GW190814: Spin and equation of state of a neutron star companion

Antonios Tsokaros, Milton Ruiz, Stuart L. Shapiro

Published 2020-07-10Version 1

The recent discovery by LIGO/Virgo of a merging binary having a $\sim 23 M_\odot$ black hole and a $\sim 2.6 M_\odot$ compact companion has triggered a debate regarding the nature of the secondary, which falls into the so-called mass gap. Here we explore some consequences of the assumption that the secondary was a neutron star (NS). We show with concrete examples of heretofore viable equations of state (EOSs) that rapid uniform rotation is certainly adequate to explain the existence of a stable $\sim 2.6 M_\odot$ NS for moderately stiff EOS but may not be adequate for soft EOSs. Moreover, rotation may not even be necessary for sufficiently stiff ones. In particular, several soft EOSs favored by GW170817 and with maximum spherical masses of $\sim 2.1 M_\odot$ \textit{cannot} be invoked to explain this object as a uniformly rotating NS. Absolute upper mass limits on the maximum spherical NS derived from GW170817 immediately suggest that this unknown compact companion can be a slowly or even a nonrotating NS. Stiff EOSs that can achieve this are neither rejected nor favored by GW170817, and they are in accord with the results of NICER.