(Sen) - The appearance of an Ultraluminous X-ray source (ULX) in the Andromeda galaxy has resulted in a long standing question being answered as to the source of these extremely luminous objects.

ULXs are quite rare and only one or two may exist in a galaxy and most of these galaxies are also at large distances, which makes studying the ULXs difficult.

There are two theories as to the source of the ULXs. Either the X-ray emission comes from the region around a stellar mass black hole or an intermediate mass black hole.

A stellar mass black hole is one that can be several times the mass of our Sun, and in this case it would need to be accreting matter at a substantial rate to account for the high luminosity. An intermediate black hole is one that is around 1,000 times the mass of our Sun.

In 2009 a ULX appeared in our nearest large galactic neighbour, Andromeda (M31). The discovery was made by NASA’s Chandra X-ray Observatory and follow up observations were performed with NASA’s Swift telescope, ESA’s XMM-Newton telescope and the NASA/ESA Hubble Space Telescope.

Andromeda is only two million light years away, which allowed astronomers to get a really good look at the ULX. It shone brightly for only a short period of time, and the light faded in around a month. This behaviour matches that of X-ray binaries in our own Galaxy, where a black hole rips material from an orbiting stellar companion, then heats the material in an accretion disc around the black hole and emits light as X-rays.

The results show that this ULX has a mass of around 13 solar masses, which rules out an intermediate mass black hole. This is the first time that the source of a low luminosity ULX has been determined.

If this X-ray binary is indeed similar to those in our own Galaxy, then astronomers might have to wait decades for the next outburst. However, the Andromeda galaxy is full of X-ray binaries so another ULX might reveal itself before then.