A Multiwavelength Study of the Massive Colliding Wind Binary WR 20a: A Possible Progenitor for Fast-Spinning LIGO Binary Black Hole Mergers

TL;DR

This study analyzes the massive binary WR 20a using X-ray and optical observations, revealing asymmetries in wind collision and suggesting it as a potential progenitor for fast-spinning binary black hole mergers detectable by LIGO.

Contribution

It provides the first detailed X-ray variability analysis of WR 20a and models its evolution as a likely progenitor of LIGO-detectable binary black hole mergers.

Findings

Asymmetric X-ray light curve peaks suggest uneven wind mass-loss or system rotation effects.

Phase shift observed between X-ray and optical light curves.

Modeling indicates WR 20a will evolve into a fast-spinning binary black hole system.

Abstract

WR 20a is the most massive close-in binary known in our Galaxy. It is composed of two $\approx$80 M$_\odot$ Wolf-Rayet stars with a short period of $\approx$3.7 days in the open cluster Westerlund 2. As such, WR 20a presents us with a unique laboratory for studying the currently uncertain physics of binary evolution and compact object formation as well as for studying the wind collision region in an massive eclipsing binary system. We use deep Chandra observations of WR 20a to study the time variability of the wind collision region between the two Wolf-Rayet stars and are able to produce an X-ray light curve covering $\approx$2/3 of its orbital period. We find that the X-ray light curve is asymmetric because the flux of one peak is 2.5$\sigma$ larger than the flux of the other peak. This asymmetry could be caused by asymmetric mass-loss from the two stars or by the lopsidedness of the wind collision region due to the unusually fast rotation of the system. The X-ray light curve is also shifted in phase space when compared to the optical light curves measured by TESS and ASAS-SN. Additionally, we explore the ultimate fate of this system by modeling the resultant binary black hole merger expected at the end of the two stars' lives. We conclude that this system will evolve to be a representative of the sub-population of LIGO progenitors of fast-spinning binary black hole merger events.