GRB/GW ASSOCIATION: LONG–SHORT GRB CANDIDATES, TIME LAG, MEASURING GRAVITATIONAL WAVE VELOCITY, AND TESTING EINSTEIN’S EQUIVALENCE PRINCIPLE

Abstract

ABSTRACT Short-duration gamma-ray bursts (SGRBs) are widely believed to be powered by the mergers of compact binaries, such as binary neutron stars or possibly neutron star–black hole binaries. Though the prospect of detecting SGRBs with gravitational wave (GW) signals by the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)/VIRGO network is promising, no known SGRB has been found within the expected advanced LIGO/VIRGO sensitivity range for binary neutron star systems. We find, however, that the two long–short GRBs (GRB 060505 and GRB 060614) may be within the horizon of advanced GW detectors. In the upcoming era of GW astronomy, the merger origin of some long–short GRBs, as favored by the macronova signature displayed in GRB 060614, can be unambiguously tested. The model-dependent time lags between the merger and the onset of the prompt emission of the GRB are estimated. The comparison of such time lags between model predictions and the real data expected in the era of the GW astronomy would be helpful in revealing the physical processes taking place at the central engine (including the launch of the relativistic outflow, the emergence of the outflow from the dense material ejected during the merger, and the radiation of gamma rays). We also show that the speed of GWs, with or without a simultaneous test of Einstein’s equivalence principle, can be directly measured to an accuracy of or even better in the advanced LIGO/VIRGO era.