Characterizing Astrophysical Binary Neutron Stars with Gravitational Waves

TL;DR

This paper introduces a new gravitational wave-based framework to characterize binary neutron stars, incorporating prior spin distributions, and applies it to GW170817 and GW190425 to infer their properties and formation scenarios.

Contribution

It proposes a novel astrophysical characterization method for binary neutron stars using gravitational wave data and a gamma-distributed prior for recycled neutron star spins.

Findings

Support for a spinning recycled neutron star in GW190425

Mass measurements of neutron stars consistent with prior expectations

Implications for binary neutron star formation scenarios

Abstract

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 spins of recycled neutron stars, given by a gamma distribution with a shape parameter of 2 and a scale parameter of 0.012, extrapolated from radio pulsar observations of Galactic binary neutron stars. Interpreting GW170817 and GW190425 in the context of the standard formation scenario and adopting the gamma-distribution prior, we find positive support (with a Bayes factor of 6, over the nonspinning hypothesis) for a spinning recycled neutron star in GW190425, whereas the spin of the recycled neutron star in GW170817 is small and consistent with our prior. We measure the masses of the recycled (slow) neutron stars 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. We discuss implications for the astrophysical origins of these two events and outline future prospects of studying binary neutron stars using our framework.