A comparison of broad iron emission lines in archival data of neutron star low-mass X-ray binaries

TL;DR

This study compares iron emission line profiles in neutron star low-mass X-ray binaries across different instruments, confirming the intrinsic broadness of lines and identifying asymmetric features better fitted by relativistic disk models.

Contribution

It provides a cross-instrument validation of broad iron line profiles and demonstrates the presence of asymmetric lines better modeled by relativistic disk-line profiles.

Findings

Good consistency between gas-based and CCD data for line profiles

Line widths depend on continuum models in low-resolution instruments

Evidence for asymmetric lines better fitted by relativistic models

Abstract

Relativistic X-ray disk-lines have been found in multiple neutron star low-mass X-ray binaries, in close analogy with black holes across the mass-scale. These lines have tremendous diagnostic power and have been used to constrain stellar radii and magnetic fields, often finding values that are consistent with independent timing techniques. Here, we compare CCD-based data from Suzaku with Fe K line profiles from archival data taken with gas-based spectrometers. In general, we find good consistency between the gas-based line profiles from EXOSAT, BeppoSAX and RXTE and the CCD data from Suzaku, demonstrating that the broad profiles seen are intrinsic to the line and not broad due to instrumental issues. However, we do find that when fitting with a Gaussian line profile, the width of the Gaussian can depend on the continuum model in instruments with low spectral resolution, though when the different models fit equally well the line widths generally agree. We also demonstrate that three BeppoSAX observations show evidence for asymmetric lines, with a relativistic disk-line model providing a significantly better fit than a Gaussian. We test this by using the posterior predictive p-value method, and bootstrapping of the spectra to show that such deviations from a Gaussian are unlikely to be observed by chance.