Orbital parameters of supergiant fast X-ray transients

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.

💡 Research Summary

Supergiant Fast X‑ray Transients (SFXTs) are a subclass of high‑mass X‑ray binaries that exhibit brief, intense X‑ray outbursts lasting from a few minutes to a few hours. The physical origin of these rapid flares remains debated, but one leading hypothesis is that dense clumps embedded in the stellar wind of the supergiant companion are sporadically captured by the neutron star (NS), producing a sudden increase in accretion rate. In this paper the authors test the viability of the clumpy‑wind scenario by focusing on how the orbital parameters of the binary affect the accretion geometry.

A simple analytic model of the wind is adopted. The wind velocity follows the standard CAK law, (v_{\rm w}(r)=v_{\infty}