Maximum mass cutoff in the neutron star mass distribution and the prospect of forming supramassive objects in the double neutron star mergers

TL;DR

This study infers a bimodal mass distribution with a sharp cutoff at about 2.26 solar masses for neutron stars, suggesting many mergers could produce supramassive remnants capable of generating ultra-high-energy cosmic rays.

Contribution

It introduces Bayesian models to robustly estimate the neutron star mass distribution and evidence for a maximum mass cutoff, impacting merger remnant predictions.

Findings

Evidence for a bimodal neutron star mass distribution.

Maximum neutron star mass cutoff at approximately 2.26 solar masses.

High-energy cosmic-ray production from neutron star mergers.

Abstract

The sample of neutron stars with a measured mass is growing quickly. With the latest sample, we adopt both a flexible Gaussian mixture model and a Gaussian plus Cauchy-Lorentz component model to infer the mass distribution of neutron stars and use the Bayesian model selection to explore evidence for multimodality and a sharp cutoff in the mass distribution. The two models yield rather similar results. Consistent with previous studies, we find evidence for a bimodal distribution together with a cutoff at a mass of $M_{\rm max}=2.26_{-0.05}^{+0.12}M_\odot$ (68% credible interval; for the Gaussian mixture model). If such a cutoff is interpreted as the maximum gravitational mass of nonrotating cold neutron stars, the prospect of forming supramassive remnants is found to be quite promising for the double neutron star mergers with a total gravitational mass less than or equal to 2.7$M_\odot$ unless the thermal pions could substantially soften the equation of state for the very hot neutron star matter. These supramassive remnants have a typical kinetic rotational energy of approximately $1-2\times 10^{53}$ ergs. Together with a high neutron star merger rate approximately $10^{3}~{\rm Gpc^{-3}~yr^{-3}}$, the neutron star mergers are expected to be significant sources of EeV($10^{18}$eV) cosmic-ray protons.