New class of medium-sized black holes identified

Astronomers led by an Indian-origin scientist have found evidence for a new intermediate-mass black hole about 5,000 times the mass of the Sun, strengthening the case for the existence of a third major class of black holes.

Nearly all black holes come in one of two sizes: stellar mass black holes that weigh up to a few dozen times the mass of our Sun or supermassive black holes ranging from a million to several billion times the Sun's mass.

Astronomers believe that medium-sized black holes between these two extremes exist, but evidence has been hard to come by, with roughly a half-dozen candidates described so far.

A team led by astronomers at the University of Maryland (UMD) and NASA's Goddard Space Flight Center found evidence for a new intermediate-mass black hole about 5,000 times the mass of the Sun.

The discovery adds one more candidate to the list of potential medium-sized black holes, while strengthening the case that these objects do exist.

The result follows up on a similar finding by some of the same scientists, using the same technique, published in August 2014.

While the previous study accurately measured a black hole weighing 400 times the mass of the Sun using data from NASA's Rossi X-ray Timing Explorer (RXTE) satellite, the current study used data from the European Space Agency's XMM-Newton satellite.

The new intermediate-mass black hole candidate, known as NGC1313X-1, is classified as an ultraluminous X-ray source, and as such is among the brightest X-ray sources in the nearby universe. It has proven hard to explain exactly why ultraluminous X-ray sources are so bright, however.

Some astronomers suspect that they are intermediate-mass black holes actively drawing in matter, producing massive amounts of friction and X-ray radiation in the process.

Against this backdrop of haphazard X-ray fireworks created by NGC1313X-1, researchers identified two repeating flares, each flashing at an unusually steady frequency.

One flashed about 27.6 times per minute and the other about 17.4 times per minute. Comparing these two rates yields a nearly perfect 3:2 ratio.

Lead author Dheeraj Pasham, a postdoctoral associate at the Joint Space-Science Institute, a research partnership between UMD's Departments of Astronomy and Physics and NASA Goddard, and his colleagues also found this 3:2 ratio in M82X-1, the black hole they identified in August 2014, although the overall frequency of flashing was much higher in M82X-1.

Although astronomers are not yet sure what causes these steady flashes, the presence of a clockwork 3:2 ratio appears to be a common feature of stellar mass black holes and possibly intermediate-mass black holes as well.

The flashes are most likely caused by activity close to the black hole, where extreme gravity keeps all surrounding matter on a very tight leash, Pasham said.
The study is published in the Astrophysical Journal Letters.