Relativistic iron emission lines in neutron star low-mass X-ray binaries as probes of neutron star radii

TL;DR

This study uses Suzaku X-ray observations to analyze relativistic iron emission lines in neutron star low-mass X-ray binaries, providing constraints on neutron star radii and insights into accretion disk properties.

Contribution

It presents the first measurement of neutron star radii using relativistic Fe K lines, linking these measurements with kHz QPOs and spin inferences.

Findings

Inner disk radii imply neutron star radii of 14.5-16.5 km for a 1.4 solar mass star.

Fe K line widths are narrower than in black holes, consistent with neutron star spin expectations.

Inner disk radii measurements agree with those inferred from kHz QPOs.

Abstract

Using Suzaku observations of three neutron star low-mass X-ray binaries (Ser X-1, 4U 1820-30 and GX 349+2) we have found broad, asymmetric, relativistic Fe K emission lines in all three objects. These Fe K lines can be well fit by a model for lines from a relativistic accretion disk ('diskline'), allowing a measurement of the inner radius of the accretion disk, and hence an upper limit on the neutron star radius. These upper limits correspond to 14.5 - 16.5 km for a 1.4 M(solar) neutron star. The inner disk radii we measure with Fe K lines are in good agreement with the inner disk radii implied by kHz QPOs observed in both 4U 1820-30 and GX 349+2, supporting the inner disk nature of kHz QPOs. Additionally, the Fe K lines observed in these neutron stars are narrower than those in the black holes that are thought to be close to maximally spinning, as one would expect if inferences for spin are robust.