July 1, 2009
A new class of black hole, more than 500 times the mass of the Sun lies approximately 290 million light-years from Earth, an international team of astronomers recently discovered.
Until now, identified black holes have been either supermassive (several million to several billion times the mass of the Sun) in the center of galaxies, or about the size of a typical star (between three and 20 solar masses).
The new discovery is the first solid evidence of a new class of medium-sized black holes. The team, led by astrophysicists at the Centre d'Etude Spatiale des Rayonnements in France, detected the new black hole with the European Space Agency's (ESA) XMM-Newton X-ray space telescope in Noordwijk, The Netherlands.
"While it is widely accepted that stellar mass black holes are created during the death throes of massive stars, it is still unknown how supermassive black holes are formed," said Sean Farrell at the Department of Physics and Astronomy at the University of Leicester.
"One theory is that supermassive black holes may be formed by the merger of a number of intermediate mass black holes," Sean said. "To ratify such a theory, however, you must first prove the existence of intermediate black holes."
"This is the best detection to date of such long sought-after intermediate mass black holes," he said. "Such detection is essential. While it is already known that stellar mass black holes are the remnants of massive stars, the formation mechanisms of supermassive black holes are still unknown."
"The identification of HLX-1 is therefore an important step towards a better understanding of the formation of the supermassive black holes that exist at the center of the Milky Way and other galaxies," Sean said.
A black hole is a remnant of a collapsed star with such a powerful gravitational field that it absorbs all the light that passes near it and reflects nothing.
It had been long believed by astrophysicists that there might be a third, intermediate class of black holes, with masses between a hundred and several hundred thousand times that of the Sun. However, such black holes had not been reliably detected until now.
This new source, dubbed Hyper-Luminous X-ray source 1 (HLX-1), lies towards the edge of the galaxy ESO 243-49. It is ultra-luminous in X-rays, with a maximum X-ray brightness of approximately 260 million times that of the Sun.
The X-ray signature of HLX-1 and the lack of a counterpart in optical images confirm that it is neither a foreground star nor a background galaxy, and its position indicates that it is not the central engine of the host galaxy.
Using XMM-Newton observations carried out November 23, 2004, and November 28, 2008, the team showed that HLX-1 displayed a variation in its X-ray signature. This indicated that it must be a single object and not a group of many fainter sources. The huge radiance observed can only be explained if HLX-1 contains a black hole more than 500 times the mass of the Sun. No other physical explanation can account for the data.