To date, only few candidates for close binary supermassive black holes have been found. But all of these are in "active galaxies" where they are constantly ripping gas clouds apart. In this process, the gas is heated considerably until it shines at many wavelengths, including X-rays. This gives these galaxies an unusually bright centre - that's why they're called "active". The new discovery is important because it is the first black hole pair to be found in a galaxy that is not active in this sense.
Normally, the observatory collects data from designated targets, one at a time. Once it completes an observation, it slews to the next. But during this movement, XMM-Newton keeps the instruments turned on and keeps recording. The result is a random sky survey, producing data that can be analysed for unknown or unexpected X-ray sources. On 10 June 2010, an event was spotted by XMM-Newton in the galaxy "SDSS J120136.02+300305.5", approximately 2 billion light years away. Komossa and her colleagues had been scanning the data for events like this and were thus able to schedule follow-up observations just days later with XMM-Newton, and with NASA's Swift satellite. The galaxy was still radiating X-rays into space, and they looked exactly like a tidal disruption event.
But as the researchers tracked the slowly fading emission day after day something strange happened: The X-rays fell below detectable levels between days 27 and 48 after the discovery. Then they re-appeared and continued to follow the expected fading rate again, as if nothing had happened. Thanks to Fukun Liu, astronomer from Peking University in China and lead author of the study, this odd behaviour could be explained: "This is exactly what you would expect from a pair of supermassive black holes orbiting one another," Liu explains. He had been working on models of black hole binary systems and had predicted a sudden plunge to darkness with a subsequent recovery, because the gravity of one of the black holes could disrupt the flow of gas onto the other, temporarily depriving it of fuel to fire the X-ray flare.
Now that astronomers have found this first candidate for a binary black hole in a quiescent galaxy, the search is on for more. XMM-Newton will continue its slew survey. "Once we have detected thousands of tidal disruption events, we can begin to extract reliable statistics about the rate at which galaxies merge," Stefanie Komossa predicts. (© Max Planck Institute for Radio Astronomy, AcademiaNet)