Scientists have long searched for proof of the decades-old theory that black holes surround the supermassive black hole known as Sagittarius A* that lies at the heart of our galaxy. By tweaking their approach just a little, astronomers have now turned up the first direct evidence that they do in fact exist, creating new opportunities to study the interplay between regular black holes and their much bigger brethren.

Sometimes when a black hole is sucked into and held in the vicinity of a supermassive black hole, they will latch onto a nearby star to form a stellar binary. This mating generates a big burst of X-ray light, which has provided black hole-hunters with a target to search for.

"It's an obvious way to want to look for black holes," explains Chuck Hailey, an astrophysicist at Columbia University and lead author on the new study. "But the Galactic Center is so far away from Earth that those bursts are only strong and bright enough to see about once every 100 to 1,000 years."

Hailey and his team found success by instead searching for the fainter but more consistent X-ray light beamed from the stellar binary after they've bonded, and are resting in an inactive state. They applied this technique to archival data gathered by NASA's Chandra X-ray Observatory, looking out for X-ray signatures of black hole-low mass binaries. They found 12 within three light years of Sagittarius A* that fit the bill nicely.

"It would be so easy if black hole binaries routinely gave off big bursts like neutron star binaries do, but they don't, so we had to come up with another way to look for them," Hailey said. "Isolated, unmated black holes are just black – they don't do anything. So looking for isolated black holes is not a smart way to find them either. But when black holes mate with a low mass star, the marriage emits X-ray bursts that are weaker, but consistent and detectable. If we could find black holes that are coupled with low mass stars and we know what fraction of black holes will mate with low mass stars, we could scientifically infer the population of isolated black holes out there."

By extrapolating their results in this way, making use of analysis and spatial distribution of already identified binary systems, they team figures there are between 300 and 500 black hole-low mass binaries in the same region, along with around 10,000 isolated black holes.

"Everything you'd ever want to learn about the way big black holes interact with little black holes, you can learn by studying this distribution," says Hailey. "The Milky Way is really the only galaxy we have where we can study how supermassive black holes interact with little ones because we simply can't see their interactions in other galaxies. In a sense, this is the only laboratory we have to study this phenomenon."

The research was published in the journal Nature.

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