Astrophysicists from Clemson and Europe unmask a black hole
February 27, 2012
CLEMSON, SC – February 27, 2012 – A study of X-rays emitted a long time ago in a galaxy far, faraway has unmasked a stellar mass black hole in Andromeda, a spiralgalaxy about 2.6 million light-years from Earth.
Two ClemsonUniversity researchers joined an an international team of astronomers,including scientists at Germany’s Max Planck Institute forExtraterrestrial Physics, in publishing their findings in a pair ofscientific journals this week.
Scientists had suspected theblack hole was possible since late 2009 when an X-ray satelliteobservatory operated by the Max Planck Institute detected an unusualX-ray transient light source in Andromeda.
The brightnesssuggested that these X-rays belonged to the class of ultraluminous X-raysources, or ULXs, said Amanpreet Kaur, a Clemson graduate student inphysics and lead author of the paper published in the Astronomy & Astrophysics Journal.But ULXs are rare. There are none at all in the Milky Way where Earthis located, and this is the first to be confirmed in Andromeda. Provingit required detailed observations.
Photo: The ultraluminousX-ray source studied by Clemson astrophysicists lies deep in the heartof the Andromeda galaxy, about 2.6 million light-years from Earth. image by: XMM-Newton observatory
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 makeconjectures.
There were two competing explanations for theirhigh luminosities, said Clemson physics professor Dieter Hartmann,Kaur’s mentor and a co-author of the paper. Either a stellar mass blackhole was accreting at extreme rates or there was a new subspecies ofintermediate mass black holes accreting at lower rates. One of thegreatest difficulties in attempting to find the right answer is thelarge distance to these objects, which makes detailed observationsdifficult or even impossible.
Working with scientists in Germanyand Spain, the Clemson researchers studied data from the Chandraobservatory and proved that the X-ray source was a stellar mass blackhole that is swallowing material at very high rates.
Follow-upobservations with the Swift and HST satellites yielded importantcomplementary data, proving that it not only is the first ULX inAndromeda but also the closest ULX ever observed. Despite its greatdistance away, Andromeda is actually the nearest major galactic neighborto our own Milky Way.
We were very lucky that we caught the ULXearly enough to see most of its light curve, which showed a verysimilar behavior to other X-ray sources from our own galaxy,” saidWolfgang Pietsch of the Max Planck Institute. The emission decayedexponentially with a characteristic timescale of about one month, whichis a common property of stellar mass X-ray binaries. This means thatthe ULX in Andromeda likely contains a normal, stellar black holeswallowing material at very high rates.
The emission of the ULXsource, the scientists said, probably originates from a system similarto X-ray binaries in our own galaxy, but with matter accreting onto ablack hole that is at least 13 times more massive than our Sun.
UnlikeX-ray binaries in our own Milky Way, this source is much less obscuredby interstellar gas and dust, allowing detailed investigations also atlow X-ray energies.
Ideally, the astronomers would liketo replicate their findings by re-observing the source in anotheroutburst. However, if it is indeed similar to the X-ray binaries in ourown Milky Way, they may be in for a long wait: Such outbursts can occurdecades apart.
On the other hand, as there are so many X-raybinaries in the Andromeda galaxy, another similar outbursting sourcecould be captured any time by the ongoing monitoring campaign, Hartmannsaid. While ‘monitoring’ may not sound exciting, the current resultsshow that these programs are often blessed with discovery and lead tobreakthroughs; in particular, if they are augmented with deep andsustained follow-up.
