July 1, 2009

Scientists Find Possible Black Hole “˜Missing Link’

Scientists have discovered a new class of black hole that stands at a mass of more than 500 times that of the Sun.

An international team of astronomers detailed the new discovery found in a galaxy some 290 million light years from Earth in the journal Nature.

The team used the European Space Agency's XMM-Newton X-ray observatory to find HLX-1 (Hyper-Luminous X-ray source 1) that could stand as the "missing link" between supermassive black holes "“ several million to several billion times the mass of the Sun "“ and stellar-mass black holes "“ about three and 20 Solar masses.

"While it is widely accepted that stellar mass black holes are created during the death throes of massive stars, it is still unknown how super-massive black holes are formed," said lead author Dr Sean Farrell, at the Department of Physics and Astronomy at the University of Leicester.

"One theory is that super-massive black holes may be formed by the merger of a number of intermediate mass black holes. To ratify such a theory, however, you must first prove the existence of intermediate black holes."

Using the XMM-Newton orbital telescope, astronomers made the discovery by measuring its brightness. They noted that the light coming from HLX-1 was too low for it to be a part of this galaxy, and "the lack of observed radio or optical emission from the location of HLX-1 in addition to the observed X-ray signature indicates that it is unlikely to be a background galaxy," said ESA.

"This means that the source of the X-ray emission must lie in ESO 243-49. Its location is too far away from the galactic centre for it to be a supermassive black hole, and too bright for a stellar-mass black hole feeding at the maximum rate."

Scientists crosschecked the discovery a second time on November 28 2008. By looking at both observations, the team showed that the "signature of X-rays originating from HLX-1 varied significantly in time and concluded from this that it must be a single object."

"This indicated that it must be a single object and not a group of many fainter sources," researchers said. "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."

"This is the best detection to date of such long sought after intermediate mass black holes. Such a 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," said Dr Farrell.

"The identification of HLX-1 is therefore an important step towards a better understanding of the formation of the super-massive black holes that exist at the centre of the Milky Way and other galaxies."


Image Caption: Illustration of HLX-1 (blue star to the upper left hand side of the galactic bulge). HLX-1, located on the outskirts of the spiral galaxy ESO 243-49, is the strongest candidate to- date of intermediate-mass black holes. Credits: Heidi Sagerud


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