Broad iron lines in neutrons stars: dynamical broadening or wind scattering?

TL;DR

This paper investigates whether broad iron lines in neutron star systems are caused by relativistic effects or wind scattering, finding evidence that supports an inner disk origin over wind scattering.

Contribution

The study provides observational evidence against wind scattering as the primary cause of broad iron lines, favoring relativistic broadening from the inner accretion disk.

Findings

No correlation between line width and column density in data

Supports inner disk origin over wind scattering for line broadening

Challenges alternative wind scattering models for broad iron lines

Abstract

Broad iron emission lines are observed in many accreting systems from black holes in AGN and X-ray binaries to neutron star low-mass X-ray binaries. The origin of the line broadening is often interpreted as due to dynamical broadening and relativistic effects. However, alternative interpretations have been proposed, included broadening due to Compton scattering in a wind or accretion disk atmosphere. Here we explore the observational signatures expected from broadening in a wind, in particular that the iron line width should increase with an increase in the column density of the absorber (due to an increase in the number of scatterings). We study the data from three neutron star low-mass X-ray binaries where both a broad iron emission line and absorption lines are seen simultaneously, and show that there is no significant correlation between line width and column density. This favors an inner disk origin for the line broadening rather than scattering in a wind.