New X-ray detections of known Wolf-Rayet stars

TL;DR

This study used X-ray observations from XMM-Newton, Swift, and Chandra, along with updated ephemerides, to investigate X-ray emissions in 18 Wolf-Rayet binary systems, revealing that not all are X-ray bright and providing evidence of colliding winds in six systems.

Contribution

First comprehensive X-ray survey of known Wolf-Rayet binaries combining multiple observatories and updated orbital data, identifying systems with and without colliding wind signatures.

Findings

Six systems show clear colliding wind evidence.

Some systems have faint or obscured X-ray emissions.

Not all Wolf-Rayet binaries are X-ray bright.

Abstract

Using XMM-Newton, we undertook a dedicated project to search for X-ray bright wind-wind collisions in 18 WR+OB systems. We complemented these observations with Swift and Chandra datasets, allowing for the study of two additional systems. We also improved the ephemerides, for these systems displaying photometric changes, using TESS, Kepler, and ASAS-SN data. Five systems displayed a very faint X-ray emission ($\log [L_{\rm X}/L_{\rm BOL}]<-8$) and three a faint one ($\log [L_{\rm X}/L_{\rm BOL}]\sim-7$), incompatible with typical colliding wind emission: not all WR binaries are thus X-ray bright. In a few other systems, X-rays from the O-star companion cannot be excluded as being the true source of X-rays (or a large contributor). In two additional cases, the emission appears faint but the observations were taken with the WR wind obscuring the line-of-sight, which could hide a colliding wind emission. Clear evidence of colliding winds was however found in the remaining six systems (WR19, 21, 31, 97, 105, 127). In WR19, increased absorption and larger emission at periastron are even detected, in line with expectations of adiabatic collisions.