Sen—The European Space Agency's X-ray observatory, XMM-Newton has spotted an unusual pair of supermassive black holes in orbit around one another. This is the first time such a pair have been seen in an ordinary galaxy.

The pair were discovered because they ripped apart a star when ESA's space observatory happened to be looking in their direction.

Most massive galaxies in the Universe are thought to harbour at least one supermassive black hole at their centre. Two supermassive black holes are evidence that the galaxy has merged with another. Finding binary supermassive black holes can tell astronomers about how galaxies evolved into their present-day shapes and sizes.

Only a few candidates for close binary supermassive black holes have been found, all in active galaxies where they are constantly ripping gas clouds apart. In the process of destruction, the gas shines at many wavelengths, including X-rays, giving the galaxy an unusually bright centre.

The new discovery, reported by Fukun Liu, of Peking University in China, and colleagues, is important because it is the first to be found in a galaxy that is not active.

In quiescent galaxies, there are no gas clouds feeding the black holes, and so the cores of these galaxies are truly dark. Astronomers have to be looking in the right direction at the moment one of the black holes rips a star to pieces. Such an occurrence is called a 'tidal disruption event'. As the star is pulled apart by the gravity of the black hole, it gives out a flare of X-rays.

Bright strips of X-ray data record the slew history of ESA's XMM-Newton as it moves its focus between different objects in the sky. The image contains information of over 1200 individual slews made between 2001 and 2012. Image credit: ESA/A. Read (University of Leicester)

XMM-Newton collects data from designated targets, one at a time. Once it completes an observation, it slews to the next. During this movement, XMM-Newton keeps the instruments turned on and recording. This surveys the sky in a random pattern, producing data that can be analysed for unknown or unexpected sources of X-rays.

On 10 June 2010, a tidal disruption event was spotted by XMM-Newton in galaxy SDSS J120136.02+300305.5. Follow-up observations were scheduled just days later with XMM-Newton and NASA's Swift satellite.

The galaxy was still spilling X-rays into space, but 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.

Liu found two possible configurations to reproduce the observations. Either 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. Or, the primary black hole was about a million solar masses and in a circular orbit. In both cases, the separation between the black holes was 0.6 milliparsecs, or about 2 thousandths of a light year, about the width of our Solar System.

Being this close the pair will radiate their orbital energy away, gradually spiralling together, until in about two million years time they will merge into a single black hole.

When binary black holes merge, they are predicted to release a massive burst of energy into the Universe but not mostly in X-rays. "The final merger is expected to be the strongest source of gravitational waves in the Universe," says Liu.