NASA Telescopes Determine Black Hole Feeding Schedule

Using new data from NASA’s Chandra X-ray Observatory and ESA’s Neil Gehrels Swift Observatory and XMM-Newton, a team of researchers has made important advances in understanding how – and when – a supermassive black hole obtains and then consumes matter.

An article describing these results appears on the arXiv pre-print server and will be published in The Journal of AstrophysicsThe authors are Dheeraj Passam (Massachusetts Institute of Technology), Eric Coughlin (Syracuse University), Muryel Guolo (Johns Hopkins University), Thomas Wevers (Space Telescope Science Institute), Chris Nixon (University of Leeds, UK), Jason Hinkle (University of Hawaii at Manoa), and Ananaya Bandopadhyay (Syracuse).

This artist’s impression above shows a star partially disrupted by such a black hole in the system known as AT2018fyk. The supermassive black hole in AT2018fyk, which has a mass about 50 million times that of the Sun, lies at the center of a galaxy about 860 million light-years from Earth.

Astronomers have determined that a star is in a highly elliptical orbit around AT2018fyk’s black hole, such that its farthest point from the black hole is much larger than its closest point. During its closest approach, the black hole’s tidal forces rip away some of the star’s material, producing two tidal tails of “stellar debris.”

The illustration shows a point in the orbit shortly after the star’s partial destruction, when the tidal tails are still close to the star. Later in the star’s orbit, the disrupted material returns to the black hole and loses energy, leading to a large increase in X-ray brightness occurring later in the orbit (not shown here).

This process repeats itself every time the star returns to its closest approach, about every 3.5 years. The illustration shows the star during its second orbit and the X-ray-emitting disk of gas around the black hole, produced by the first encounter with the star.

Researchers first noticed AT2018fyk in 2018 when the ground-based ASAS-SN optical survey detected that the system had become much brighter. After observing it with NASA’s NICER and Chandra, as well as XMM-Newton, researchers determined that the increase in brightness came from a “tidal disruption event,” or TDE, which signals that a star has been completely torn apart and partially ingested after flying too close to a black hole. Chandra data from AT2018fyk are shown in the inset of an optical image of a wider field of view.

As the material from the destroyed star approached the black hole, it became hotter and produced X-rays and ultraviolet (UV) light. These signals then faded, consistent with the idea that there was nothing left of the star for the black hole to digest.

However, about two years later, the galaxy’s X-rays and UV rays became much brighter again. Astronomers believe this means that the star likely survived the black hole’s initial gravitational pull and then entered a highly elliptical orbit with the black hole. During its second close encounter with the black hole, more material was torn away, producing more X-rays and UV rays.

These results were published in a 2023 article in the Astrophysical Journal Letters led by Thomas Wevers from the space telescope Science Institute in Baltimore.

“We initially thought this was a run-of-the-mill case of a black hole destroying a star,” Wevers said. “But instead, the star seems to live to die another day.”

Based on what they had learned about the star and its orbit, Wevers and his team predicted that the black hole’s second meal would end in August 2023, and requested observing time from Chandra to verify.

“The telltale sign that this stellar collation is over would be a sudden drop in X-rays, and that’s exactly what we see in our observations of Chandra on August 14, 2023,” said Dheeraj Pasham of the Massachusetts Institute of Technology, who led the new paper on these results. “Our data show that in August of last year, the black hole was essentially wiping its mouth and walking away from the table.”

New data obtained by Chandra and Swift after the 2023 paper was published give the researchers an even more precise estimate of how long it will take the star to complete a full orbit and the black hole’s future meals. They determine that the star makes its closest approach to the black hole about once every three and a half years.

“We think a third meal of the black hole, if there is anything left of the star, will begin between May and August 2025 and last about two years,” said Eric Coughlin, a co-author of the new study, of Syracuse University in New York. “It will probably be more of a snack than a full meal, because the second meal was smaller than the first, and the star is shrinking.”

The authors believe that the doomed star originally had another star as a companion when it approached the black hole. However, when the pair of stars got too close to the black hole, the black hole’s gravity pulled the two stars apart. One entered into orbit with the black hole, and the other was thrown into space at high speed.

“The doomed star was forced to radically change companions, going from another star to a giant black hole,” said study co-author Muryel Guolo, a fellow at Johns Hopkins University in Baltimore. “Its stellar partner escaped, but it did not.”

The team plans to continue monitoring AT2018fyk for as long as possible to study the behavior of such an exotic system.