Mergers of black hole-neutron star binaries and rates of associated electromagnetic counterparts

Abstract

Black hole-neutron star (BHNS) binaries are amongst promising candidates for the joint detection of electromagnetic (EM) signals with gravitational waves (GWs) and are expected to be detected in the near future. Here we study the effect of the BHNS binary parameters on the merger ejecta properties and associated EM signals. We estimate the remnant disc and unbound ejecta masses for BH mass and spin distributions motivated from the observations of transient low-mass X-ray binaries and a specific NS equation of state (EoS). The amount of r-process elements synthesized in BHNS mergers is estimated to be a factor of ~102-104 smaller than BNSmergers, due to the smaller dynamical ejecta and merger rates for the former. We compute the EM luminosities and light curves for the early- and late-time emissions from the ultrarelativistic jet, sub-relativistic dynamical ejecta and wind, and the mildly relativistic cocoon for typical ejecta parameters. We then evaluate the low-latency EM follow-up rates of the GWtriggers in terms of the GW detection rate NGW for current telescope sensitivities and typical BHNS binary parameters to find that most of the EM counterparts are detectable for high BH spin, small BH mass, and a stiffer NS EoS when NS disruption is significant. Based on the relative detection rates for given binary parameters, we find the ease of EMfollow-up to be: ejecta afterglow > cocoon afterglow ≳ jet prompt > ejecta macronova > cocoon prompt > jet afterglow ≫ wind macronova ≫ wind afterglow.