The delay time of gravitational wave - gamma-ray burst associations

The first gravitational wave (GW) - gamma-ray burst (GRB) association, GW170817/GRB 170817A, had an offset in time, with the GRB trigger time delayed by similar to 1.7 s with respect to the merger time of the GW signal. We generally discuss the astrophysical origin of the delay time, Delta t, of GW-GRB associations within the context of compact binary coalescence (CBC) - short GRB (sGRB) associations and GW burst - long GRB (lGRB) associations. In general, the delay time should include three terms, the time to launch a clean (relativistic) jet, Delta t(jet); the time for the jet to break out from the surrounding medium, Delta t(bo); and the time for the jet to reach the energy dissipation and GRB emission site, Delta t(GRB). For CBC-sGRB associations, Delta t(jet) and Delta t(bo) are correlated, and the final delay can be from 10 ms to a few seconds. For GWB-lGRB associations, Delta t(jet) and Delta t(bo) are independent. The latter is at least similar to 10 s, so that Delta t of these associations is at least this long. For certain jet launching mechanisms of lGRBs, Delta t can be minutes or even hours long due to the extended engine waiting time to launch a jet. We discuss the cases of GW170817/GRB 170817A and GW150914/GW150914-GBM within this theoretical framework and suggest that the delay times of future GW/GRB associations will shed light into the jet launching mechanisms of GRBs.