Probing clumpy stellar winds with a neutron star

TL;DR

This paper investigates the properties of clumpy stellar winds in super-giant high-mass X-ray binaries using X-ray variability data from the INTEGRAL observatory, revealing insights into wind structure and clump characteristics.

Contribution

It provides quantitative constraints on wind clumping in sgHMXB systems through analysis of hard X-ray flares and quiescent emission, supporting macro-clumping models.

Findings

Hard X-ray flares last about 3 ks with a frequency of 7 days.

Clump densities are 100 to 10,000 times the inter-clump medium.

Clump parameters align with macro-clumping and line-driven instability simulations.

Abstract

INTEGRAL, the European Space Agency's gamma-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. 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. Hard X-ray flares and quiescent emission of SFXT systems have been characterized and used to derive wind clump parameters. 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 1E36 erg/s. Such flares are most probably emitted by the interaction of a compact object orbiting at ~10 R* with wind clumps (1E(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 1E(2-4) in SFXT systems. 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.