Stochastic accretion and the variability of supergiant fast X-ray transients

Abstract

In this paper we consider the variability of the luminosity of a compact object (CO) powered by the accretion of an extremely inhomogeneous ("clumpy") stream of matter. The accretion of a single clump results in an X-ray flare: we adopt a simple model for the response of the CO to the accretion of a single clump, and derive a stochastic differential equation (SDE) for the accretion powered luminosity $L(t)$. We put the SDE in the equivalent form of an equation for the flares' luminosity distribution (FLD), and discuss its solution in the stationary case. As a case study, we apply our formalism to the analysis of the FLDs of Super-Giant Fast X-ray Transients (SFXTs), a peculiar sub-class of High Mass X-ray Binary Systems (HMXBs). We compare our theoretical FLDs to the distributions observed in the SFXTs IGR J16479-4514, IGR J17544-2619 and XTE J1739-302. Despite its simplicity, our model fairly agrees with the observed distributions, and allows to predict some properties of the stellar wind. Finally, we discuss how our model may explain the difference between the broad FLD of SFXTs and the much narrower distribution of persistent HMXBs.

ApJ accepted

References in corpus (9)

• Mass loss from hot massive stars
• Unveiling supergiant fast X-ray transient sources with INTEGRAL
• An alternative hypothesis for the outburst mechanism in Supergiant Fast X-ray Transients: the case of IGR J11215-5952
• Clumped stellar winds in supergiant high-mass X-ray binaries: X-ray variability and photoionization
• Monitoring Supergiant Fast X-ray Transients with Swift. I. Behavior outside outbursts
• Generalized Fokker-Planck equation: Derivation and exact solutions
• Swift/XRT observes the fifth outburst of the periodic Supergiant Fast X-ray Transient IGR J11215-5952
• Two years of monitoring Supergiant Fast X-ray Transients with Swift
• Supergiant Fast X-ray Transients as transient sources in High Mass X-ray Binaries