Orbital parameters of supergiant fast X-ray transients

TL;DR

This paper investigates the orbital parameters of supergiant fast X-ray transients (SFXTs) to understand their short X-ray flares, using a simple clumpy wind accretion model to constrain the conditions under which these flares occur.

Contribution

It provides new constraints on the orbital parameters of SFXTs within a simple analytic clumpy wind model, linking orbital eccentricity and period to flare activity.

Findings

SFXTs are likely to have orbital periods around 10 days and eccentricities greater than 0.4.

Systems with orbital periods longer than 100 days are unlikely to produce bright flares in this model.

High eccentricity and moderate orbital period systems are prone to periodic X-ray outbursts.

Abstract

Supergiant fast X-ray transient (SFXT) is a new class of the high mass X-ray binary that shows short X-ray flares. The physical mechanism of SFXT short flares is still open for discussion. The accretion process of dense clumps in stellar wind onto neutron star (NS) has been proposed as the origin of such short flares. In order to examine the applicability of the clumpy wind scenario, we focus on the accretion mode that depends on orbital parameters. Our goal is to impose restrictions on the orbital parameters of SFXT. Assuming a simple analytic model of clumpy wind, we investigate the condition where the size of accretion cylinder overcomes the clump size. The allowed parameter region for SFXT is restricted in a relatively narrow window in $P_{\rm{orb}} - e$ diagram. Binary systems with large eccentricities ($e \ga 0.4$) and moderate orbital periods ($P_{\rm{orb}} \sim 10$ d) are prone to show periodic X-ray outbursts which are characteristic for SFXT. We confirm that systems with a long orbital period of more than 100 days cannot produce bright X-ray flares in the simple clumpy wind scenario.