A study of X-rays emitted a long time ago in a galaxy far, far away has unmasked a stellar mass black hole in Andromeda, a spiral galaxy about 2.6 million light-years from Earth.

Two Clemson University researchers joined an an international team of astronomers, including scientists at Germany's Max Planck Institute for Extraterrestrial Physics, in publishing their findings in a pair of scientific journals this week.

Scientists had suspected the black hole was possible since late 2009 when an X-ray satellite observatory operated by the Max Planck Institute detected an unusual X-ray transient light source in Andromeda.

"The brightness suggested that these X-rays belonged to the class of ultraluminous X-ray sources, or ULXs," said Amanpreet Kaur, a Clemson graduate student in physics and lead author of the paper published in the Astronomy and Astrophysics Journal. "But ULXs are rare. There are none at all in the Milky Way where Earth is located, and this is the first to be confirmed in Andromeda. Proving it required detailed observations."

Because ULX sources are rare - usually with just one or two in a galaxy, if they are present at all - there was very little data with which astronomers could make conjectures.

"There were two competing explanations for their high luminosities," said Clemson physics professor Dieter Hartmann, Kaur's mentor and a co-author of the paper. "Either a stellar mass black hole was accreting at extreme rates or there was a new subspecies of intermediate mass black holes accreting at lower rates. One of the greatest difficulties in attempting to find the right answer is the large distance to these objects, which makes detailed observations difficult or even impossible."

Working with scientists in Germany and Spain, the Clemson researchers studied data from the Chandra observatory and proved that the X-ray source was a stellar mass black hole that is swallowing material at very high rates.

Follow-up observations with the Swift and HST satellites yielded important complementary data, proving that it not only is the first ULX in Andromeda but also the closest ULX ever observed. Despite its great distance away, Andromeda is actually the nearest major galactic neighbor to our own Milky Way.

"We were very lucky that we caught the ULX early enough to see most of its light curve, which showed a very similar behavior to other X-ray sources from our own galaxy," said Wolfgang Pietsch of the Max Planck Institute. The emission decayed exponentially with a characteristic timescale of about one month, which is a common property of stellar mass X-ray binaries. "This means that the ULX in Andromeda likely contains a normal, stellar black hole swallowing material at very high rates."

The emission of the ULX source, the scientists said, probably originates from a system similar to X-ray binaries in our own galaxy, but with matter accreting onto a black hole that is at least 13 times more massive than our Sun.

Unlike X-ray binaries in our own Milky Way, this source is much less obscured by interstellar gas and dust, allowing detailed investigations also at low X-ray energies.

Ideally, the astronomers would like to replicate their findings by re-observing the source in another outburst. However, if it is indeed similar to the X-ray binaries in our own Milky Way, they may be in for a long wait: Such outbursts can occur decades apart.

"On the other hand, as there are so many X-ray binaries in the Andromeda galaxy, another similar outbursting source could be captured any time by the ongoing monitoring campaign," Hartmann said. "While 'monitoring' may not sound exciting, the current results show that these programs are often blessed with discovery and lead to breakthroughs; in particular, if they are augmented with deep and sustained follow-up."