Soft X-ray emission lines in the X-ray binary Swift J1858.6-0814 observed with XMM-Newton-RGS: disc atmosphere or wind?

TL;DR

This study analyzes soft X-ray emission lines in the X-ray binary Swift J1858.6-0814 using XMM-Newton-RGS data, exploring whether they originate from the disc atmosphere or a wind, and infers properties of the emitting plasma.

Contribution

It provides the first detailed characterization of soft X-ray emission lines in Swift J1858.6-0814, including plasma parameters and spatial constraints, to determine their origin.

Findings

Detected N VII, O VII, O VIII emission lines.

Inferred plasma ionization and density parameters.

Estimated the location of the emitting plasma relative to the neutron star.

Abstract

We find soft X-ray emission lines from the X-ray binary Swift J1858.6-0814 in data from XMM-Newton-RGS: N VII, O VII and O VIII, as well as notable residuals short of a detection at Ne IX and other higher ionisation transitions. These could be associated with the disc atmosphere, as in accretion disc corona sources, or with a wind, as has been detected in Swift J1858.6-0814 in emission lines at optical wavelengths. Indeed, the N VII line is redshifted, consistent with being the emitting component of a P-Cygni profile. We find that the emitting plasma has an ionisation parameter $\log(\xi)=1.35\pm0.2$ and a density $n>1.5\times10^{11}$ cm$^{-3}$. From this, we infer that the emitting plasma must be within $10^{13}$ cm of the ionising source, $\sim5\times10^{7}r_{\rm g}$ for a $1.4M_{\odot}$ neutron star, and from the line width that it is at least $10^4r_{\rm g}$ away ($2\times10^{9}(M/1.4M_{\odot})$ cm). We compare this with known classes of emission line regions in other X-ray binaries and active galactic nuclei.