The X-ray emission of Be+stripped star binaries★

Using observations from Chandra, Swift, and XMM-Newton, we investigate the high-energy properties of all known (18) Be+sdO systems as well as seven additional Be binaries suspected to harbour stripped stars. The observed X-ray properties are found to be similar to those observed for other Be samples. The vast majority of these systems (15 out of 25) display very faint (and soft) X-ray emission, and six others are certainly not bright X-ray sources. Only two systems display γ Cas characteristics (i.e. bright and hard X-rays), and one of them is a new detection: HD 37202 (ζ Tau). It presents an extremely hard spectrum due to a combination of high temperature and high absorption (possibly due to its high inclination). In parallel, it may be noted that the previously reported cyclic behaviour of this Be star has disappeared in recent years. Instead, shorter cycles and symmetric line profiles are observed for the Hα line. It had been recently suggested that the peculiar X-ray emissions observed in γ Cas stars could arise from a collision between the disc of a Be star and the wind of its hot, stripped-star companion. The small fraction of γ Cas analogues in this sample, as well as the properties of the known companions of the γ Cas cases (low mass or not extremely hot, contrary to predictions), combined to the actual stripped-star and colliding-wind empirical knowledge, make the disc-wind collision an unlikely scenario to explain the γ Cas phenomenon.