Relativistic Lines and Reflection from the Inner Accretion Disks Around Neutron Stars

TL;DR

This study systematically analyzes X-ray spectra of 10 neutron star low-mass X-ray binaries, revealing broad Fe K emission lines consistent with relativistic effects, indicating the accretion disk extends close to the neutron star surface across various luminosities.

Contribution

It provides the first comprehensive analysis of relativistic Fe K lines in multiple neutron star binaries, confirming the inner disk radius range and the boundary layer's role in disk illumination.

Findings

Fe K lines fit by relativistic models imply inner disk radii of 6-15 GM/c^2.

The accretion disk extends close to the neutron star surface across different luminosities.

A blackbody component likely dominates the ionizing flux illuminating the disk.

Abstract

A number of neutron star low-mass X-ray binaries have recently been discovered to show broad, asymmetric Fe K emission lines in their X-ray spectra. These lines are generally thought to be the most prominent part of a reflection spectrum, originating in the inner part of the accretion disk where strong relativistic effects can broaden emission lines. We present a comprehensive, systematic analysis of Suzaku and XMM-Newton spectra of 10 neutron star low-mass X-ray binaries, all of which display broad Fe K emission lines. Of the 10 sources, 4 are Z sources, 4 are atolls and 2 are accreting millisecond X-ray pulsars (also atolls). The Fe K lines are well fit by a relativistic line model for a Schwarzschild metric, and imply a narrow range of inner disk radii (6 - 15 GM/c^2) in most cases. This implies that the accretion disk extends close to the neutron star surface over a range of luminosities. Continuum modeling shows that for the majority of observations, a blackbody component (plausibly associated with the boundary layer) dominates the X-ray emission from 8 - 20 keV. Thus it appears likely that this spectral component produces the majority of the ionizing flux that illuminates the accretion disk. Therefore, we also fit the spectra with a blurred reflection model, wherein a blackbody component illuminates the disk. This model fits well in most cases, supporting the idea that the boundary layer is illuminating a geometrically thin disk.