April 1, 2008 -- The discovery of a black hole just 15 miles across and 3.8 times the mass of our sun was announced by astronomers today at a meeting of the American Astronomical Society's High Energy Astrophysics Division.
The newfound mini black hole has pushed the lower limit of the incredibly dense objects, which are the product of large stars running out of fuel and then collapsing.
"This (lower) limit is not theoretically well-known," said black hole researcher Nikolai Shaposhnikov of NASA's Goddard Space Flight Center.
Black holes have been thought to be no smaller than four to five times the mass of the sun, but until now no one has been able to find any evidence to back up the prediction.
The black hole, called XTE J1650-500, is in the southern constellation Ara and was detected using NASA's Rossi X-ray Timing Explorer (RXTE) satellite. The intense X-ray emissions of the two-star system gave away the black hole. The X-rays mean the black hole is feeding on material from its companion star.
When the gases crowd around the black hole, they are squeezed so violently on their way in that they heat up to tens of millions of degrees, causing them to emit loads of X-rays, explained Shaposhnikov.
The trick to determining the size of J1650 to high enough precision came from a new method developed by Shaposhnikov and his colleague Lev Titarchuk.
The method takes advantage of a relationship between black holes and the congestion of doomed stolen stellar gases.
Simply put, the smaller the black hole, the more regularly it gets backed up with hot, X-ray-emitting gases. So by measuring the "quasi-periodic oscillations" (QPOs) of black hole X-rays, they can work out the size of the disk, and therefore the size of the black hole.
The researchers used archived X-ray data of QPOs to test their method on 15 known black holes. The method worked like a charm, and gave their current mini-hole measurement a lot of weight.
"I was especially intrigued when I heard their result," said black hole researcher Vicky Kalogera from Northwestern University.
Past attempts to find the smallest black holes had large error bars, which made them hard to trust. The QPO method appears to be far more precise, she said.
Kalogera hopes the new method will turn up more small black holes and solve another puzzle: why theory predicts many small black holes while observations have turned up very few.
"I would be very interested to see what else we see in coming years and how the sample size changes as a result of this method," said Kalogera.
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