The spectral and timing behaviour of HMXBs offers a unique opportunity for the investigation of accretion onto compact objects and of wind structure in massive stars. The bright and persistent neutron star HMXB Vela X-1 is one of the key systems for such studies with both current and future instruments. It has a complex clumpy stellar wind, prominent cyclotron resonant scattering features (CRSFs) and strong flares. Understanding the variability of the systems on both short time scales of a few hundreds seconds and along its 9d orbit with current instruments enables us to make predictions for future observations with XRISM and Athena and to devise the best observational strategy for Vela X-1. Here, we analyse two new observations taken with NuSTAR and XMM-Newton at orbital phases ~0.5 and ~0.75 and follow the evolution of spectral parameters down to the pulse period (~300s) time-scale. The flux-dependency we observe in the spectral shape implies a change in the properties of the Comptonising plasma and the observed drop of the CRSF energy following a strong flare may indicate a change in the accretion geometry. The strong variability of absorption is due to the presence of a large-scale wind structure, such as accretion- and photoionisation wakes, combined with the variable line of sight as the neutron star moves along the orbit. In particular, we, for the first time, are able to trace the onset of the wakes with high time resolution and compare to predictions from simulations.
Local contact: Jérôme RODRIGUEZ
Organizer: Frédéric GALLIANO
