Probing clumpy stellar winds with a neutron star

¹ INTEGRAL Science Data Centre, Chemin d'Ecogia 16, 1290 Versoix, Switzerland e-mail: Roland.Walter@obs.unige.ch
² Observatoire de Genève, Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
³ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, DAPNIA/Service d'Astrophysique, 91191 Gif-sur-Yvette, France e-mail: juan-antonio.zurita-heras@cea.fr

Received: 25 July 2007
Accepted: 8 October 2007

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 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.