Astronomers have detected an elusive, little-described kind of black hole that isn't supermassive, nor is it of the smaller stellar variety. It's more like a medium, or intermediate black hole, and its discovery could help answer questions about the origins and growth of larger ones.

Intermediate black holes, between a mass of 100 and 100,000 suns, have been thought to exist, and were described for the first time in 2020. But now, using methods they believe are more conclusive than prior studies, researchers from the University of Melbourne and Monash University in Australia have uncovered what could be a black hole with a mass of less than or equal to 55,000 suns. 

The researchers detailed their findings in a new study published March 29 in Nature Astronomy. 

"We don't see any black holes with this mass," said lead author James Paynter, a Ph.D. student at the University of Melbourne. "There's just no information, no observations. And so despite the fact that it doesn't really make sense for these things not to exist, we just haven't seen them before."

Paynter and his colleagues identified such an intermediate black hole by analyzing about 2,500 gamma ray bursts. 

When a massive star collapses at the end of its life or collides into another one, it creates a black hole that releases a burst of gamma rays. These gamma rays travel through the universe, occasionally encountering celestial bodies such as black holes. As the rays travel around these bodies, which can act as gravitational lenses, they may become distorted and "gravitationally lensed." Because the line of sight between a gamma ray burst, a gravitational lens and gamma ray detectors on Earth is unlikely to be completely straight, the gamma rays create a double image as they pass a black hole.

When this happens, the gravitational pull of a black hole pulls one of these images closer than the other, creating a discernible time dilation: The image pulled closer to the black hole arrives at its destination sooner than its counterpart. These time differences can help scientists determine how massive these lenses are.

Objects with greater mass are more powerful lenses, so they create larger time differentials between signals. In his analysis of these bursts, Paynter saw a 400-millisecond time difference in a gamma ray signal, suggesting the presence of an intermediate black hole. A supermassive black hole of 1 million solar masses, meanwhile, would create a signal delay of 50 seconds. 

Intermediate black holes may be able to help scientists account for the existence of supermassive black holes, which can be millions to billions of times greater than the mass of the sun. According to Paynter, current astronomical models simply cannot account for their size based on typical black hole formation alone. Usually, a stellar black hole forms with the gravitational collapse of a star, and grows to about five to tens of times larger than the sun, and they grow larger by sucking up nearby matter. But theoretically, not enough time has passed for these black holes to grow to a supermassive size. 

"With supermassive black holes currently, we don't have a good grasp of where they're coming from, because it's not possible for them to grow to that size if they start off so small," Paynter said in an interview with The Academic Times. "So you need a seed which is quite large for them to create and grow, because they need to have enough gravitational pull to, to pull more matter in, like a snowball effect."

These "seeds" could be intermediate black holes. While the source of intermediate black holes also remains uncertain, they may have formed from collapsing clusters of enormous hydrogen-based stars in the early universe, according to Paynter. They could also be primordial, having formed extremely early. If they are, then they may be rather static, as the LIGO gravitational wave observatory recently proved that primordial black holes lack spin, a common feature of most black holes. 

Having already seemed to have found one of these rare black holes, Paynter is setting out to find more, which could help provide a sense of how many of these intermediate mass black holes there could be. He is going to start by going over his dataset again, looking for more examples of gravitational lensing. 

"We don't know where they are; we don't know where they're hiding," Paynter said. "But by finding more gravitational lensing events, which would identify more intermediate mass black holes, then we'd get a better idea of what the true number density of these things are, because the uncertainty is quite large."

The study, "Evidence for an intermediate-mass black hole from a gravitationally lensed gamma-ray burst," published March 29 in Nature Astronomy, was authored by James Paynter and Rachel Webster, University of Melbourne; and Eric Thrane, Monash University and OzGrav.