TUSCALOOSA, Ala. — An astronomer at The University of Alabama is part of an international team of researchers who found a mid-sized black hole, a cosmic oddity in observations of the universe.
The finding, which demonstrate an effective method to detect this class of black holes, was announced today in the journal Nature Astronomy.
Two types of black holes are well-known. Massive stars create stellar-mass black holes when they die, while galaxies host supermassive black holes at their centers, with masses equivalent to millions or billions of Suns.
Between these extremes are intermediate-mass black holes. They could grow into supermassive black holes as they take in more mass from stars and cosmic particles that pass too close, but few robust candidates have been found.
“It’s a class of black of hole that we don’t have a lot of data evidence they exist,” said Dr. Jimmy Irwin, UA associate professor of astronomy and physics. “If we understand how intermediate-mass black holes form, we can understand how large black holes form.”
Irwin, who has a background in these types of black holes, was an adviser for a team of researchers using data from the European Space Agency’s XMM-Newton X-ray space observatory, as well as NASA’s Chandra X-Ray Observatory and Swift X-Ray Telescope, to find a rare telltale sign of activity. They detected an enormous flare of radiation in the outskirts of a distant galaxy, thrown off as a star passed too close to a black hole and was subsequently devoured.
“This is incredibly exciting: this type of black hole hasn’t been spotted so clearly before,” says lead scientist Dr. Dacheng Lin of the University of New Hampshire and a former post-doctoral researcher under Irwin at UA. “A few candidates have been found, but on the whole they’re extremely rare and very sought after. This is the best intermediate black hole candidate observed so far.”
One way intermediate-mass black holes are thought to form is the merger of massive stars lying within dense star clusters, making the centers of these clusters one of the best places to hunt for them. However, by the time such black holes have formed, these sites tend to be devoid of gas, leaving the black holes with no material to consume and thus little radiation to emit, making them difficult to spot.
To find a good candidate, Lin and colleagues sifted through data taken by XMM-Newton, Chandra and Swift along with other telescopes. They identified it in observations of a large galaxy some 740 million light-years away.
The data showed the star likely began falling into the black hole in October 2003 in observations from Earth, producing a burst of recordable and visible energy that tapered off over the next decade, Irwin said.
The scientists believe the star was disrupted and torn apart by a black hole with a mass of around 50,000 times that of the Sun.
Such star-triggered outbursts are rare for this type of black hole, so the discovery suggests many more might lurk in a dormant state in galaxy peripheries across the local universe.
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