On GW170817 and the Galactic Binary Neutron Star Population

Abstract

GW170817/GRB170817A, a short gamma-ray burst arising from a low-mass compact object merger was the first multi-messenger discovery of a compact binary system outside the local galactic neighborhood. From gravitational-wave measurements, we know GW170817 has a wide range of plausible component masses, depending also on less well-constrained properties such as the spin and tidal deformability of the component stars. The kilonova light curve --- and hence the total ejecta mass from a given source --- depends on the relative contribution of dynamical ejecta and other sources such as disk winds. Electromagnetic observations and model fitting of the ejecta properties from the subsequent kilonova detection provided values of the ejecta mass from the merger. These values, when combined with the gravitational-wave measurement disfavors an equal-mass configuration, with the level of disagreement dependent on the assumed amount of ejecta mass of dynamical origin. Within the confines of our own galaxy, several binary neutron star systems along with measurements of their component masses have been made. If those distributions are indicative of a universal distribution, the joint measurement of the component masses of GW170817 represents an outlier. This tension is not easily resolvable from physical arguments, as the proposed pathways which form binary neutron stars do not often produce very asymmetrical pairs. Even accounting for the uncertainty associated with the total mass of the dynamical ejecta, this tension suggests that the distribution of binary neutron star masses in the galaxy is not indicative of those in other galaxies.