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

An article describing these results appears on the arXiv Preprint server and will be published in The Astrophysical Journal. The 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).

The artist’s impression above shows a star partially destroyed by such a black hole in the AT2018fyk system. The supermassive black hole in AT2018fyk – with about 50 million times more mass than the Sun – is located at the center of a galaxy about 860 million light-years from Earth.

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

The figure shows a point in the orbit shortly after the partial destruction of the star, when the tidal tails are still in close proximity to the star. Later in the star’s orbit, the destroyed material returns to the black hole and loses energy, causing a large increase in X-ray brightness that occurs later in the orbit (not shown here).

This process repeats itself every time the star returns to its point of closest approach, which happens about every 3.5 years. The image shows the star during its second orbit and the disk of X-ray gas around the black hole that is formed as a byproduct of the first tidal encounter.

Researchers became aware of AT2018fyk in 2018 when the ASAS-SN ground-based optical survey found that the system had become much brighter. After observing with NASA’s NICER and Chandra, as well as XMM-Newton, researchers determined that the increase in brightness came from a “tidal disruption event” (TDE), which indicates that a star was completely ripped apart and partially swallowed after passing too close to a black hole. Chandra data from AT2018fyk are shown in the inset of a larger field-of-view optical image.

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

However, about two years later, the galaxy’s X-ray and UV light became significantly brighter again. According to astronomers, this meant that the star had likely survived the black hole’s initial gravitational grip and then entered a highly elliptical orbit with the black hole. On its second approach to the black hole, more material was ripped off, producing more X-ray and UV light.

These results were published in 2023 in an article in Astrophysical Journal Letters under the direction of Thomas Wevers from the space telescope Science Institute in Baltimore.

“At first we thought this was a very ordinary case of a black hole tearing a star completely apart,” Wevers said. “But instead the star seems to live and die another day.”

Based on their knowledge of the star and its orbit, Wevers and his team predicted that the black hole’s second meal would end in August 2023 and requested Chandra observing time to verify this.

“The telltale sign that this stellar snack is coming to an end would be a sudden drop in X-rays, and that’s exactly what we see in our Chandra observations 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 the black hole essentially wiped its mouth and moved away from the table in August of last year.”

The new data Chandra and Swift obtained after completing the 2023 study allows the researchers to make an even better estimate of the time it takes the star to complete a full orbit and the black hole’s future meals. They conclude that the star comes closest to the black hole about 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 for nearly two years,” said Eric Coughlin of Syracuse University in New York, a co-author of the new study. “This will likely be more of a snack than a full meal, since the second meal was smaller than the first and the star continues to be eroded.”

The authors believe that the doomed star originally had another star as a companion as 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 the black hole’s orbit, and the other was hurled into space at high speed.

“The doomed star was forced to drastically change its companion stars – from another star to a giant black hole,” said co-author Muryel Guolo of Johns Hopkins University in Baltimore. “Its stellar partner escaped, but it did not.”

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