Black holes are some of the most amazing structures in the universe. Sadly, we don't know all that much about them. At least, when you compare black holes to other cosmic structures, our data seems to be somewhat lacking. However, we are constantly gathering new information, which brings us closer and closer to attaining a comprehensive understanding of the tyrants of the cosmos. Case in point, in a recent edition of Astrophysical Journal, an international research team announced the discovery of the first pair of supermassive black holes found in orbit around one another in a dormant galaxy. The team responsible for the find announced that the pair was discovered because the black holes happened to be ripping apart a star when the European Space Agency’s XMM-Newton space observatory was looking their way.
The discovery was reported by Fukun Liu from Peking University in China, along with a team of colleagues. The scientists note that it is entirely possible that a number of inactive galaxies host black hole binaries; however, it is hard to locate them as they give off little electromagnetic radiation (or light). Consequently, the only way that we can hope to uncover these black holes is if we just happen to be looking in the right direction when a star wanders in too close to the pair, which is precisely what happened in this case.
In June of 2010, the XMM-Newton observatory spotted a tidal disruption event in a galaxy approximately 2 billion light-years away. Initially, it looked like the kind of tidal disruption event that is caused by a supermassive black hole; however, there was an uncharacteristic "wobble" in the data. The X-rays that were being emitted fell below detectable levels between days 27 and 48 when the team was monitoring the emissions. After this, the X-rays re-appeared and followed the expected fading pattern. Initially, scientists were puzzled by the find. However, thanks to Fukun Liu, this behavior can be explained. “This is exactly what you would expect from a pair of supermassive black holes orbiting one another,” Liu stated.
Liu worked out two possible solutions in order to explain this surprising data. According to the configuration of the first solution, the primary black hole contains 10 million solar masses and is orbited by a black hole that is about a million solar masses and orbits in an elliptical orbit. In the second, the primary black hole is about a million solar masses in a circular orbit. Amazingly, in each of the solutions, the separation between the black holes is relatively small (just 2 thousandths of a light-year, or about the width of our solar system).
This find is exciting for a number of reasons. First, the merger of supermassive black holes is expected to be the strongest source of gravitational waves in the universe. Second, as the ESA's Norbert Schartel notes, “The innovative use of XMM-Newton’s slew observations made the detection of this binary supermassive black hole system possible. This demonstrates the important role that long-lasting space observatories have in detecting such rare events that can potentially open new areas in astronomy.”