An XMM-Newton view of FeK{\alpha} in HMXBs

TL;DR

This study analyzes XMM-Newton observations of 46 High Mass X-ray Binaries to investigate the properties and origins of the FeKα emission line, revealing its characteristics and relation to the circumstellar environment.

Contribution

It provides a comprehensive analysis of FeKα in HMXBs, including its centroid, width, and correlation with continuum flux, highlighting the donor wind's role in emission and absorption.

Findings

FeKα centroid at 6.42 keV, indicating low ionization states.

Line flux correlates with continuum flux.

FeKα is narrow, suggesting low-velocity reprocessing material.

Abstract

We present a comprehensive analysis of the whole sample of available XMM-Newton observations of High Mass X-ray Binaries (HMXBs) until August, 2013, focusing on the FeK{\alpha} emission line. This line is a key tool to better understand the physical properties of the material surrounding the X-ray source within a few stellar radii (the circumstellar medium). We have collected observations from 46 HMXBs, detecting FeK{\alpha} in 21 of them. We have used the standard classification of HMXBs to divide the sample in different groups. We find that: (1) FeK{\alpha} is centred at a mean value of 6.42 keV. Considering the instrumental and fits uncertainties, this value is compatible with ionization states lower than FeXVIII. (2) The flux of the continuum is well correlated with the flux of the line, as expected. Eclipse observations show that the Fe fluorescence emission comes from an extended region surrounding the X-ray source. (3) FeK{\alpha} is narrow (width lower than 0.15keV), reflecting that the reprocessing material does not move at high speeds. We attempt to explain the broadness of the line in terms of three possible broadening phenomena: line blending, Compton scattering and Doppler shifts (with velocities of the reprocessing material V=1000-2000 km/s). (4) The equivalent hydrogen column (NH) directly correlates with the EW of FeK{\alpha}, displaying clear similarities to numerical simulations. It highlights the strong link between the absorbing and the fluorescent matter. The obtained results clearly point to a very important contribution of the donors wind in the FeK{\alpha} emission and the absorption when the donor is a supergiant massive star.