High resolution X-ray spectroscopy of Supergiant HMXB 4U1700$-$37 during the compact object eclipse

TL;DR

This study uses high-resolution X-ray spectroscopy during an eclipse of the supergiant HMXB 4U1700-37 to analyze the stellar wind structure and ionization conditions, revealing complex multi-ionization regions and wind clumping.

Contribution

It provides the first detailed high-resolution spectral analysis during eclipse, identifying multiple ionization zones and wind clumping in the HMXB, which advances understanding of stellar wind environments.

Findings

Emission lines from near-neutral species are unaffected during eclipse, indicating wind origin.

Highly ionized lines diminish during eclipse, produced near the compact object.

A combination of low and high ionization models explains the emission spectrum.

Abstract

We present an analysis of the first observation of the iconic High Mass X-ray Binary \so with the \chandra High Energy Transmission Gratings during an X-ray eclipse. The goal of the observation was to study the structure/physical conditions in the clumpy stellar wind through high resolution spectroscopy. We find that: a) emission line brightness from K shell transitions, corresponding to near neutral species, directly correlates with continuum illumination. However, these lines do not greatly diminish during eclipse. This is readily explained if fluorescence K$\alpha$ emission comes from the bulk of the wind. b) The highly ionised Fexxv and Fexxvi Ly$\alpha$ diminish during eclipse. Thus, they must be produced in the vicinity of the compact object where $\log \xi >3$. c) to describe the emission line spectrum, the sum of two self consistent photo ionisation models with low ionisation ($\log \xi\sim -1$) and high ionisation ($\log \xi\sim 2.4$) is required. From their emission measures, the clump-to-interclump density ratio can be estimated to be $n_c/n_i\sim 300$. To fit the complex He-like \ion{Si}{xiii}{} profile, the plasma requires a broadening with $v_{\rm bulk}\sim 840$ km s$^{-1}$. Reproducing the observed $r\approx f$ line fluxes requires the addition of a third collisionally ionised plasma. d) Emission lines widths appear unresolved at the \textsc{hetg} gratings resolution with exception of Silicon. There is no clear radial segregation between (quasi)neutral and ionised species, consistent with cold wind clumps interspersed in a hot rarefied interclump medium.