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

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.