Characterizing Astrophysical Binary Neutron Stars with Gravitational Waves.

Merging binary neutron stars are thought to be formed predominantly via isolated binary evolution. In this standard formation scenario, the first-born neutron star goes through a recycling process and might be rapidly spinning during the final inspiral, whereas the second-born star is expected to have effectively zero spin at merger. Based on this feature, we propose a new framework for the astrophysical characterization of binary neutron stars observed from their gravitational wave emission. We further propose a prior for the dimensionless spin magnitudes of recycled neutron stars, given by a gamma distribution with a shape parameter of 2 and a scale parameter of 0.012, from radio pulsar observations of Galactic binary neutron stars. Interpreting GW170817 and GW190425 in the context of the standard formation scenario, we find positive support for a spinning recycled neutron star (with a spin tilt angle $\lesssim 60^{\circ}$) in GW190425 with a Bayes factor of 6, whereas the spin of the recycled neutron star in GW170817 is small and consistent with our prior. We measure the mass of the recycled (slow) neutron star in GW170817 and GW190425 to be $1.34_{-0.09}^{+0.12}$ ($1.38_{-0.11}^{+0.11}$) $M_{\odot}$ and $1.64_{-0.11}^{+0.13}$ ($1.66_{-0.12}^{+0.12}$) $M_{\odot}$, with $68\%$ credibility, respectively. The mass ratio of GW170817 (GW190425) is constrained to be between 0.79 (0.80) and 1, with 90\% credibility.