Theoretical X-ray properties of colliding stellar winds in O+O star binaries

Abstract

This is the first in a series of papers on colliding wind binaries which aim to present a detailed and quantitative comparison between theoretical models of colliding winds and X-ray observations. In this paper we concentrate on colliding stellar winds in O+O star binaries. We calculate the expected X-ray spectra and orbital light curves, and investigate the effect of different parameters (mass-loss rate, orbital separation, system inclination etc.) on the expected X-ray properties. In particular we investigate three model systems; one representing HD 165052 with equal winds, another representing HD 57060 with very unequal winds, and a third representing an intermediate case. We demonstrate the sensitive relationship between system wind parameters and the shape of the X-ray light curve, and discuss these models in relation to recent ROSAT results on O+O star colliding wind systems. We find that the variation in X-ray luminosity during the orbit is primarily a result of varying extinction of the intrinsic X-ray luminosity by the more massive wind in the system. Secondary effects such as the intrinsic variation of the luminosity from instabilities in the shocked wind and variation with orbital separation are also quantified. We also present the results of spectral fitting to our synthetic spectra. This is the first time that this method has been applied to colliding wind systems and promises to be an important future tool. Our theoretical results are intended primarily as examples but good agreement is found with observations. We find that a Raymond-Smith model fitted to our simulations has a characteristic temperature of between 0.4 and 3.0 keV. This is comparable to typical observational results of ∼ 0.1-0.6 keV. Future papers will sequentially add more physics into the theoretical models and comparisons with the latest observational results will be made. More detailed results for individual systems will follow in due course. © 1997 RAS.