Andromeda yields cache of stellar black holes

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.