Observational evidence for supermassive black hole binaries

Coalescing massive black hole binaries are powerful emitters of gravitational waves, in the LISA sensitivity range for masses M-BH approximate to 10(4-7) M-circle dot. According to hierarchical galaxy merger models, binary black holes should form frequently, and should be common in the cores of galaxies. The presence of massive black hole binaries has been invoked to explain a number of class properties of different types of galaxies, and in triggering various forms of activity. The search for such binary black holes is therefore of great interest for key topics in astrophysics ranging from galaxy formation to activity in galaxies. A number of phenomena were attributed to the presence of supermassive binary black holes, including X-shaped radio galaxies and double-double radio galaxies, helical radio-jets, periodicities in the lightcurves of blazars, (double-horned emission-line profiles), binary galaxies with radio-jet cores, binary quasars, and the X-ray active binary black hole at the center of the galaxy NGC 6240. Here, I review the observational evidence for the presence of supermassive binary black holes in galaxies, and the scenarios which have been discussed to explain these observations.