Looking through the photoionisation wake: Vela X-1 at phi orb ~ 0.75 with Chandra/HETG

Context. The supergiant X-ray binary Vela X−1 represents one of the best astrophysical sources to investigate the wind environment of an O/B star irradiated by an accreting neutron star. Previous studies and hydrodynamic simulations of the system have revealed a clumpy environment and the presence of two wakes: an accretion wake surrounding the compact object and a photoionisation wake trailing it along the orbit.Aims. Our goal is to conduct, for the first time, high-resolution spectroscopy on Chandra /HETGS data at the orbital phase φ orb  ≈ 0.75, when the line of sight is crossing the photoionisation wake. We aim to conduct plasma diagnostics, inferring the structure and the geometry of the wind.Methods. We performed a blind search employing a Bayesian block algorithm to find discrete spectral features and identify them thanks to the most recent laboratory results or through atomic databases. Plasma properties were inferred both with empirical techniques and with photoionisation models within CLOUDY and SPEX.Results. We detect and identify five narrow radiative recombination continua (Mg XI-XII, Ne IX-X, O VIII) and several emission lines from Fe, S, Si, Mg, Ne, Al, and Na, including four He-like triplets (S XV, Si XIII, Mg XI, and Ne IX). Photoionisation models reproduce the overall spectrum well, except for the near-neutral fluorescence lines of Fe, S, and Si.Conclusions. We conclude that the plasma is mainly photoionised, but more than one component is most likely present, which is consistent with a multi-phase plasma scenario, where denser and colder clumps of matter are embedded in the hot, photoionised wind of the companion star. Simulations with the future X-ray satellites Athena and XRISM show that a few hundred seconds of exposure is sufficient to disentangle the lines of the Fe Kα doublet and the He-like Fe XXV, improving, in general, the determination of the plasma parameters.