Where Are the Electromagnetic-wave Counterparts of Stellar-mass Binary Black Hole Mergers?

Multimessenger astronomy, combining gravitational-wave (GW) and electromagnetic-wave (EM) observations, has a huge impact on physics, astrophysics, and cosmology. However, the majority of sources detected with currently running ground-based GW observatories are binary black hole (BBH) mergers, which disappointingly were expected to have no EM counterparts. In this Letter, we propose that if a BBH merger happens in a gaseous disk around a supermassive black hole, the merger can be accompanied by a transient radio flare like a fast radio burst (FRB). We argue that the total mass and the effective spin derived from GW detection can be used to distinguish such a source from other channels of BBH mergers. If this prediction is confirmed with future observations, multimessenger astronomy can be greatly improved. The mystery of the origin of FRBs could also be revealed partially.