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 and NASA's Goddard Space Flight Center has 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 team reported its findings in the September 21, 2015 online edition of Astrophysical Journal Letters. 

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. 

"This result provides support to the idea that black holes exist on all size scales. When you describe something for the first time, there is always some doubt," said 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. "Identifying a second candidate with a different instrument puts weight behind both findings and gives us confidence in our technique."

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.