XMM-Newton and Swift observations of supergiant high mass X-ray binaries

TL;DR

This study presents extensive XMM-Newton and Swift observations of supergiant high mass X-ray binaries, analyzing spectral variations and large-scale structures in stellar winds to better understand accretion processes and wind properties.

Contribution

It provides new detailed spectral and long-term observational data on multiple supergiant X-ray binaries, clarifies the nature of a candidate source, and enhances understanding of stellar wind structures.

Findings

Detection of spectral variations linked to wind clumps.

Identification of large-scale wind structures affecting X-ray emission.

Disregard of a previously suspected supergiant X-ray binary due to data artifact.

Abstract

Wind-fed supergiant X-ray binaries are precious laboratories not only to study accretion under extreme gravity and magnetic field conditions, but also to probe still highly debated properties of massive star winds. These includes the so-called clumps, originated from the inherent instability of line driven winds, and larger structures. In this paper, we report on the results of the last (and not yet published) monitoring campaigns that our group has been carrying out since 2007 with both XMM-Newton and the Swift Neil Gehrels observatory. Data collected with the EPIC cameras on-board XMM-Newton allow us to carry out a detailed hardness ratio-resolved spectral analysis that can be used as an efficient way to detect spectral variations associated to the presence of clumps. Long-term observations with the XRT on-board Swift, evenly sampling the X-ray emission of supergiant X-ray binaries over many different orbital cycles, are exploited to look for the presence of large scale structures in the medium surrounding the compact objects. The results reported in this paper represent the outcomes of the concluded observational campaigns we carried out on the supergiant X-ray binaries 4U 1907+09, IGR J16393-4643, IGR J19140+0951, and XTE J1855-026, as well as the supergiant fast X-ray transients IGR J17503-2636, IGR J18410-0535, and IGR J11215-5952. All results are discussed in the context of wind-fed supergiant X-ray binaries and shall ideally serve to optimally shape the next observational campaigns aimed at sources in the same classes. We show in one of the paper appendices that IGR J17315-3221, preliminary classified in the literature as a possible supergiant X-ray binary discovered by INTEGRAL, is the product of a data analysis artifact and should thus be disregarded for future studies.