THE black hole family has a middle child, if an otherwise unexplained source of fluctuating X-rays is anything to go by. Small black holes the size of stars and the supermassive variety are familiar, but until now there have only been tentative signs of intermediate-mass black holes . For example, in 2008 a source of X-rays seemed to be a middleweight black hole, but only one measurement was made and the finding was not confirmed. Now Sean Farrell’s team at the CESR astrophysics lab in Toulouse, France, have stumbled across a source of variable X-rays they think is a black hole of around 500 solar masses. They were cataloguing various sources of X-rays from 2000 onwards, when they found an object 400 times as bright as the maximum value for a stellar black hole. And the X-rays did not come from the centre of a galaxy, the usual home of a supermassive black hole. Other celestial phenomena were also ruled out. A jet from a small black hole could have made that object seem bigger if directly focused at Earth, but the characteristics of the X-ray spectrum did not suggest this, says Farrell. A blazar, a galaxy that kicks out fluctuating X-rays, might also have been responsible, but blazars are easily identified by their strong radio emissions, something not apparent in this source (Nature, DOI: 10.1038/ nature08083) . The existence of middleweights could explain how black holes grow from small to supermassive. “We are progressively filling the gap,” says Farrell.
Source of Information : New Scientist July 4 2009