Astronomers have discovered ten previously unknown likely black holes in the Andromeda Galaxy by means of a powerful new search technique they have devised. The Andromeda Galaxy is the nearest neighbouring spiral galaxy, 2.5 million light years away. Drs Robin Barnard, Ulrich Kolb and Carole Haswell of the Open University and Dr Julian Osborne of The University of Leicester used the European Space Agency's XMM-Newton orbiting X-ray observatory to find what are probably black holes lurking in double star systems known as low mass X-ray binaries (LMXBs).

Most LMXBs consist of a neutron star together with a normal star similar to our Sun but a few have black holes instead of neutron stars, and it is difficult to distinguish between them. Now, by identifying a particular signature in the X-rays these systems emit, the astronomers believe they can determine whether the small dense star is massive enough to be a black hole. Dr Barnard will describe the team's fruitful hunt in the Andromeda Galaxy for these rare stellar black holes in a talk at the RAS National Astronomy Meeting at the Open University on Tuesday 30 March.

The astronomers cautiously refer to their discoveries as 'candidate' black holes. 'Black holes are elusive beasts,' says Dr Barnard. 'We can never see them directly only the effects they have on the stars and gas around them.' For example, in LMXBs, material from the companion star spiralling onto the black hole or neutron star gives out X-rays as a result of being heated to over a million degrees. 'But if black holes exist, the 10 X-ray sources we have singled out are very likely black holes,' Barnard says. And the astronomers are pleased with their haul of 10 so far since LMXBs are very rare, and it only took 18 months to find them, compared with the decades taken to find 10 candidate black holes in the Milky Way.

Another feature of these X-ray stars makes the discovery particularly interesting. Though black hole candidates in LMXBs have been identified in galaxies beyond the Milky Way by astronomers employing other techniques, they have all been ones that fluctuate markedly, by a factor of 10 or much more. 'Eight out of 10 of the sources we have identified as likely black holes appear to be persistently bright the first such LMXBs to be found anywhere, including in our own Galaxy,' explains Dr Barnard.

The technique the group have developed to identify LXMBs containing black holes involves looking at the characteristics of the X-ray emission for a particular kind of variability indicating that material is being transferred at a relatively low rate for the system. The black hole systems can show this signature while they are considerably more luminous than the neutron star ones. In particular, the researchers have found that the LXMBs are running at less than 10% of their theoretical maximum power when the characteristic variability is observed. From that it is possible to estimate the mass of the compact star. If it is over 3 times the mass of the Sun, it is too high for a neutron star and the object is most likely a black hole. Of course, other reasons for the high luminosity must be ruled out before a black hole is identied.

With the powerful capabilities of XMM-Newton, the team say they expect to be able to detect stellar black holes in galaxies even farther away than Andromeda, as well as revealing previously unknown ones in our own.

Notes

What are black holes?
Black holes are stars so compact that they are smaller than their "event horizon"; nothing that gets nearer to the black hole than this can ever escape - not even light. For example, if we could somehow squash the Sun down to less than 4 miles across, then it would turn into a black hole. The gravity of the Sun would be the same, but the sky would be a lot darker!

What is an X-ray binary?
Most stars exist in pairs. If one of the two stars is extremely massive, then it will die in a fiery supernova, leaving behind a compact star (either a neutron star, or a black hole). If the pair survives, it might form an X-ray binary. These are very rare; we know of only 150 LMXB in the Milky Way, which contains 100 billion stars. The gravity of the compact star is so strong that material from its companion "falls" onto it, often spiralling in gradually, forming an "accretion disc". By the time it reaches the compact star, the doomed material is travelling at almost the speed of light, and heated to over 1 million degrees - so hot that it shines out in X-rays. An apple sucked into an X-ray binary would release more energy than 500 million Hiroshima bombs. An X-ray binary that is only twice as massive as the Sun can be up to 500,000 times more powerful.

The power of an X-ray binary depends on how quickly material is being transferred from the companion to the compact star - the "accretion rate". The maximum power is known as the Eddington limit; this occurs when the X-ray radiation is so powerful that it balances the gravitational pull of the compact star on the infalling material. If the radiation were any brighter it would blow the infalling material away.