Speed of sound constraints on maximally rotating neutron stars

TL;DR

This paper investigates how upper bounds on the speed of sound in dense nuclear matter affect the properties of maximally rotating neutron stars, providing theoretical constraints and implications for astrophysical observations.

Contribution

It offers a theoretical analysis of the impact of sound speed limits on neutron star characteristics, focusing on the lower bound $v_s = c/\sqrt{3}$ and its astrophysical consequences.

Findings

The $v_s=c/\sqrt{3}$ limit significantly reduces maximum neutron star mass and moment of inertia.

Constraints on the Kerr parameter can inform the true upper bound of sound speed.

Relations between bulk properties and transition density are derived and compared with nonrotating cases.

Abstract

The observation of maximally rotating neutron stars (in comparison to nonrotating ones) may provide more information on the behavior of nuclear matter at high densities. We provide a theoretical treatment concerning the effects of the upper bound of the sound speed in dense matter on the bulk properties of maximally rotating (at mass-shedding limit) neutron stars. In particular, we consider two upper bounds for the speed of sound, $v_s = c$ and $v_s = c/\sqrt{3}$, and the one provided by the relativistic kinetic theory. We investigate to what extent the possible predicted (from various theories and conjectures) upper bounds on the speed of sound constrain the ones of various key quantities, including the maximum mass and the corresponding radius, Keplerian frequency, Kerr parameter and moment of inertia. We mainly focus on the lower proposed limit, $v_{s}=c/\sqrt{3}$, and we explore in which mass region a rotating neutron star collapses to a black hole. In any case, useful relations of the mentioned bulk properties with the transition density are derived and compared with the corresponding nonrotating cases. We concluded that the proposed limit $v_{s}=c/\sqrt{3}$ leads to dramatic decrease on the values of the maximum mass, Kerr parameter and moment of inertia preventing a neutron star to reach values which derived with the consideration of realistic equations of state or from other constraints. Possible measurements of the Kerr parameter and moment of inertia would shed light on these issues and help to reveal the speed of sound bound in dense matter.