Ways to constrain neutron star equation of state models using relativistic disc lines

TL;DR

This paper explores how relativistic spectral lines from neutron star accretion discs can be used to constrain neutron star equations of state by comparing observed disc inner edges with theoretical models.

Contribution

The authors perform extensive general relativistic calculations of neutron star structures to evaluate and improve methods for constraining EoS models using relativistic disc lines.

Findings

Kerr spacetime is inadequate for modeling neutron star discs.

Systematic calculations of 16,000 neutron star structures were performed.

The study clarifies the relationship between disc inner edge and neutron star properties.

Abstract

Relativistic spectral lines from the accretion disc of a neutron star low-mass X-ray binary can be modelled to infer the disc inner edge radius. A small value of this radius tentatively implies that the disc terminates either at the neutron star hard surface, or at the innermost stable circular orbit (ISCO). Therefore an inferred disc inner edge radius either provides the stellar radius, or can directly constrain stellar equation of state (EoS) models using the theoretically computed ISCO radius for the spacetime of a rapidly spinning neutron star. However, this procedure requires numerical computation of stellar and ISCO radii for various EoS models and neutron star configurations using an appropriate rapidly spinning stellar spacetime. We have fully general relativistically calculated about 16000 stable neutron star structures to explore and establish the above mentioned procedure, and to show that the Kerr spacetime is inadequate for this purpose. Our work systematically studies the methods to constrain EoS models using relativistic disc lines, and will motivate future X-ray astronomy instruments.