Competitive X-ray and Optical Cooling in the Collisionless Shocks of WR 140

TL;DR

This study analyzes 20 years of X-ray and optical data from WR 140, revealing how colliding stellar winds produce variable X-ray emission, and provides insights into the shock physics, system morphology, and dust formation in this binary system.

Contribution

It offers the first comprehensive long-term observational analysis of WR 140's colliding-wind shocks, linking X-ray variability with optical emission and shock physics.

Findings

X-ray luminosity inversely correlates with stellar separation.

Absorption peaks at inferior conjunction, indicating collisionless shock formation.

Dust formation occurs shortly after periastron within shocked gas.

Abstract

WR 140 is a long-period, highly eccentric Wolf-Rayet star binary system with exceptionally well-determined orbital and stellar parameters. Bright, variable X-ray emission is generated in shocks produced by the collision of the winds of the WC7pd+O5.5fc component stars. We discuss the variations in the context of the colliding-wind model using broad-band spectrometry from the RXTE, SWIFT, and NICER observatories obtained over 20 years and nearly 1000 observations through 3 consecutive 7.94-year orbits including 3 periastron passages. The X-ray luminosity varies as expected with the inverse of the stellar separation over most of the orbit: departures near periastron are produced when cooling shifts to excess optical emission in CIII $\lambda5696$ in particular. We use X-ray absorption to estimate mass-loss rates for both stars and to constrain the system morphology. The absorption maximum coincides closely with inferior conjunction of the WC star and provides evidence of the ion-reflection mechanism that underlies the formation of collisionless shocks governed by magnetic fields probably generated by the Weibel instability. Comparisons with K-band emission and HeI $\lambda$10830 absorption show that both are correlated after periastron with the asymmetric X-ray absorption. Dust appears within a few days of periastron suggesting formation within shocked gas near the stagnation point. X-ray flares seen in $\eta$ Carinae have not occurred in WR 140, suggesting the absence of large-scale wind inhomogeneities. Relatively constant soft emission revealed during the X-ray minimum is probably not from recombining plasma entrained in outflowing shocked gas.