Rare ‘Goldilocks’ black hole discovered after extreme deep space explosion

Going through a catalog of over 2,700 deep-space energy explosions captured by NASA’s Compton Gamma-Ray Observatory, James Paynter’s task was to find a needle in a pile of needles. The doctoral student, from the University of Melbourne, wanted to find one of the explosions, a gamma-ray burst, which had been “lens” – its path interrupted by a gigantic cosmic object on its way to Earth.

By the end of his master’s degree, he had narrowed the candidates down to a handful, and after creating sophisticated analytical techniques, he was left with just one.

This candidate, known as GRB 950830, is detailed in a new study, published Monday in the journal Nature Astronomy, and the gigantic cosmic object it had to meander to reach Earth is a “mass black hole. intermediate”. a rare race of cosmic gravity wells that astronomers are only now beginning to understand.

Goldilocks

In space, there are two well-known types of black holes: stellar black holes, formed when a huge star explodes and then collapses on itself, and supermassive black holes, which are found in the heart of galaxies. the supermassive black hole in the heart of the galaxy Messier 87 was the subject of the very first image of the black hole, broken in 2019.

Stellar black holes only reach masses about 10 times more massive than the sun. Supermassive black holes can be billions of times more massive. The region between them? “Between those two boundaries, we don’t see anything,” says Paynter.

Astrophysicists theorize – and have started to detect – black holes in this so-called “mass gap,” the “Goldilocks” area where intermediate-mass black holes, or IMBHs, are believed to exist. These black holes are 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 holes in the center of galaxies.

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

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

“Our IMBH is much bigger,” says Paynter. “If it is formed by stellar black holes, it has undergone many, many generations of fusions to reach its observed mass.”

A similar candidate weighing around 50,000 suns, 3XMM J215022.4−055108, was published in the Astrophysical Journal Letters in March 2020, but was discovered when he tore up a nearby star. The method used by Paynter and his colleagues takes advantage of the gravitational lens.

Gravity lens

GRB 950830 is a short gamma-ray burst from a very long distance. Researchers aren’t sure where it came from, but it is somewhere deep in the dark forest of the cosmos. The explosion, they suspect, was created when two neutron stars crashed, releasing extreme amounts of energy.

Paynter was looking for a “gravitational lens” gamma-ray burst from the BATSE instrument at NASA’s Compton Gamma-Ray Observatory, which picked up around 2,700 gamma pings between 1990 and 1999. “We wanted the best chance of finding one. goal, So BATSE is what we have chosen, ”says Paynter.

The GRB 950830 is the only one the team found to have a lens.

“My original enthusiasm was to have found the first GRB with gravitational lenses, because despite 30 years of research, no statistically robust candidate has been found,” notes Paynter. “We have developed sophisticated analysis software that dramatically improves older lens detection methods, which has allowed us to strongly reject applicants as well.”

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

IMBH’s incredible gravitational pull bent all forms of light and electromagnetic radiation around it. When the gamma-ray burst exploded, it crossed the universe towards us. But when it hit the IMBH, the signal split, a phenomenon known as the gravitational lens.

The lens has a telltale sign, an echo of a signal, because some of the gamma-ray burst radiation takes longer to reach the Observatory’s detector as it curves around the IMBH. It’s a very subtle difference. “Although we have traveled billions of light years to reach us, this difference in arrival time is only 400 ms in our case,” says Paynter.

This is not a definitive detection of an IMBH, but it is another good candidate. It also helps astronomers estimate how many of these IMBHs might be lurking in deep space. Their calculations suggest it could be around 46,000 in our neighborhood in the Milky Way, but Paynter says there’s a lot of uncertainty in the measurement. Finding more targeted candidates would help reduce uncertainty.

“Ideally, we will also analyze other catalogs in the future,” says Paynter.

Perhaps, Thrane suggests, IMBHs may be ancient relics of the early universe from an era before the formation of stars and galaxies. If so, they could be “the seeds of the supermassive black holes that now live in the heart of galaxies”.

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