Supermassive black hole binaries form naturally when galaxies merge, but scientists have only confidently observed a very few of these systems that are widely separated. Black hole binaries that closely orbit each other have not yet been measured. In a paper published today in Physical Review Letters, the researchers suggest hunting down the hidden systems by searching for repeating flashes of light from individual stars lying behind the black holes as they are temporarily magnified by gravitational lensing as the binary orbits.
Supermassive black holes reside at the centers of most galaxies. When two galaxies collide and merge, their central black holes eventually form a bound pair, known as a supermassive black hole binary. These systems play a crucial role in galaxy evolution and are among the most powerful sources of gravitational waves in the universe. While future space-based gravitational-wave observatories like LISA will be able to probe such binaries directly, researchers are now showing that they may already be detectable using existing and upcoming electromagnetic surveys.









