Origin of the X-ray off-states in Vela X-1

Context. Vela X-1 is the prototype of the classical super-giant high mass X-ray binary systems. Recent continuous and long monitoring campaigns revealed a large hard X-rays variability amplitude with strong flares and off-states. This activity has been interpreted by invoking clumpy stellar winds and/or magnetic gating mechanisms. Aims. We investigate whether the observed behavior may be explained by unstable hydrodynamic flows close to the neutron star instead of by the more exotic phenomena. Methods. We used the hydrodynamic code VH-1 to simulate the flow of the stellar wind with high temporal resolution and to compare the predicted accretion rate with the observed light curves. Results. The simulation results are similar to the observed variability. Off-states are predicted with a duration of 5 to 120 min corresponding to transient low-density bubbles forming around the neutron star. Oscillations of the accretion rate with a typical period of ~6800 s are generated in our simulations and observed. They correspond to the complex motion of a bow shock, moving either toward or away from the neutron star. Flares are also produced by the simulations up to a level of 1037 erg/s. Conclusions. We have qualitatively reproduced the hard X-ray variations observed in Vela X-1 with hydrodynamic instabilities predicted by a simple model. More sophisticated phenomena, such as clumpy winds or the magnetic gating mechanism, are not excluded, but are not required either to explain the basic phenomenology