A pair of supermassive black holes in orbit around one another have been discovered by Prof. Fukun Liu of the Department of Astronomy and The Kavli Institute for Astronomy and Astrophysics of Peking University, and his colleagues. This is the first time such a pair have been found in an ordinary galaxy. They were discovered because they ripped apart a star when ESA’s space observatory XMM-Newton happened to be looking in their direction. The findings also validates the predictions of the tidal disruption by supermassive binary black holes given by Fukun Liu and his colleagues in 2009.The findings are published in the May 10 issue of the “Astrophysical Journal”, and appeared online on the astrophysics preprint server. [Link to full paper].Most massive galaxies in the Universe are thought to harbour at least one supermassive black hole at their centre. Two supermassive black holes are the smoking gun that the galaxy has merged with another. Thus, finding binary supermassive black holes can tell astronomers about how galaxies evolved into their present-day shapes and sizes. On 10 June 2010, a tidal disruption event was spotted by XMM-Newton in galaxy SDSS J120136.02+300305.5, approximately 2 billion light years away. Co-author Stefanie Komossa (Max-Planck-Institut für Radioastronomie) and her colleagues were scanning the data for such events and scheduled follow-up observations just days later with XMM-Newton and NASA’s Swift satellite. The galaxy was still spilling X-rays into space. It looked exactly like a tidal disruption event caused by a supermassive black hole but as they 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 a more expected fading rate, as if nothing had happened.Now, thanks to Fukun Liu, this behaviour can be explained. “This is exactly what you would expect from a pair of supermassive black holes orbiting one another,” says Liu.Liu had been working on models of black hole binary systems that predicted a sudden plunge to darkness and then the recovery because the gravity of one of the black holes disrupted the flow of gas onto the other, temporarily depriving it of fuel to fire the X-ray flare. He found that two possible configurations were possible to reproduce the observations of J120136.In the first, the primary black hole contained 10 million solar masses and was orbited by a black hole of about a million solar masses in an elliptical orbit. In the second solution, the primary black hole was about a million solar masses in a circular orbit.In both cases, the separation between the black holes was relatively small: 0.6 milliparsecs, or about 2 thousandths of a light year. This is about the width of our Solar System.Being this close, the fate of this newly discovered black hole pair is sealed. They will radiate their orbital energy away, gradually spiralling together, until in about two million years time they will merge into a single black hole.Now that astronomers have found this first candidate for a binary black hole in a quiescent galaxy, the search is inevitably on for more. XMM-Newton will continue its slew survey. This detection will also spur interest in a network of telescopes that search the whole sky for tidal disruption events.“Once we have detected thousands of tidal disruption events, we can begin to extract reliable statistics about the rate at which galaxies merge,” says Stefanie Komossa. Other links: The ESA news release, the MPIfR news release. Image: Artist's impression of a binary supermassive black hole system. Credit: ESA - C. Carreau.