SIDOLI, LaraLaraSIDOLIPAIZIS, ADAMANTIAADAMANTIAPAIZISFürst, F.F.FürstTorrejón, J. M.J. M.TorrejónKretschmar, P.P.KretschmarBozzo, E.E.BozzoPottschmidt, K.K.Pottschmidt2020-03-192020-03-1920150035-8711http://hdl.handle.net/20.500.12386/23384Based on observations with INTEGRAL , an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Spain and Switzerland), Czech Republic and Poland, and with the participation of Russia and the USA. This work has made use of the INTEGRAL archive developed at INAF-IASF Milano, http://www.iasf-milano.inaf.it/ ∼ada/GOLIA.html. We acknowledge support from ISSI through funding for the International Team on ‘Unified View of Stellar Winds in Massive X-ray Binaries’ (PI: S. Martínez-Nuez). LS thanks L. Oskinova for interesting discussions. JMT acknowledges grant AYA2010-15431. LS and AP acknowledge the Italian Space Agency financial support INTEGRAL ASI/INAF agreement no. 2013-025.R.0.Vela X-1 is the prototype of the class of wind-fed accreting pulsars in high-mass X-ray binaries hosting a supergiant donor. We have analysed in a systematic way 10 years of INTEGRAL data of Vela X-1 (22-50 keV) and we found that when outside the X-ray eclipse, the source undergoes several luminosity drops where the hard X-rays luminosity goes below ̃3 × 10<SUP>35</SUP> erg s<SUP>-1</SUP>, becoming undetected by INTEGRAL. These drops in the X-ray flux are usually referred to as `off-states' in the literature. We have investigated the distribution of these off-states along the Vela X-1 ̃ 8.9 d orbit, finding that their orbital occurrence displays an asymmetric distribution, with a higher probability to observe an off-state near the pre-eclipse than during the post-eclipse. This asymmetry can be explained by scattering of hard X-rays in a region of ionized wind, able to reduce the source hard X-ray brightness preferentially near eclipse ingress. We associate this ionized large-scale wind structure with the photoionization wake produced by the interaction of the supergiant wind with the X-ray emission from the neutron star. We emphasize that this observational result could be obtained thanks to the accumulation of a decade of INTEGRAL data, with observations covering the whole orbit several times, allowing us to detect an asymmetric pattern in the orbital distribution of off-states in Vela X-1.STAMPAenArticle10.1093/mnras/stu25332-s2.0-84922569226000350272900022https://academic.oup.com/mnras/article/447/2/1299/25937602015MNRAS.447.1299SFIS/05 - ASTRONOMIA E ASTROFISICA