X (rays) mark the spot where a black hole ate a star

An artist’s impression of a star being torn apart by a black hole in a tidal disruption event. Image: NASA/JPL-Caltech

When a star is ripped apart in the gravitational maw of a black hole – a tidal disruption event – a torrent of high-energy radiation is released that can outshine the combined light of every star in the host galaxy for months if not years.

Re-examining X-ray emissions from one such event, known as J2150, and comparing the data with theoretical models, a team led by Sixiang Wen, a postdoctoral research associate at the University of Arizona, concluded the radiation was, in fact, generated in a TDE. They found the unlucky star in question was devoured by an intermediate-mass black hold tipping the scales at roughly 10,000 times the mass of the Sun.

Intermediate-mass black holes, bridging the gap between stellar-mass supernova remnants and the supermassive black holes lurking in the hearts of most, if not all, large galaxies have long eluded observation.

“The fact that we were able to catch this black hole while it was devouring a star offers a remarkable opportunity to observe what otherwise would be invisible,” said Ann Zabludoff, professor of astronomy at the University of Arizona and co-author of a paper in The Astrophysical Journal.

“Not only that, by analysing the flare we were able to better understand this elusive category of black holes, which may well account for the majority of black holes in the centres of galaxies.”

Dozens of tidal disruption events have been observed in galaxies hosting supermassive black holes, and a handful have been spotted in smaller galaxies that might host intermediate-mass black holes. But J2150 is the first confirmation of an individual TDE generated by a middle-weight black hole.

“We know that the centres of almost all galaxies that are similar to or larger in size than our Milky Way host central supermassive black holes,” said co-author Nicholas Stone, a senior lecturer at Hebrew University in Jerusalem. “These behemoths range in size from 1 million to 10 billion times the mass of our sun.”

The mass of a supermassive black hole correlates closely with the total mass of the host galaxy – the more massive the black hole, the larger the galaxy. Intermediate-mass black holes could be the seeds of future supermassive holes.

“We still know very little about the existence of black holes in the centres of galaxies smaller than the Milky Way,” said co-author Peter Jonker of Radboud University in the Netherlands. “If we get a better handle of how many bona fide intermediate black holes are out there, it can help determine which theories of supermassive black hole formation are correct.”

The team also was able to determine the spin of the J2150 black hole, finding it to be fast but not as fast as theoretically possible. The spin measurement holds clues about how black holes grow, but it’s not yet known how J2150’s hole ended up with the observed value.

“It’s possible that the black hole formed that way and hasn’t changed much since, or that two intermediate-mass black holes merged recently to form this one,” Zabludoff said. “We do know that the spin we measured excludes scenarios where the black hole grows over a long time from steadily eating gas or from many quick gas snacks that arrive from random directions.”

The Vera C. Rubin Observatory and other new instruments on Earth and in space are expected to discover thousands of tidal disruption events per year.

“If it turns out that most dwarf galaxies contain intermediate-mass black holes, then they will dominate the rate of stellar tidal disruption,” Stone said. “By fitting the X-ray emission from these flares to theoretical models, we can conduct a census of the intermediate-mass black hole population in the universe.”