Black holes can be secretive about their past, but now there may be an easy way to tell if a monster black hole was once a pair that got cosy and fused together.

Computer simulations suggest that when two black holes spiral towards each other on a collision course, much of the gas and dust in the spinning accretion disc surrounding each of them is ripped away by the gravity of the other. Some of this material fuses into a larger third disc that surrounds both.

According to a model created by Kimitake Hayasaki at Kyoto University in Japan and colleagues, this third disc should periodically return material to the depleted original accretion discs, which survive for a time. Hayasaki's model, details of which will appear in The Astrophysical Journal, shows this shuffling of gas and dust should create spikes in the ultraviolet emissions from the system, which existing spaceborne telescopes could spot.

By contrast, gravity waves, the only other telltale signal from black hole mergers, are notoriously difficult to detect and have yet to be observed.