Revisiting the Rhoades-Ruffini bound

TL;DR

This paper revisits the Rhoades-Ruffini bound on neutron star maximum mass, showing that relaxing certain assumptions can significantly increase the upper mass limit to over 4 solar masses.

Contribution

It demonstrates that the original bound is not strict and provides a new upper limit considering non-nucleonic matter onset at lower densities.

Findings

Upper mass limit can exceed 4 solar masses.

Relaxing assumptions increases the maximum neutron star mass.

Provides a formula linking maximum mass to sound speed and transition density.

Abstract

We revisit the derivation of the Rhoades-Ruffini bound on the upper limit for the maximum mass of neutron stars and find that the assumption made there for the onset of an ultimately stiff phase of high-density matter is not stringent. Relaxing this assumption and allowing for an onset of stiff non-nucleonic matter under neutron star constraints at the saturation density or below boost the upper limit of the theoretically possible maximum mass to $4~M_\odot$ or higher, in the mass-gap region between neutron stars and stellar-mass black holes. We provide a fit formula for the dependence of this upper limit on the speed of sound and the onset density of the deconfinement transition.