Mergers of Charged Black Holes: Gravitational Wave Events, Short Gamma-Ray Bursts, and Fast Radio Bursts

Abstract

The discovery of GW 150914 suggests that double black hole (BH-BH) mergers are common in the universe and are the dominant population of the detectable gravitational wave events. Unlike double neutron star (NS-NS) mergers and black hole - neutron star (BH-NS) mergers that are believed to be accompanied with rich electromagnetic (EM) counterparts, the BH-BH merger systems are not expected to have copious surrounding materials to power bright EM counterparts due to accretion. However, the Fermi GBM team surprisingly detected a putative short gamma-ray burst 0.4 seconds after the GW chirp signal. On the other hand, at least some fast radio bursts (FRBs), mysterious milliseconds-duration radio transients, are recently identified to have a cosmological origin, but their physical origin remains unknown. Here I show that if at least one of the two merging black holes carries a small amount of charge, the inspiral of the BH-BH system would drive a magnetic dipole normal to the orbital plane. A magnetosphere would be developed, and the system would behave like a giant pulsar with increasing wind power. The magnetospheric activities during the final merging phase would make an FRB if the BH charge can be as large as a factor of \(\hat q \sim 10^{-6}\) of the critical charge \(Q_c\) of the BH. At large radii outside the magnetosphere, dissipation of the Poynting flux energy in the outflow would power a short duration high-energy transient, which would appear as a detectable short GRB if the charge can be as large as \(\hat q \sim 10^{-4}\). The putative short GRB coincident with GW 150914 can be interpreted with this model. Future joint GW - GRB - FRB searches would verify this model and lead to measurements or constraints on the charges carried by isolate black holes.