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

Abstract

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.