2017ApJ...844L..22L

Neutron star-black hole (NS-BH) coalescences are widely believed to be promising gravitational-wave sources in the era of advanced detectors of LIGO/Virgo, but the rate of this population is highly uncertain due to the lack of direct detection of such binaries. There is growing evidence for the connection between the observed three luminous macronova (also known as kilonova) events and NS-BH mergers. In this work, we propose, for the first time based on such a link, a fiducial lower limit of NS-BH coalescence rate density R_nsbh ~18.8_–8.6^+12.5 Gpc^–3 yr^–1 (θ_j/0.1rad)^–2, where θ_j is the typical half-opening angle of the GRB ejecta. After marginalizing over distributions of black hole masses and spins, we find a rate density R_nsbh 10² Gpc^–3 yr^–1, depending upon the equation of state (EoS) of NS material and the properties of the NS-BH system. With the O1 non-observation by advanced LIGO, we show a preference for NS-BH systems with a stiffer NS EoS and a larger contribution from low-mass/high-spin BHs. Our estimate predicts the first detection of an NS-BH system can be as early as the late O2 run or the early O3 run. We expect that future multi-messenger observations can much better constrain NS-BH systems' properties.