Investigation of the nature of the wind interaction in HD93205 based on multi-epoch X-ray observations

Abstract

The study of the X-ray emission from massive binaries constitutes a relevant approach to investigate shock physics. The case of short period binaries may turn out to be quite challenging, especially in very asymmetric systems where the primary wind may overwhelm that of the secondary in the wind interaction. Our objective consists in providing an observational diagnostic of the X-ray behaviour of HD93205, that is a very good candidate to investigate these aspects. We analysed 31 epochs of XMM-Newton X-ray data spanning about two decades to investigate its spectral and timing behaviour. The X-ray spectrum is very soft along the full orbit, with a luminosity exclusively from the wind interaction region in the range of 2.3 -- 5.4\,$\times$10${}^{32}$\,erg\,s${}^{-1}$. The light curve peaks close to periastron, with a rather wide pre-periastron low-state coincident with the secondary's body hiding a part of the X-ray emitting region close to its surface. We determined a variability time scale of 6.0807\,$\pm$\,0.0013\,d, in full agreement with the orbital period. Making use of a one-dimensional approach to deal with mutual radiative effects, our results point to a very likely hybrid wind interaction, with a wind-photosphere occurring along most of the orbit, while a brief episode of wind-wind interaction may still develop close to apastron. Beside mutual radiative effects, the radiative nature of the shock that leads to some additional pre-shock obliquitity of the primary wind flow certainly explains the very soft emission. HD93205 constitutes a relevant target to investigate shock physics in short period, asymmetric massive binary systems, where various mutual radiative effects and radiative shocks concur to display an instructive soft X-ray behaviour. HD93205 should be considered as a valid, though challenging target for future three-dimensional modelling initiatives.

arXiv comment17 pages, 15 figures, accepted for publication in Astronomy & Astrophysics

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