

The detection of an ultraluminous X-ray source is now the strongest observational evidence for the existence of intermediate-mass black holes, as reported in a paper titled 'An intermediate-mass black hole of over 500 solar masses in the galaxy ESO 243-49' in the most recent issue (2 July) of the journal Nature.
Black holes vary in mass, with the supermassive types ranging from several million to several billion times the Sun's mass, while the stellar-mass ones have masses several to several tens that of the Sun. The existence intermediate-mass black holes is in dispute, and though many candidates have been proposed, none are widely accepted as definitive.
Now Sean Farrell of the Universite de Toulouse and colleagues detected X-rays from the spiral galaxy ESO 243-49 that exceed the upper limit of luminosity for normal stellar processes. Analysis of the X-ray data shows that the black hole outside the host galaxy's centre has a lower mass limit that is 500 times the mass of the Sun.
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