On the maximum mass and oblateness of rotating neutron stars with generic equations of state

TL;DR

This study uses over a million generic equations of state to analyze the maximum mass and shape of rotating neutron stars, revealing new universal relations and improving constraints on neutron star properties.

Contribution

It introduces a new relation for surface oblateness and provides highly accurate quasi-universal maximum-mass ratios across rotation rates.

Findings

Established a new universal relation for surface oblateness.

Determined maximum-mass ratios with unprecedented accuracy.

Enhanced understanding of neutron star structure for gravitational wave and X-ray observations.

Abstract

A considerable effort has been dedicated recently to the construction of generic equations of state (EOSs) for matter in neutron stars. The advantage of these approaches is that they can provide model-independent information on the interior structure and global properties of neutron stars. Making use of more than $10^6$ generic EOSs, we asses the validity of quasi-universal relations of neutron star properties for a broad range of rotation rates, from slow-rotation up to the mass-shedding limit. In this way, we are able to determine with unprecedented accuracy the quasi-universal maximum-mass ratio between rotating and nonrotating stars and reveal the existence of a new relation for the surface oblateness, i.e., the ratio between the polar and equatorial proper radii. We discuss the impact that our findings have on the imminent detection of new binary neutron-star mergers and how they can be used to set new and more stringent limits on the maximum mass of nonrotating neutron stars, as well as to improve the modelling of the X-ray emission from the surface of rotating stars.