Probing clumpy stellar winds with a neutron star

Abstract

Context: INTEGRAL, the European Space Agency's γ-ray observatory, tripled the number of super-giant high-mass X-ray binaries (sgHMXB) known in the Galaxy by revealing absorbed and fast transient (SFXT) systems.
Aims: In these sources, quantitative constraints on the wind clumping of the massive stars could be obtained from the study of the hard X-ray variability of the compact accreting object.
Methods: Hard X-ray flares and quiescent emission of SFXT systems have been characterized and used to derive wind clump parameters.
Results: A large fraction of the hard X-ray emission is emitted in the form of flares with a typical duration of 3 ks, frequency of 7 days and luminosity of 10³⁶ erg/s. Such flares are most probably emitted by the interaction of a compact object orbiting at 10 R_ast with wind clumps (10^22-23 g) representing a large fraction of the stellar mass-loss rate. The density ratio between the clumps and the inter-clump medium is 10^2-4 in SFXT systems.
Conclusions: The parameters of the clumps and of the inter-clump medium, derived from the SFXT flaring behavior, are in good agreement with macro-clumping scenario and line driven instability simulations. SFXT have probably a larger orbital radius than classical sgHMXB.