Twin black holes, so close that they are gravitationally bound and orbiting around one another in the final stages before they merge to form one colossal black hole, have been found in a quasar that existed around 10.3 billion years ago.

“We believe we have observed two supermassive black holes in closer proximity than ever before,” says Suvi Gezari of the University of Maryland, USA, who participated in the study of these rare black holes.

The two black holes share the same material falling onto them, and so they swallow matter in cyclical episodes, leading to periodic brightening and fading. Quasars are known to change in brightness as their activity varies from one day to the next, but these variations are usually random. Using the Pan-STARRS1 Medium-Deep Survey, a team of astronomers including Gezari spotted that a quasar, designated PSO J334.2028+01.4075, had a periodic cycle of brightening and dimming instead, repeating this cycle every 542 days. The astronomers quickly realised that what they were looking at was not one active black hole, but two very close together, making one orbit of each other over this 542 day period. If they were in our Solar System, that would put them within the orbit of Mars. Follow up observations with the Catalina Real Time Transient Survey and the FIRST Bright Quasar Survey confirmed the finding.

Quasars are distant galaxies with at least one monster black hole at their centre that is so active it completely outshines the rest of the galaxy. The black hole is fed large servings of material –interstellar gas and dust, stars –that winds up in a spiralling disc around the black hole, waiting its turn to be eaten or ejected. It is this disc, which reaches temperatures of many millions of degrees Celsius, that shines so brightly. Sometimes the material comes from clouds of intergalactic gas falling onto the black hole, but in other circumstances quasars can be fuelled by galaxy mergers, which disrupt the gas in the colliding galaxies. Each galaxy has a supermassive black hole at its heart, and when the galaxies merge, the black holes eventually find their way into each other’s vicinity to merge too.

The combined mass of the binary black hole system in PSO J334 is ten billion times the mass of our Sun, which is enormous compared to the mass of the black hole at the centre of the Milky Way Galaxy, which has between 4.1 and 4.3 million times the mass of the Sun. The existence of this binary black hole possibly paves the way for testing one of Albert Einstein’s predictions from the General Theory of Relativity: gravitational waves.

“This pair of black holes may be so close together that they are emitting gravitational waves,” says Gezari. Although gravitational waves have never been detected before, our detectors are growing more sensitive. Merging black holes would create high frequency gravitational waves that are easier to spot in space than by ground-based detectors such as Advanced LIGO (the Laser Interferometer Gravitational Wave Observatory). Plans for a space-based gravitational detector, called LISA (Laser Interferometer Space Antenna), are already well advanced, with the European Space Agency launching the technology-testing LISA Pathfinder mission in September as a pre-cursor to the full blown mission.

A paper describing the results of the binary black hole system was published online in Astrophysical Journal Letters.