Total gravitational mass of the Galactic Double Neutron Star systems: evidence for a bimodal distribution

TL;DR

This study analyzes the total gravitational mass distribution of Galactic double neutron star systems, revealing a bimodal distribution with implications for neutron star mergers and the equation of state.

Contribution

It provides the first evidence for a bimodal distribution of total neutron star masses in the Galaxy, with potential to inform neutron star physics and merger observations.

Findings

Distribution is better described by a double Gaussian than a single Gaussian.

Low and high mass populations center at approximately 2.58 and 2.72 solar masses.

Upcoming gravitational wave detections will help further probe the mass distribution.

Abstract

So far, in total 15 double neutron star systems (DNSs) with a reliable measurement of the total gravitational mass ($M_{\rm T}$) have been detected in the Galaxy. In this work we study the distribution of $M_{\rm T}$. The data prefer the double Gaussian distribution over a single Gaussian distribution and the low and high mass populations center at $M_{\rm T}\sim 2.58M_\odot$ and $\sim 2.72M_\odot$, respectively. The progenitor stars of GW170817 have a $M_{\rm T}=2.74^{+0.04}_{-0.01}M_\odot$, falling into the high mass population. With a local neutron star merger rate of $\sim 10^{3}~{\rm Gpc^{-3}~yr^{-1}}$, supposing the $M_{\rm T}$ of those merging neutron stars also follow the double Gaussian distribution, the upcoming runs of the advanced LIGO/Virgo will soon detect some events with a $M_{\rm T}\lesssim 2.6M_\odot$ that can effectively probe the equation of state of the neutron stars and the distribution function is expected to be reliably reconstructed in the next decade.