Constraining the neutron star equation of state using XMM-Newton

TL;DR

This paper discusses three future XMM-Newton observational strategies to better constrain the neutron star equation of state by analyzing spectral features, burst properties, and system inclination.

Contribution

It proposes new observational methods using XMM-Newton data to improve measurements of neutron star properties relevant to the equation of state.

Findings

Potential to detect redshifted spectral lines from neutron star surfaces.

Ability to constrain interstellar extinction affecting spectral analysis.

Precise measurement of neutron star mass through eclipse duration and optical data.

Abstract

We have identified three possible ways in which future XMM-Newton observations can provide significant constraints on the equation of state of neutron stars. First, using a long observation of the neutron star X-ray transient CenX-4 in quiescence one can use the RGS spectrum to constrain the interstellar extinction to the source. This removes this parameter from the X-ray spectral fitting of the pn and MOS spectra and allows us 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 spectral component can indeed be due to the hot thermal glow of the neutron star. Potentially such an observation could also reveal redshifted spectral lines from the neutron star surface. Second, XMM-Newton observations of radius expansion type I X-ray bursts might reveal redshifted absorption lines from the surface of the neutron star. Third, XMM-Newton observations of eclipsing quiescent low-mass X-ray binaries provide the eclipse duration. With this the system inclination can be determined accurately. The inclination determined from the X-ray eclipse duration in quiescence, the rotational velocity of the companion star and the semi-amplitude of the radial velocity curve determined through optical spectroscopy, yield the neutron star mass.