Probing Clumpy Stellar Winds in SFXTs

Quantitative constraints on the wind clumping of massive stars can be obtained from the study of the hard X-ray variability of SFXTs. In these systems, a large fraction of the hard X-ray emission is emitted in the form of flares with typical duration of 3 ksec, frequency of 7 days and luminosity of 1036 ergs/s. Such flares are most probably emitted by the interaction of a compact object orbiting at ∼10 R* with wind clumps (1022-23 g). The density ratio between the clumps and the inter-clump medium is 102-4. The parameters of the clumps and of the inter-clump medium are in good agreement with macro-clumping scenario and line-driven instability simulations