The X-ray emission of Be+stripped star binaries

TL;DR

This study examines the X-ray emissions of Be+stripped star binaries, revealing most are faint, with only a few showing gamma-Cas-like characteristics, challenging previous collision-based explanations for their high-energy emissions.

Contribution

It provides the first comprehensive X-ray analysis of all known Be+stripped star systems, challenging existing theories about gamma-Cas phenomena.

Findings

Most systems show faint, soft X-ray emission.

Only two systems exhibit gamma-Cas-like hard X-ray emission.

The disk-wind collision scenario is unlikely to explain gamma-Cas emissions.

Abstract

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.