
These results were published in a 2023 paper in the Astrophysical Journal Letters led by Thomas Wevers from the Space Telescope Science Institute in Baltimore.
“Initially we thought this was a garden-variety case of a black hole totally ripping a star apart,” says Wevers.
“But instead, the star appears to be living to die another day.”
“The telltale sign of this stellar snack ending would be a sudden drop in the X-rays and that’s exactly what we see in our Chandra observations on Aug. 14, 2023,” says Dheeraj Pasham of the Massachusetts Institute of Technology, the leader of a new paper on these results.
“Our data shows that in August last year, the black hole was essentially wiping its mouth and pushing back from the table.”
Using data from the observations of AT2018fyk, the team predict the star makes its closest approach to the black hole about once every 3.5 years.
“We think that a third meal by the black hole, if anything is left of the star, will begin between May and August of 2025 and last for almost two years,” said Eric Coughlin, a co-author of the new paper, from Syracuse University in New York.
“This will probably be more of a snack than a full meal because the second meal was smaller than the first, and the star is being whittled away.”
Yet there’s a twist
One final discovery about this black hole system and its stellar victim, is that it seems the star in question may actually have been a binary star.
This means it could have been two stars in orbit around each other.
When the binary pair ventured too close to the star, the black hole may have pulled them apart, causing one to enter orbit around the black hole and the other to be ejected into space.
“The doomed star was forced to make a drastic change in companions — from another star to a giant black hole,” says co-author Muryel Guolo of Johns Hopkins University in Baltimore.
“Its stellar partner escaped, but it did not.”
Read the full paper at arxiv.org/abs/2406.18124