Mass/Radius Constraints on the Quiescent Neutron Star in M13 Using Hydrogen and Helium Atmospheres

TL;DR

This study refines neutron star mass and radius estimates in M13 by analyzing X-ray spectra with different atmospheric compositions, revealing that assumptions about the atmosphere significantly affect inferred properties.

Contribution

It provides the first detailed spectral analysis considering both hydrogen and helium atmospheres for the neutron star in M13, highlighting the impact of atmospheric composition on radius estimates.

Findings

Hydrogen atmosphere models yield larger, more consistent radii.

Helium atmosphere models produce even larger radius estimates.

Atmospheric composition critically influences neutron star property inferences.

Abstract

The mass and radius of the neutron star (NS) in low-mass X-ray binaries can be obtained by fitting the X-ray spectrum of the NS in quiescence, and the mass and radius constrains the properties of dense matter in NS cores. A critical ingredient for spectral fits is the composition of the NS atmosphere: hydrogen atmospheres are assumed in most prior work, but helium atmospheres are possible if the donor star is a helium white dwarf. Here we perform spectral fits to XMM, Chandra, and ROSAT data of a quiescent NS in the globular cluster M13. This NS has the smallest inferred radius from previous spectral fitting. Assuming an atmosphere composed of hydrogen, we find a significantly larger radius, more consistent with those from other quiescent NSs. With a helium atmosphere (an equally acceptable fit), we find even larger values for the radius.