Iron line diagnostics of the stellar wind in X1908+075

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.