Constraining neutron star radii in black hole-neutron star mergers from their electromagnetic counterparts

TL;DR

This paper proposes a method to constrain neutron star radii in black hole-neutron star mergers by combining gravitational wave data and electromagnetic observations, aiding in understanding the neutron star equation of state.

Contribution

It introduces a novel approach to estimate neutron star radii using EM counterparts and GW measurements, especially focusing on the presence or absence of a remnant disk.

Findings

Current GW data limitations prevent precise radius constraints.

Method is most effective with large aligned black hole spins.

Can complement GW data to improve neutron star equation of state understanding.

Abstract

Mergers of black hole (BH) and neutron star (NS) binaries are of interest since the emission of gravitational waves (GWs) can be followed by an electromagnetic (EM) counterpart, which could power short gamma-ray bursts. Until now, LIGO/Virgo has only observed a candidate BH-NS event, GW190426\_152155, which was not followed by any EM counterpart. We discuss how the presence (absence) of a remnant disk, which powers the EM counterpart, can be used along with spin measurements by LIGO/Virgo to derive a lower (upper) limit on the radius of the NS. For the case of GW190426\_152155, large measurement errors on the spin and mass ratio prevent from placing an upper limit on the NS radius. Our proposed method works best when the aligned component of the BH spin (with respect to the orbital angular momentum) is the largest, and can be used to complement the information that can be extracted from the GW signal to derive valuable information on the NS equation of state.