Numerical simulations of black hole-neutron star mergers with equal and near-equal mass ratios

TL;DR

This study conducts numerical simulations of black hole-neutron star mergers with near-equal masses to improve gravitational waveform models, analyze remnant properties, and predict electromagnetic signals like kilonovae for better detection and understanding.

Contribution

It provides new numerical simulations for symmetric mass ratio mergers, evaluates the accuracy of existing models, and explores electromagnetic counterparts, filling a critical gap in current research.

Findings

Waveform models show ~1 rad dephasing at merger.

Remnant mass predictions agree well with formulas.

Kilonovae are potentially detectable within days after merger.

Abstract

The detection of GW230529_181500 suggested the existence of more symmetric black hole-neutron star mergers where the black hole mass can be as low as 2.6 times that of the neutron star. Black hole-neutron star binaries with even more symmetric mass ratios are expected to leave behind massive disks capable of driving bright electromagnetic transients like kilonovae. Currently, there is only a limited number of numerical-relativity simulations of black hole-neutron star mergers in this regime, which are vital for accurate gravitational waveform models and analytical fitting formulas for the remnant properties. Insufficient accuracy of these may lead to misclassification of real events and potentially missed opportunities to locate their electromagnetic counterparts. To fill this gap in the parameter space coverage, we perform simulations of black hole-neutron star mergers with mass ratios $q \in \{1, 1/2, 1/3\}$. We find the gravitational waveform models do not show good agreement with the numerical waveforms, with dephasing at the level of around 1 rad at the merger. We find that the masses of the dynamical ejecta and disk are in good agreement with the available fitting formulas. The analytical formulas for the remnant black hole are in excellent agreement for the black hole mass, but are less accurate with the predictions for its spin. Moreover, we analyze the remnant disk structure and dynamics, deriving the rotation law and identifying global trapped $g$-mode density oscillations. We distinguish three types of accretion in the postmerger and find modulation of the accretion rate by the global oscillations of the disk. Finally, we model the kilonova emission these systems would produce and find that most of them are potentially detectable by Vera C. Rubin Observatory within four days after merger, and by DECam within two days after merger if located at a distance of 200 Mpc.