Isotropic detectable X-ray counterparts to gravitational waves from neutron star binary mergers

Abstract

Neutron star binary mergers are strong sources of gravitational waves (GWs). Promising electromagnetic counterparts are short gamma-ray bursts (GRBs), but the emission is highly collimated. We propose that the scattering of the long-lasting plateau emission in short GRBs by the merger ejecta produces nearly isotropic emission for ~10 4 s with flux 10 -13 -10 -10 erg cm -2 s -1 at 100Mpc in X-ray. This is detectable by Swift/XRT and wide field X-ray detectors such as ISS-Lobster, Einstein Probe, eROSITA, and WF-MAXI, which are desired by the infrared and optical follow-ups to localize and measure the distance to the host galaxy. The scattered X-rays obtain linear polarization, which correlates with the jet direction, X-ray luminosity, and GW polarizations. The activity of the plateau emission is also a natural energy source of a macronova (or kilonova) detected in short GRB 130603B without the r-process radioactivity.