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

WR 140 is a long-period, highly eccentric massive Wolf-Rayet star binary system with exceptionally well-determined orbital and stellar parameters. Bright, periodically-varying X-ray emission is primarily generated in the hot shocked gas produced by the collision of the winds of the WC7pd+O5.5fc component stars. We define and discuss the X-ray variations in the context of the colliding-wind model using time-resolved broad-band X-ray spectrometry from the RXTE, Swift, and NICER observatories obtained over 20 years and nearly 1000 observations through 3 consecutive 7.94-year orbital cycles 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 from this simple dependence are produced when the shock energy balance shifts to excess optical emission in CIII 5696 in particular. We use X-ray column density measures in a point-source approximation to determine mass-loss-rate estimates for both stars and to constrain the overall system morphology. The absorbing-column maximum coincides closely with inferior conjunction of the WC star and provides evidence of the ion-reflection mechanism that underlie the formation of collisionless shocks governed by magnetic fields probably generated by the Weibel instability. Comparisons after periastron with K-band emission and He I10830 absorption show that both are correlated with the asymmetric X-ray absorption. The K-band monitoring shows that dust appears within only a few days of periastron passage, suggesting that the dust-forming gas is embedded within shocked gas near the stagnation point. Comparisons with Eta Carinae show that X-ray flares seen there in every orbit have not occurred in WR 140, suggesting the absence of large-scale wind inhomogeneities. Soft emission appearing during the X-ray minimum is relatively constant and therefore unlikely to be produced by recombining plasma entrained in outflowing shocked gas.