Models of Kilonova/macronova emission from black hole-neutron star mergers

TL;DR

This paper develops a semi-analytic model for kilonova/macronova emission from black hole-neutron star mergers, based on numerical relativity simulations, enabling predictions of electromagnetic signals for future gravitational-wave detections.

Contribution

The authors derive new fitting formulas for ejecta properties and create a semi-analytic lightcurve model that accurately reproduces radiation-transfer simulation results.

Findings

Kilonova/macronova brightness varies with black hole spin and neutron star mass.

At 400 Mpc, brightness ranges from 22-24 mag for high spin/small mass to >28 mag for low spin/large mass.

Application to GRB130603B suggests a rapidly spinning black hole and large neutron star radius are favored.

Abstract

Black hole-neutron star mergers are among the promising gravitational-wave sources for ground-based detectors, and gravitational waves from black hole-neutron star mergers are expected to be detected in the next few years. Simultaneous detection of electromagnetic counterparts with gravitational-wave detection provides rich information about the merger events. Among the possible electromagnetic counterparts from the black hole-neutron star merger, the emission powered by the decay of radioactive r-process nuclei, so called kilonova/macronova, is one of the best targets for follow-up observation. We derive fitting formulas for the mass and the velocity of ejecta from a generic black hole-neutron star merger based on recently performed numerical relativity simulations. We combined these fitting formulas with a new semi-analytic model for a black hole-neutron star kilonova/macronova lightcurve which reproduces the results of radiation-transfer simulations. Specifically, the semi-analytic model reproduces the result of each band magnitude obtained by the radiation transfer simulations within ~1 mag. By using this semi-analytic model, we found that, at 400 Mpc, the kilonova/macronova is as bright as 22-24 mag for the cases with a small chirp mass and a high black hole spin, and >28 mag for a large chirp mass and a low black hole spin. We also apply our model to GRB130603B as an illustration, and show that a black hole-neutron star merger with a rapidly spinning black hole and a large neutron star radius is favored.