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 7 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 gamma-Cas characteristics (i.e. bright and hard X-rays), and
one of them is a new detection: HD37202 (zeta 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
Halpha line. It had been recently suggested that the peculiar X-ray emissions
observed in gamma-Cas stars could arise from a collision between the disk of a
Be star and the wind of its hot, stripped-star companion. The small fraction of
gamma-Cas analogs in this sample, as well as the properties of the known
companions of the gamma-Cas cases (low mass or not extremely hot, contrary to
predictions), combined to the actual stripped-star and colliding-wind empirical
knowledge, make the disk-wind collision an unlikely scenario to explain the
gamma-Cas phenomenon.