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

TL;DR

This paper models the luminosity variability of supergiant fast X-ray transients (SFXTs) using a stochastic approach to accretion of clumpy stellar winds, successfully matching observed flare distributions and explaining differences with persistent HMXBs.

Contribution

It introduces a stochastic differential equation model for accretion-driven luminosity variability, applied specifically to SFXTs, linking wind properties to observed flare distributions.

Findings

Model reproduces observed flare luminosity distributions of SFXTs.

Predicts properties of stellar winds from flare data.

Explains the difference between SFXT and persistent HMXB luminosity distributions.

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.