SFXTs versus classical SgXBs: Does the difference lie in the companion wind?

TL;DR

This study compares stellar wind properties in classical supergiant high mass X-ray binaries and supergiant fast X-ray transients, revealing differences in absorption, luminosity, and iron line features that suggest varying accretion mechanisms.

Contribution

It provides a comparative analysis of wind characteristics in SgXBs and SFXTs using Suzaku and XMM-Newton data, highlighting differences in accretion inhibition.

Findings

Classical SgXBs show higher absorption and luminosity than SFXTs.

Equivalent width of iron line is larger in SgXBs compared to SFXTs.

No evidence found for anti-correlation between iron line strength and luminosity.

Abstract

We present a comparative study of stellar winds in classical supergiant high mass X-ray binaries (SgXBs) and supergiant fast X-ray transients (SFXTs) based on the analysis of publicly available out-of-eclipse observations performed with Suzaku and XMM-Newton. Our data-set includes 55 observations of classical SgXBs and 21 observations of SFXTs. We found that classical SgXBs are characterized by a systematically higher absorption and luminosity compared to the SFXTs, confirming the results of previous works in the literature. Additionally, we show that the equivalent width of the fluorescence K{\alpha} iron line in the classical SgXBs is significantly larger than that of the SFXTs (outside X-ray eclipses). Based on our current understanding of the physics of accretion in these systems, we conclude that the most likely explanation of these differences is to be ascribed to the presence of mechanisms inhibiting accretion for most of the time in the SFXTs and leading to a much less efficient photoionization of the stellar wind compared to classical SgXBs.We do not find evidence for the previously reported anti-correlation between the equivalent width of the fluorescence iron line and the luminosity of SgXBs.