2007ApJ...663..522H

We report the results of an observing campaign on η Car around the 2003 X-ray minimum, mainly using the XMM-Newton observatory. These are the first spatially resolved X-ray monitoring observations of the stellar X-ray spectrum during the minimum. The hard X-ray emission, associated with the wind-wind collision (WWC) in the binary system, varied strongly in flux on timescales of days, but not significantly on timescales of hours. The X-ray flux in the 2-10 keV band seen by XMM-Newton was only 0.7% of the flux maximum seen by RXTE. The slope of the X-ray continuum above 5 keV did not vary in any observation, which suggests that the electron temperature of the hottest plasma did not vary significantly at any phase. Through the minimum, the absorption to the stellar source increased by a factor of 5-10 to N_H~(3-4)x10²³/cm2. These variations were qualitatively consistent with emission from the WWC plasma entering into the dense wind of the massive primary star. During the minimum, X-ray spectra also showed significant excesses in the thermal Fe XXV emission line on the red side, while they showed only a factor of 2 increase in equivalent width of the Fe fluorescence line at 6.4 keV. These features are not fully consistent with the eclipse of the X-ray plasma and may suggest an intrinsic fading of the X-ray emissivity. The drop in the WWC emission revealed the presence of an additional X-ray component that exhibited no variation on timescales of weeks to years. This component may be produced by the collision of high-speed outflows at v∼1000-2000 km/s from η Car with ambient gas within a few thousand AU from the star.