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WR 140 is a long-period, highly eccentric Wolf-Rayet star binary system withexceptionally well-determined orbital and stellar parameters. Bright, variableX-ray emission is generated in shocks produced by the collision of the winds ofthe WC7pd+O5.5fc component stars. We discuss the variations in the context ofthe colliding-wind model using broad-band spectrometry from the RXTE, SWIFT,and NICER observatories obtained over 20 years and nearly 1000 observationsthrough 3 consecutive 7.94-year orbits including 3 periastron passages. TheX-ray luminosity varies as expected with the inverse of the stellar separationover most of the orbit: departures near periastron are produced when coolingshifts to excess optical emission in CIII $\lambda5696$ in particular. We useX-ray absorption to estimate mass-loss rates for both stars and to constrainthe system morphology. The absorption maximum coincides closely with inferiorconjunction of the WC star and provides evidence of the ion-reflectionmechanism that underlies the formation of collisionless shocks governed bymagnetic fields probably generated by the Weibel instability. Comparisons withK-band emission and HeI $\lambda$10830 absorption show that both are correlatedafter periastron with the asymmetric X-ray absorption. Dust appears within afew days of periastron suggesting formation within shocked gas near thestagnation point. X-ray flares seen in $\eta$ Carinae have not occurred in WR140, suggesting the absence of large-scale wind inhomogeneities. Relativelyconstant soft emission revealed during the X-ray minimum is probably not fromrecombining plasma entrained in outflowing shocked gas.

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