Iron line diagnostics of the stellar wind in X1908+075

TL;DR

This study analyzes Chandra X-ray observations of the supergiant X-ray binary X1908+075 to characterize its stellar wind, investigate Fe Kα line variability, and constrain system geometry and wind properties.

Contribution

It provides the first detailed characterization of the stellar wind and system geometry of X1908+075 using high-resolution X-ray spectroscopy and modeling.

Findings

Fe Kα emission detected in all observations with significant statistical confidence.

The Fe Kα flux correlates with continuum flux, and its equivalent width anticorrelates with $N_{H}$ and orbital phase.

Modeling yields an inclination of 46 degrees and a donor mass-loss rate of approximately 9.1 x 10^{-7} solar masses per year.

Abstract

Aims. X1908+075 is a supergiant X-ray binary (SgXB) composed of an evolved OB star and a neutron star (NS) in a 4.4-day orbit. We aim to characterize the stellar wind, constrain the system geometry, and investigate the origin and variability of the Fe K$\alpha$ fluorescence line. Methods. We analyzed three Chandra HETGS observations at different orbital phases. The continuum was modeled with a bulk motion Comptonization (Bmc) model including partial-covering absorption. We performed a blind line search with Monte Carlo simulations accounting for the look-elsewhere effect. The orbital modulation of $N_{\rm H}$ was modeled through particle swarm optimization to constrain the inclination and donor mass-loss rate. Results. Fe K$\alpha$ emission is detected in all observations and remains significant after correcting for multiple trials (global $p < 0.005$). Tentative features include a Compton shoulder and Fe K$\beta$ emission, indicating dense reprocessing material. No highly ionized Fe XXV or Fe XXVI lines are confirmed. The Fe K$\alpha$ flux correlates with the continuum flux, while its equivalent width anticorrelates with both $N_{\rm H}$ and orbital phase, opposite to the canonical curve of growth seen in many HMXBs. Line broadening reaches velocities of $\sim3000$ km s$^{-1}$. Modeling of the $N_{\rm H}$ variability yields an inclination $i = 46 \pm 3^\circ$ and a donor mass-loss rate $\dot{M}_{\rm W} = (9.1 \pm 1.6) \times 10^{-7}\,M_\odot\,{\rm yr}^{-1}$. Conclusions. X1908+075 is a classical wind-fed SgXB in the direct accretion regime. The observed continuum and line variability reflect the complex density and ionization structure of the stellar wind.