A new class of black hole, more than 500 times the mass of the Sun, has been discovered using ESA's XMM-Newton space telescope.

Up until now the black hole inventory contained either monstrous million-to-billion times the mass of our Sun objects or those a relatively tame three to twenty solar masses, or about the size of a typical star. The discovery of a 500 solar mass black hole provides the first solid evidence of a new type of intermediate black hole.

"This is the best detection to date of such long sought after intermediate mass black holes," says the lead author of the research announced in today's issue of the journal Nature, Sean Farrell. "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."

Farrell adds that 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," he says.

The identification of the new source, nicknamed HLX-1 (short for Hyper-Luminous X-ray source 1), provides an important stepping stone in understanding the formation mechanisms of the type of black hole that exists at the centre of our own Milky Way Galaxy.

But just how do the intermediate mass black holes form? "It is likely that these intermediate mass black holes are also formed through mergers and accretion of matter around them, where the seed may be a stellar mass black hole," co-authors Olivier Godet and Natalie Webb tell Astronomy Now. "This would then be an intermediate stage to forming supermassive black holes found in quasars and our own Galaxy, the Milky Way."

The team think that at least one intermediate black hole exists per galaxy. "One place that could privilege intermediate black hole formation is globular clusters – spherical dense clusters of old stars," says Godet. "There are likely to be hundreds of globular clusters in most galaxies."

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 XMM-Newton X-ray space telescope. It lies towards the edge of the galaxy ESO 243-49 and is ultra-luminous in X-rays, with a maximum X-ray brightness of approximately 260 million times that of the Sun.

By comparing observations carried out on the 23rd November 2004 and the 28th November 2008, the team showed that HLX-1 displayed a variation in its X-ray signature such 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.