Electromagnetic Signatures from the Tidal Tail of a Black Hole - Neutron Star Merger

TL;DR

This study models the electromagnetic signatures of kilonovae from black hole-neutron star mergers, revealing how ejecta shape, heating, and viewing angle influence observable light curves and spectra.

Contribution

It introduces a detailed simulation of kilonova emission from BH-NS merger ejecta, incorporating relativistic hydrodynamics and radiative transfer to analyze viewing angle effects.

Findings

Light curve brightness varies by factor of 3 with viewing angle.

Optical peak magnitude can reach -14 despite lanthanide-rich ejecta.

Spectrum is blue-shifted along the bulk motion direction.

Abstract

Black hole - neutron star (BH-NS) mergers are a major target for ground-based gravitational wave (GW) observatories. A merger can also produce an electromagnetic counterpart (a kilonova) if it ejects neutron-rich matter that assembles into heavy elements through r-process nucleosynthesis. We study the kilonova signatures of the unbound dynamical ejecta of a BH-NS merger. We take as our initial state the results from a numerical relativity simulation, and then use a general relativistic hydrodynamics code to study the evolution of the ejecta with parameterized r-process heating models. The unbound dynamical ejecta is initially a flattened, directed tidal tail largely confined to a plane. Heating from the r-process inflates the ejecta into a more spherical shape and smooths its small-scale structure, though the ejecta retains its bulk directed motion. We calculate the electromagnetic signatures using a 3D radiative transfer code and a parameterized opacity model for lanthanide-rich matter. The light curve varies with viewing angle due to two effects: asphericity results in brighter emission for orientations with larger projected areas, while Doppler boosting results in brighter emission for viewing angles more aligned with the direction of bulk motion. For typical r-process heating rates, the peak bolometric luminosity varies by a factor of $\sim 3$ with orientation while the peak in the optical bands varies by $\sim 3$ magnitudes. The spectrum is blue-shifted at viewing angles along the bulk motion, which increases the $V$-band peak magnitude to $\sim -14$ despite the lanthanide-rich composition.