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In high mass X-ray binaries (HMXBs), an accreting compact object orbits ahigh mass star which loses mass through a dense and inhomogeneous wind. Usingthe compact object as an X-ray backlight, the time variability of the absorbingcolumn density in the wind can be exploited in order to shed light on themicro-structure of the wind and obtain unbiased stellar mass loss rates forhigh mass stars. We explore the impact of clumpiness on the variability of thecolumn density with a simplified wind model. In particular, we focus on thestandard deviation of the column density and the characteristic duration ofenhanced absorption episodes, and compare them with analytical predictionsbased on the porosity length. We identified the favorable systems and orbitalphases to determine the wind micro-structure. The coherence time scale of thecolumn density is shown to be the self-crossing time of a clump in front of thecompact object. We provide a recipe to get accurate measurements of the sizeand of the mass of the clumps, purely based on the observable time variabilityof the column density. The coherence time scale grants direct access to thesize of the clumps while their mass can be deduced separately from theamplitude of the variability. If it is due to unaccreted passing-by clumps, thehigh column density variations in some HMXBs requires high mass clumps toreproduce the observed peak-to-peak amplitude and coherence time scales. Theseclump properties are hardly compatible with the ones derived from firstprinciples. Alternatively, other components could contribute to the variabilityof the column density: larger orbital scale structures produced by a mechanismstill to be identified, or a dense environment in the immediate vicinity of theaccretor such as an accretion disk, an outflow or a spherical shell around themagnetosphere of the accreting neutron star.

Radiography in high mass X-ray binaries -- Micro-structure of the stellar wind through variability of the column density