Understanding the Unusual X-Ray Emission Properties of the Massive, Close Binary WR 20a: A High Energy Window into the Stellar Wind Initiation Region

TL;DR

This study investigates the unusual X-ray emission of the massive binary WR 20a, revealing that wind-wind collisions during wind acceleration phases explain the observations and offer insights into stellar wind initiation.

Contribution

The paper introduces hydrodynamical models including radiative effects that successfully explain WR 20a's X-ray properties and shed light on stellar wind initiation in massive binaries.

Findings

X-ray emission is explained by wind collisions during acceleration phase.

WR 20a's X-ray luminosity increases during optical eclipse, contrary to typical binaries.

Models predict detailed X-ray behavior over the entire orbit.

Abstract

The problem of explaining the X-ray emission properties of the massive, close binary WR 20a is discussed. Located near the cluster core of Westerlund 2, WR 20a is composed of two nearly identical Wolf- Rayet stars of 82 and 83 solar masses orbiting with a period of only 3.7 days. Although Chandra observations were taken during the secondary optical eclipse, the X-ray light curve shows no signs of a flux decrement. In fact, WR 20a appears slightly more X-ray luminous and softer during the optical eclipse, opposite to what has been observed in other binary systems. To aid in our interpretation of the data, we compare with the results of hydrodynamical simulations using the adaptive mesh refinement code Mezcal that includes radiative cooling and a radiative acceleration force term. It is shown that the X-ray emission can be successfully explained in models where the wind-wind collision interface in this system occurs while the outflowing material is still being accelerated. Consequently, WR 20a serves as a critical test-case for how radiatively-driven stellar winds initiate and interact. Our models not only procure a robust description of current Chandra data, which cover the orbital phases between 0.3 to 0.6, but provide detailed predictions over the entire orbit.