First look at Vela X-1 with XRISM. A simultaneous campaign with XMM-Newton and NuSTAR

High-mass X-ray binaries (HMXBs) can serve as useful laboratories for exploring the behaviour of accreted matter onto compact objects and for probing the complex wind environments of massive stars. These investigations are essential for understanding stellar life cycles and the dynamics of the Milky Way, and they are prominent topics in the science cases for and newathena. We report, for the first time, on a observation of the HMXB Vela X-1, conducted during the first cycle of the general observer programme and complemented by simultaneous and νstar coverage. This campaign targeted a critical orbital phase -- when the neutron star is in inferior conjunction -- during which significant changes in absorption are expected. We performed absorption-resolved spectral analyses during two time intervals of interest: the soft and hard hardness ratio (HR) intervals, as it is strongly correlated with absorption variability. We observed a sudden transition in the HR from a soft to a hard state coinciding with an increase in the absorption column density. This is likely attributable to the onset of the accretion structure crossing our line of sight. With we also investigated the Fe K region, and we report for the first time the presence of a Fe Kα doublet in the spectrum of Vela X-1 together with the presence of already known Fe Kβ and Ni Kα lines that are produced in cold clumps embedded in the hot ionised wind. The measured line velocities of the order of $10^2 $ are consistent with production sites in the vicinity of the neutron star. This precursor study with Vela X-1 shows the potential of to study in unprecedented detail the spectral evolution of wind-accreting X-ray binaries.