Weighing a Black Hole

Scientists have weighed a black hole by observing strong X-ray outbursts from it. The timing and regularity of the bursts – seen by NASA’s Chandra X-ray Observatory – imply an object 10,000 times more massive than our Sun. This might sound like a bundle, but in the boxing ring of black holes, it qualifies only as middleweight.

Astronomers have previously observed stellar-mass black holes – with about 10 solar masses – and supermassive black holes – with a million or more solar masses. The recent measurements of a black hole in M74, a galaxy 32 million light years away, are the best evidence so far for an intermediate-mass black hole.

"It is important to verify the existence of intermediate-mass black holes, because they would bridge the gap between stellar-mass black holes and supermassive black holes in the centers of galaxies," said Jifeng Liu of the University of Michigan in Ann Arbor.

Liu and his colleagues found that the M74 source varied in its X-ray brightness every two hours, providing an important clue to the black holes' mass.

Some scientists had speculated that ultraluminous X-ray sources (ULXs), of which the M74 object is one, are stellar-mass black holes that look brighter because they are beaming X-rays directly at Earth. However, the variation pattern observed by Liu’s team seems to require a bigger black hole.

If the object is indeed one of the elusive intermediate-mass black holes, the next question is how did it form. One of the leading theories is that hundreds of stellar-mass black holes (which form out of the deaths of massive stars) merge together at the center of a dense star cluster.

Another possibility is that the intermediate object was the central black hole of a small galaxy that is being eaten by the larger M74 galaxy.

The results appeared in the March 1 issue of the Astrophysical Journal Letters.