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

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 $\lambda 5696$ 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.

52 pages, 17+1 figures, Accepted for publication in The Astrophysical Journal 23 August 2021

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Cited by in corpus (8)

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• Exploring dust growth in the episodic WCd system WR140
• Inverse Compton cooling of thermal plasma in colliding-wind binaries
• X-ray plasma flow and turbulence in the colliding winds of WR140
• Revealing the accelerating wind in the inner region of the colliding-wind binary WR 112
• Particle Acceleration in Colliding Flows: Binary Star Winds and Other Double-Shock Structures