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

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/Virgohas 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 disc, 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.

Automated summary

Mergers of black hole (BH) and neutron star (NS) binaries are of interest due to the potential for electromagnetic counterparts following gravitational wave emissions. A proposed method utilizes the presence (absence) of a remnant disc and spin measurements to derive lower (upper) limits on the radius of the NS. This method is most effective when the aligned component of the BH spin is the largest, providing valuable information on the NS equation of state beyond what can be extracted from the GW signal.