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

TL;DR

This study targets BHNS mergers with symmetric and near-symmetric mass ratios by performing twelve NR simulations for $q \in \{1,1/2,1/3\}$ across two NS EOSs and multiple NS masses. It assesses waveform-model accuracy by comparing NR waveforms to standard BHNS models, finding typically $\mathcal{O}(1)$ radian dephasing near merger and limitations when models are used outside their calibration region. The work also characterizes remnant properties (dynamical ejecta, disk, BH mass/spin), reveals disk dynamics including evolving rotation laws and global $g$-mode oscillations that modulate accretion, and models kilonova emission, concluding that such events at 200 Mpc would be detectable by wide-field surveys within days after merger. Overall, the results highlight the need for broader NR coverage to calibrate waveform models across symmetric mass ratios and improve predictions for electromagnetic counterparts.

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.

Figures (9)

• Figure 1: Numerical waveforms and their dephasing with the waveform models. Upper panels: real part of the (2,2)-mode of the GW strain. Lower panels: phase difference $\phi_\mathrm{NR} - \phi_\mathrm{model}$ between the numerical waveform and each of the waveform models. The gray area represents the NR waveform error, estimated as the absolute phase difference between the waveform phases at the highest and medium resolutions. The waveform model for the DD2_mNS0.8_mBH1.6 is shown in dashed lines to highlight that it was used outside of its allowed parameter space.
• Figure 2: The predictions of the dynamical ejecta mass of the fitting formula of Ref. Kruger:2020gig plotted against the corresponding simulation data of this work and of Refs. Kawaguchi:2015bwaFoucart:2019bxj.
• Figure 3: Remnant baryonic mass normalized by the initial baryonic mass of the NS predicted by the fitting formula of Ref. Foucart:2018rjc, $\hat{M}^\mathrm{rem}_\mathrm{fit}$, plotted against the values obtained from the simulations in this work, $\hat{M}^\mathrm{rem}_\mathrm{NR}$. For comparison, the configurations used for calibration of the Foucart et al. (2018) Foucart:2018rjc model are plotted additionally.
• Figure 4: Disk matter density in the equatorial plane at selected times for two BHNS configurations with $m_\mathrm{NS}=1.4$ and $q=\{1, 1/2\}$, with the black circles with white outline showing the apparent horizons. In the equal-mass ratio, the disk circularizes almost immediately, whereas in the $q=1/2$ case, the disk is highly perturbed by spiral density wave interaction and the fallback matter.
• Figure 5: Azimuthal angle- and time-averaged radial profiles of the normalized disk density, specific angular momentum $j$ normalized to the BH mass, and the inferred rotation law power $n$. The gray horizontal lines in the last panel delineate constant-$j$ ($n=0$) and Keplerian ($n=1/3$) regimes. The solid gray vertical line shows the ISCO radius, and the dashed vertical lines show the radius at the maximum density of the disk.