Scientists have just reported the discovery of a rare black hole. It may shed light on the formation of supermassive black holes.

A new rare black hole breaks the record for being a middle-sized black hole.

There are two well-known kinds of black holes: stellar black holes, typically three to ten times the mass of our Sun, formed when an enormous star explodes and then collapses in on itself, and supermassive black holes which are millions to billions of times heavier and lie at the heart of galaxies.

In 2019 we got the first image of a supermassive black hole at the heart of the Messier 87 galaxy.

Astronomers can’t seem to find anything between the mass of a stellar black hole and a supermassive black hole.

But they have previously theorized that these black holes exist and they range in size from 100 to 100,000 times the mass of the Sun. These Goldilocks black holes could be a missing link between the small black holes that litter the cosmos and the supermassive ones at the center of galaxies.

Up to now, only a handful of intermediate-mass black holes – between 100 and 100,000 solar masses – have been detected. And none have been squarely in the middle of that range.

Astronomers discovered another smaller black hole in September 2020. While studying gravitational waves astronomers found an intermediate-mass black hole about 150 times the mass of the Sun, created by the merger of two smaller black holes.

But the new-found black hole is monstruous in comparison.

Now, in a joint effort, researchers from the University of Melbourne and Monash University have uncovered a black hole about 55,000 times the mass of the sun, an “intermediate-mass” black hole.

A similar candidate weighing in at around 50,000 suns, 3XMM J215022.4−055108, was published in the Astrophysical Journal Letters in March 2020. But that was found when it tore apart a nearby star.

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Lead author and University of Melbourne Ph.D. student, James Paynter, said the latest discovery sheds new light on how supermassive black holes form. “While we know that these supermassive black holes lurk in the cores of most, if not all galaxies, we don’t understand how these behemoths are able to grow so large within the age of the Universe,” he said.

Black holes are huge cosmic objects that compress enormous masses into an extremely small space. They are so unimaginably big, their gravity literally bends light around them.

The researchers detected the new black hole through the detection of a gravitationally lensed gamma-ray burst called GRB 950830. The researchers aren’t sure exactly where the burst came from, but it’s somewhere deep in the dark forest of space. 

Gamma-ray bursts are half-second flashes of high-energy light that are the strongest and brightest explosions in the universe.

Paynter was looking for a “gravitationally lensed” gamma-ray burst from the BATSE instrument at NASA’s Compton Gamma-Ray Observatory, which picked up around 2,700 gamma-ray pings between 1990 and 1999. “We wanted the best chance at finding a lens, so BATSE was the one we chose,” says Paynter. 

GRB 950830 is the only one the team found that was lensed.

After a little more analysis, an even more exciting discovery emerged. The energetic explosion had occurred behind an intermediate-mass black hole. 

When a pair of neutron stars merged, they emitted a gamma-ray burst. The light from the burst traveled across the universe toward us. But when it ran into the new-found black hole, the path of the light was bent. Scientists call this phenomenon gravitational lensing.

The intermediate black hole acted as a lens. Thus, bending the path of the light as it was traveling toward the Earth. Thus, astronomers saw the same flash twice.

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To establish that the two flashes are images of the same object, scientists adapted powerful software developed to detect black holes from gravitational waves.

The new-found black hole could be an ancient relic, a primordial black hole created before the first stars and galaxies formed.

“These early black holes may be the seeds of the supermassive black holes that live in the hearts of galaxies today,” said senior author Eric Thrane, a professor at Monash University.

Scientists were able to measure the mass of their intermediate black hole with precision. But they could only speculate on its formation.

It could have been the result of the merger between two lesser black holes. Or it might have been born as a stellar-class black hole and slowly accumulated mass as it sucked matter into its maw.

But the accumulation is a slow process. That’s because it’s hard to grow supermassive black holes from a solar mass seed over the age of the Universe.

But a more likely scenario is that their discovery “was born that way,” said co-author Rachel Webster.

The authors estimate that there are about 46,000 intermediate black holes in our own galaxy alone.

Scientists published the discovery today, in the journal Nature Astronomy.

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