Coordinated monitoring of the eccentric O-star binary Iota Orionis: the X-ray analysis

Abstract

We analyse two ASCA observations of the highly eccentric O9III + B1III binary Iota Orionis obtained at periastron and apastron. Based on the assumption of a strong colliding winds shock between the stellar components, we expected to see significant variation in the X-ray emission between these phases. The observations proved otherwise: the X-ray luminosities and spectral distributions were remarkably similar. The only noteworthy feature was the hint of a proximity effect during periastron passage, supported also in the optical. We discuss the accuracy of our results, and also analyse archival ROSAT observations. We investigate why we do not see a clear colliding winds signature. A simple model shows that the wind attenuation to the expected position of the shock apex is negligible throughout the orbit, which poses the puzzling question of why the expected 1/D variation (ie. a factor of 7.5) in the intrinsic luminosity is not seen in the data. Two scenarios are proposed: either the colliding winds emission is unexpectedly weak such that intrinsic shocks in the winds dominate the emission, or, alternatively, that the emission observed is colliding winds emission but in a more complex form than we would naively expect. Complex hydrodynamical models are then analyzed. Despite strongly phase-variable emission from the models, both were consistent with the observations. We find that if the mass-loss rates of the stars are low then intrinsic wind shocks could dominate the emission. However, when we assume higher mass-loss rates of the stars, we find that the observed emission could also be consistent with a purely colliding winds origin. To distinguish between the different models X-ray observations with improved phase coverage will be necessary.