Models of Kilonova/Macronova Emission From Black Hole–neutron Star Mergers

Abstract

Black hole-neutron star (BH-NS) mergers are among the most promising gravitational-wave sources for groundbased detectors, and gravitational waves from BH-NS mergers are expected to be detected in the next few years. The simultaneous detection of electromagnetic counterparts with gravitational waves would provide rich information about merger events. Among the possible electromagnetic counterparts from BH-NS mergers is the so-called kilonova/macronova, emission powered by the decay of radioactive r-process nuclei, which is one of the best targets for follow-up observations. We derive fitting formulas for the mass and the velocity of ejecta from a generic BH-NS merger based on recently performed numerical-relativity simulations. We combine these fitting formulas with a new semi-analytic model for a BH-NS kilonova/macronova lightcurve, which reproduces the results of radiation-transfer simulations. Specifically, the semi-analytic model reproduces the results of each band magnitude obtained by the previous 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 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 GRB 130603B as an illustration, and show that a BH-NS merger with a rapidly spinning black hole and a large neutron star radius is favored.

Gravitational waves and mass ejecta from binary neutron star mergers: Effect of the mass ratio

“…Similar statements hold for other configurations employing stiff EOSs. For this reason we expect that the model of [75] is appropriate for the MS1b as well as other stiff configurations and systems without shocks.…”

“…17) we see that the peak temperature decreases from ∼ 2500K to ∼ 1500K when the mass ratio increases, except for SLy for which the temperature for all mass ratios is around ∼ 1700K. Light curves are computed following [75]. The model assumes homologous expansion of the ejecta, a gray opacity, diffusion approximation for the radiation transfer and that the photons diffuse only from the latitudinal edge.…”

“…We are very grateful to Kyohei Kawaguchi for discussions about the ejecta during BHNS mergers and for making his model and code publicly available [75]. Computations where performed on SuperMUC at the LRZ (Munich) under the project numbers pr87nu and pr48pu, Juropa/Jureca (Jülich) under the project number HJN26 and HPO21, Stampede (Texas, XSEDE allocation -TG-PHY140019), and the Jena group local cluster quadler.…”

“…Thus, the mass ratio has a clear observational imprint on the EM counterparts. 4 Notice that the model of [75] was tested for ejecta masses above 0.01M and that higher uncertainties and errors might be present for low mass ejecta. …”

“…We use in particular the analytical model of [89] which assumes that the diffusion time is less than the dynamical times for computing the peak luminosity and temperature. Light-curves are computed with the model described in [75], originally introduced for the merger of BHNS systems. Radio flares peak fluxes are instead computed following [10].…”