3D models of radiatively driven colliding winds in massive O+O star binaries - III. Thermal X-ray emission

TL;DR

This study models the X-ray emission from wind collisions in massive O+O star binaries, revealing phase-dependent variability, hysteresis effects, and energy-dependent absorption, with simulated observations matching real system spectra.

Contribution

It presents detailed 3D hydrodynamical models of colliding stellar winds, incorporating orbital effects and radiative cooling, and compares simulated X-ray spectra with observations.

Findings

X-ray emission varies with orbital phase and eccentricity.

Hysteresis causes softer emission after periastron.

Simulated spectra match observed temperature estimates.

Abstract

The X-ray emission from the wind-wind collision in short-period massive O+O-star binaries is investigated. The emission is calculated from three-dimensional hydrodynamical models which incorporate gravity, the driving of the winds, orbital motion of the stars, and radiative cooling of the shocked plasma. Changes in the amount of stellar occultation and circumstellar attenuation introduce phase-dependent X-ray variability in systems with circular orbits, while strong variations in the intrinsic emission also occur in systems with eccentric orbits. The X-ray emission in eccentric systems can display strong hysteresis, with the emission softer after periastron than at corresponding orbital phases prior to periastron, reflecting the physical state of the shocked plasma at these times. Furthermore, the rise of the luminosity to maximum does not necessarily follow a 1/D law. Our models further demonstrate that the effective circumstellar column can be highly energy dependent. We simulate Chandra and Suzaku observations, and fit these using standard XSPEC models. We find that the recovered temperatures from two or three-temperature mekal fits are comparable to those from fits to the emission from real systems with similar stellar and orbital parameters/nature. We also find that when the global abundance is thawed in the spectral fits, sub-solar values are exclusively returned, despite the calculations using solar values as input (abridged).