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Vela X-1 is the prototype of the class of wind-fed accreting pulsars in highmass X-ray binaries hosting a supergiant donor. We have analyzed in asystematic way ten years of INTEGRAL data of Vela X-1 (22-50 keV) and we foundthat when outside the X-ray eclipse, the source undergoes several luminositydrops where the hard X-rays luminosity goes below 3x10^35 erg/s, becomingundetected by INTEGRAL. These drops in the X-ray flux are usually referred toas "off-states" in the literature. We have investigated the distribution ofthese off-states along the Vela X-1 ~8.9 d orbit, finding that their orbitaloccurrence displays an asymmetric distribution, with a higher probability toobserve an off-state near the pre-eclipse than during the post-eclipse. Thisasymmetry can be explained by scattering of hard X-rays in a region of ionizedwind, able to reduce the source hard X-ray brightness preferentially neareclipse ingress. We associate this ionized large-scale wind structure with thephotoionization wake produced by the interaction of the supergiant wind withthe X-ray emission from the neutron star. We emphasize that this observationalresult could be obtained thanks to the accumulation of a decade of INTEGRALdata, with observations covering the whole orbit several times, allowing us todetect an asymmetric pattern in the orbital distribution of off-states in VelaX-1.

Probing large-scale wind structures in Vela X-1 using off-states with INTEGRAL