X-rays from Colliding Stellar Winds: the case of close WR+O binary systems

TL;DR

This study analyzes X-ray emissions from close WR+O binary systems, revealing that their colliding stellar wind shocks are adiabatic, likely due to winds being two-component flows with smaller mass-loss rates than previously thought.

Contribution

It is the first detailed spectral analysis indicating that CSW shocks in close WR+O binaries are adiabatic, suggesting winds are two-component flows with reduced mass-loss rates.

Findings

X-ray spectra require two-temperature plasma models.

CSW shocks are found to be adiabatic, not radiative.

Winds likely consist of dense clumps and a smooth component.

Abstract

We have analysed the X-ray emission from a sample of close WR+O binaries using data from the public Chandra and XMM-Newton archives. Global spectral fits show that two-temperature plasma is needed to match the X-ray emission from these objects as the hot component (kT > 2 keV) is an important ingredient of the spectral models. In close WR+O binaries, X-rays likely originate in colliding stellar wind (CSW) shocks driven by the massive winds of the binary components. CSW shocks in these objects are expected to be radiative due to the high density of the plasma in the interaction region. Opposite to this, our analysis shows that the CSW shocks in the sample of close WR+O binaries are adiabatic. This is possible only if the mass-loss rates of the stellar components in the binary are at least one order of magnitude smaller than the values currently accepted. The most likely explanation for the X-ray properties of close WR+O binaries could be that their winds are two-component flows. The more massive component (dense clumps) play role for the optical/UV emission from these objects, while the smooth rarefied component is a key factor for their X-ray emission.