Constraining the neutron star equation of state using quiescent low-mass X-ray binaries

TL;DR

This paper discusses how X-ray observations of quiescent low-mass X-ray binaries can constrain neutron star equations of state by measuring mass and radius, emphasizing the importance of accurate distance and atmosphere modeling.

Contribution

It proposes a detailed observational strategy using XMM-Newton to distinguish thermal and non-thermal spectral components, improving neutron star parameter estimates.

Findings

Potential to constrain neutron star radii with improved spectral modeling.

Assessment of thermal component variability in quiescent X-ray spectra.

Identification of promising sources for mass and radius determination.

Abstract

Chandra or XMM-Newton observations of quiescent low-mass X-ray binaries can provide important constraints on the equation of state of neutron stars. The mass and radius of the neutron star can potentially be determined from fitting a neutron star atmosphere model to the observed X-ray spectrum. For a radius measurement it is of critical importance that the distance to the source is well constrained since the fractional uncertainty in the radius is at least as large as the fractional uncertainty in the distance. Uncertainties in modelling the neutron star atmosphere remain. At this stage it is not yet clear if the soft thermal component in the spectra of many quiescent X-ray binaries is variable on timescales too short to be accommodated by the cooling neutron star scenario. This can be tested with a long XMM-Newton observation of the neutron star X-ray transient CenX-4 in quiescence. With such an observation one can use the Reflection Grating Spectrometer spectrum to constrain the interstellar extinction to the source. This removes this parameter from the X-ray spectral fitting of the EPIC pn and MOS spectra and allows one to investigate whether the variability observed in the quiescent X-ray spectrum of this source is due to variations in the soft thermal spectral component or variations in the power law spectral component coupled with variations in N_H. This will test whether the soft thermal component can indeed be due to the hot thermal glow of the neutron star. Irrespective of the outcome of such a study, the observed cooling in quiescence in sources for which the crust is significantly out of thermal equilibrium with the core due to a prolonged outburst, such as KS 1731-260, seem excellent candidates for mass and radius determinations through modelling the observed X-rays with a neutron star atmosphere model.