Browsing by Author "Corcoran, M. F."
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Item Competitive X-ray and Optical Cooling in the Collisionless Shocks of WR 140(2021-09-20) Pollock, A. M. T.; Corcoran, M. F.; Stevens, I. R.; Russell, C. M. P.; Hamaguchi, K.; Williams, P. M.; Moffat, A. F. J.; Weigelt, G.; Shenavrin, V.; Richardson, N. D.; Espinoza, D.; Drake, S. A.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 λ5696 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 λ10830 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 η 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.Item NICER X-Ray Observations of Eta Carinae during Its Most Recent Periastron Passage(IOP, 2022-07-08) Espinoza-Galeas, David; Corcoran, M. F.; Hamaguchi, Kenji; Russell, C. M. P.; Gull, T. R.; Moffat, A. F. J.; Richardson, N. D.; Weigelt, G.; Hillier, D. John; Damineli, Augusto; Stevens, Ian R.; Madura, Thomas; Gendreau, K.; Arzoumanian, Z.; Navarete, FelipeWe report high-precision X-ray monitoring observations in the 0.4–10 keV band of the luminous, long-period colliding wind binary Eta Carinae, up to and through its most recent X-ray minimum/periastron passage in 2020 February. Eta Carinae reached its observed maximum X-ray flux on 2020 January 7, at a flux level of 3.30 ×10−10 ergs s−1 cm−2, followed by a rapid plunge to its observed minimum flux, 0.03 × 10−10 ergs s−1 cm−2, near 2020 February 17. The NICER observations show an X-ray recovery from the minimum of only ∼16 days, the shortest X-ray minimum observed so far. We provide new constraints for the "deep" and "shallow" minimum intervals. Variations in the characteristic X-ray temperatures of the hottest observed X-ray emission indicate that the apex of the wind–wind "bow shock" enters the companion's wind acceleration zone about 81 days before the start of the X-ray minimum. There is a steplike increase in column density just before the X-ray minimum, probably associated with the presence of dense clumps near the shock apex. During the recovery and after, the column density shows a smooth decline, which agrees with previous NH measurements made by Swift at the same orbital phase, indicating that the changes in the mass-loss rate are only a few percent over the two cycles. Finally, we use the variations in the X-ray flux of the outer ejecta seen by NICER to derive a kinetic X-ray luminosity of the ejecta of ∼1041 ergs s−1 near the time of the "Great Eruption."