# The mass distribution of Galactic double neutron stars

## TL;DR

This study revisits the mass distribution of Galactic double neutron stars using Bayesian inference with an expanded sample, suggesting possible differences between recycled and non-recycled neutron star masses and exploring various distribution models.

## Contribution

It introduces a Bayesian framework with expanded models to analyze neutron star mass distributions, considering separate distributions for recycled and non-recycled stars.

## Key findings

- Support for different mass distributions for recycled and non-recycled neutron stars (Bayes factor 9).
- Most probable model involves a two-Gaussian distribution for recycled stars and a uniform distribution for non-recycled stars.
- Approximately 20 precise measurements are needed to confidently distinguish distribution differences.

## Abstract

The conventional wisdom, dating back to 2012, is that the mass distribution of Galactic double neutron stars is well-fit by a Gaussian distribution with a mean of $1.33 M_\odot$ and a width of $0.09 M_\odot$. With the recent discovery of new Galactic double neutron stars and GW170817, the first neutron star merger event to be observed with gravitational waves, it is timely to revisit this model. In order to constrain the mass distribution of double neutron stars, we perform Bayesian inference using a sample of 17 Galactic double neutron stars effectively doubling the sample used in previous studies. We expand the space of models so that the recycled neutron star need not be drawn from the same distribution as the non-recycled companion. Moreover, we consider different functional forms including uniform, single-Gaussian, and two-Gaussian distributions. While there is insufficient data to draw firm conclusions, we find positive support (a Bayes factor of 9) for the hypothesis that recycled and non-recycled neutron stars have distinct mass distributions. The most probable model---preferred with a Bayes factor of 29 over the conventional model---is one in which the recycled neutron star mass is distributed according to a two-Gaussian distribution and the non-recycled neutron star mass is distributed uniformly. We show that precise component mass measurements of $\approx 20$ double neutron stars are required in order to determine with high confidence (a Bayes factor of 150) if recycled and non-recycled neutron stars come from a common distribution. Approximately $60$ are needed in order to establish the detailed shape of the distributions.