A study of X-rays emitted a long time ago in a galaxy far, far away has positively identified a stellar mass black hole in Andromeda, a spiral galaxy located about 2.6 million light-years from Earth.
Astronomers have suspected the presence of a black hole 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,” explained Amanpreet Kaur, a Clemson graduate student in physics.
“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.”
However, because ULX sources are rare, there was very little data with which astronomers could make conjectures.
“There were two competing explanations for their high luminosities. 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,” said Clemson physics professor Dieter Hartmann.
“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 proed over data from the Chandra observatory – ultimately concluding 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,” noted 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, say the scientists, 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 the 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 are interested in replicating 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, as such outbursts are likely to 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.
“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,” Hartmann added.