Three dimensional end-to-end simulation for kilonova emission from a black-hole neutron-star merger

TL;DR

This study uses long-term simulations to model kilonova emissions from a black-hole neutron-star merger, revealing that such events produce fainter but longer-lasting near-infrared signals compared to neutron star mergers.

Contribution

It provides the first detailed long-term simulation of kilonova emission from a BH-NS merger, highlighting the impact of ejecta morphology and radioactive heating on observable signals.

Findings

Kilonova from BH-NS mergers is fainter but lasts longer than from NS-NS mergers.

Emission is mainly powered by lanthanide-rich dynamical ejecta with high opacity.

Long-lasting (>2 weeks) near-infrared emission can help identify BH-NS mergers observationally.

Abstract

We study long-term evolution of the matter ejected in a black-hole neutron-star (BH-NS) merger employing the results of a long-term numerical-relativity simulation and nucleosynthesis calculation, in which both dynamical and post-merger ejecta formation is consistently followed. In particular, we employ the results for the merger of a $1.35\,M_\odot$ NS and a $5.4\,M_\odot$ BH with the dimensionless spin of 0.75. We confirm the finding in the previous studies that thermal pressure induced by radioactive heating in the ejecta significantly modifies the morphology of the ejecta. We then compute the kilonova (KN) light curves employing the ejecta profile obtained by the long-term evolution. We find that our present BH-NS model results in a KN light curve that is fainter yet more enduring than that observed in AT2017gfo. This is due to the fact that the emission is primarily powered by the lanthanide-rich dynamical ejecta, in which a long photon diffusion time scale is realized by the large mass and high opacity. While the peak brightness of the KN emission in both the optical and near-infrared bands is fainter than or comparable to those of binary NS models, the time-scale maintaining the peak brightness is much longer in the near-infrared band for the BH-NS KN model. Our result indicates that a BH-NS merger with massive ejecta can observationally be identified by the long lasting ($>$two weeks) near-infrared emission.