Probing Clumpy Stellar Winds in SFXTs

TL;DR

This paper investigates the variability in hard X-ray emissions of Supergiant Fast X-ray Transients (SFXTs) to constrain the properties of stellar wind clumping, revealing insights into the interaction between compact objects and stellar wind structures.

Contribution

It provides quantitative constraints on wind clumping in massive stars through analysis of X-ray variability in SFXTs, supporting macro-clumping and line-driven instability models.

Findings

Flares last about 3 ksec with a frequency of 7 days.

Clump densities are 10^{22-23} g, with a density ratio of 10^{2-4} compared to inter-clump medium.

Results align with macro-clumping scenario and line-driven instability simulations.

Abstract

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 $10^{36}$ ergs/s. Such flares are most probably emitted by the interaction of a compact object orbiting at $\sim10$ R$_*$ with wind clumps ($10^{22-23}$ g). The density ratio between the clumps and the inter-clump medium is $10^{2-4}$ . The parameters of the clumps and of the inter-clump medium are in good agreement with macro-clumping scenario and line-driven instability simulations.