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Rare 'Goldilocks' black hole discovered after extreme explosion in deep space

A gamma-ray burst provides strong evidence for an elusive and highly sought after breed of black hole.

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Beams of light bending around a black hole

An artist's impression of a lensing event. Light (purple) bends around an object in space, like a black hole, splitting into different paths -- one arrives faster than the other.

Carl Knox, OzGrav

Scanning through a catalog of over 2,700 energetic deep space explosions captured by NASA's Compton Gamma-Ray Observatory, James Paynter's task was to find a needle in a needle stack. The doctoral student, from the University of Melbourne, wanted to find one of the explosions, a gamma-ray burst, that had been "lensed" -- its path interrupted by a mammoth cosmic object on its way to the Earth. 

By the end of his master's, he'd whittled down the candidates to just a handful and, after creating some sophisticated analysis techniques, he was left with only one. 

That candidate, known as GRB 950830, is detailed in a new study, published in the journal Nature Astronomy on Monday, and the mammoth cosmic object it had to snake around to make its way to Earth is an "intermediate-mass black hole," a rare breed of the cosmic gravity sinks that astronomers have only just started to understand. 

Goldilocks

In space, there are two well-known kinds of black holes: stellar black holes, formed when an enormous star explodes and then collapses in on itself, and supermassive black holes, which sit at the heart of galaxies. The supermassive black hole at the heart of the Messier 87 galaxy was the subject of the very first black hole image, snapped in 2019. 

Stellar black holes only get to masses around 10 times as massive as the sun. Supermassive black holes can be billions of times more massive. The region between them? "Between these two limits we see nothing," says Paynter. 

Astrophysicists theorize -- and have begun to detect -- black holes in this so-called "mass gap," the "Goldilocks" zone where intermediate-mass black holes, or IMBH, are believed to exist. These black holes range in size from 100 to 100,000 times the mass of the sun and are a missing link between the small black holes that litter the cosmos and the supermassive ones at the center of galaxies.

Very few intermediate-mass black holes have been definitively detected. Another, smaller black hole was discovered in September 2020: Astronomers studying gravitational waves announced they'd found an IMBH with a mass around 150 times that of the sun, created by the merger of two smaller black holes.

But the one discovered by Paynter and co-authors Rachel Webster, an astrophysicist at the University of Melbourne, and Eric Thrane, an astrophysicist at Monash University, is monstrous in comparison.

"Our IMBH is much larger," says Paynter. "If it is formed from stellar black holes, it has been through many, many generations of mergers to reach its observed mass."  

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. The method Paynter and colleagues used takes advantage of gravitational lensing.

Gravity's lens

GRB 950830 is a short gamma-ray burst from a long, long way away. The researchers aren't sure exactly where it came from, but it's somewhere deep in the dark forest of the cosmos. The burst, they suspect, was created when two neutron stars smashed into each other, releasing extreme amounts of energy. 

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.

"My original excitement was having found the first gravitationally lensed GRB, as despite 30 years of searching, no statistically robust candidates have been found," Paynter notes. "We developed a sophisticated analysis software which greatly improves on past lensing detection methods, which allowed us to strongly reject candidates too."

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

The incredible gravitational pull of the IMBH bent all forms of light and electromagnetic radiation around it. When the gamma-ray burst exploded, it traveled across the universe toward us. But when it ran into the IMBH, the signal was split, a phenomenon known as gravitational lensing.

The lensing has a telltale sign, an echo of a signal, because some of the radiation from the gamma-ray burst takes longer to reach the detector on the Observatory as it bends around the IMBH. It's a very subtle difference. "Despite traveling many billions of light-years to reach us, this difference in arrival time is just 400ms for our case," explains Paynter.  

It's not a definite detection of an IMBH, but it's another good candidate. It also helps astronomers estimate how many of these IMBHs might lurk in deep space. Their calculations suggest their could be around 46,000 within our vicinity in the Milky Way, but Paynter says there's a lot of uncertainty in the measurement. Searching for more lensing candidates would help reduce the uncertainty.

"Ideally we will analyse other catalogs in the future too," Paynter says. 

Perhaps, Thrane suggests, the IMBHs may be ancient relics of the early universe from a time before stars and galaxies formed. If that's the case, they could be "the seeds of the supermassive black holes that live in the hearts of galaxies today."

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