Tidal disruption event AT2018fyk | BBC Sky at Night magazine

A star with a near-lucky escape after an encounter with a black hole has become the second course of the cosmic behemoth.

What’s more, the team uses their observations to predict when the black hole will feast again.

The study examines a black hole 50 million times the size of the Sun at the center of a galaxy 860 million light-years away.

That means it is so far away that it takes 860 million years for light from this galaxy to reach us.

Learn about Chandra’s observations of the Death Star black hole

Supermassive black holes and tidal disruption

Astronomers now know that most massive galaxies have a supermassive black hole at their center, including our own, and that they are key to a galaxy’s formation and evolution.

Understanding more about what happens in galactic centers and how superfast black holes behave can tell us more about the evolution of the universe.

In 2018, the Optical All Sky Automated Survey for Supernova observed that the black hole system in question was bright.

A team of scientists studied the black hole using NASA’s Chandra X-ray Observatory and Neil Gehrels SWIFT Observatory and the European Space Agency’s XMM-Newton.

They determined that the increase in brightness was the result of a 'tidal disruption event’, where a star was torn apart by drifting too close to the black hole.

This tidal disruption event was cataloged as AT2018fyk.

The glow is caused by the star getting closer and closer to the black hole, and the energy generated during this event causes the star to heat up and shine brightly in X-ray and ultraviolet light.

The signal then faded, suggesting that the star had been swallowed whole.

Back to seconds

Two years after the AT2018fyk event, the X-ray and UV signals brightened again.

Astronomers believe this is most likely due to the star not being fully engulfed in the first place.

The star must have escaped the encounter with the black hole and entered an elliptical (egg-shaped) orbit around it.

Then, during the second close approach, more material was injected by the black hole, increasing in brightness again and again.

The results were published in 2023 in the Astrophysical Journal Letters, led by Thomas Wevers of the Space Telescope Science Institute in Baltimore.

„Initially we thought it was a garden-variety case of a black hole ripping apart a star,” Wevers says.

„But instead, the star lives on until it dies another day.”

„The telltale sign of the end of this galactic snack would be a sudden drop in X-rays, which is what we see in Chandra observations on August 14, 2023,” says Dheeraj Basam, president of the Massachusetts Institute of Technology. A new paper reports on these results.

„Our data shows that in August of last year, the black hole was wiping its mouth and retreating from the table.”

Using data from observations of AT2018fyk, the team predicts that the star will make its closest approach to the black hole once every 3.5 years.

„We think that the third feeding by the black hole, if any of the star, will begin between May and August 2025 and last for almost two years,” said Eric Coughlin, co-author of the new paper. Syracuse University in New York.

„It was probably more of a snack than a full meal because the second meal was smaller than the first meal, and the star was moving away.”

But a twist

A final discovery about this black hole system and its stellar impact is that the star in question may actually have been a binary star.

That means two stars may have orbited each other.

As the binary pair moves too close to the star, the black hole may tear them apart, causing one to enter orbit around the black hole and the other to be ejected into space.

Co-author Muriel Guolow of Johns Hopkins University in Baltimore said, „The dying star forced a drastic change in companions — from another star to a supermassive black hole.”

„Its star partner escaped, but it didn’t.”

Read the full paper arxiv.org/abs/2406.18124