X-Ray Monitoring of η Carinae: Variations on a Theme

We present monitoring observations by the Rossi X-Ray Timing Explorer of the 2-10 keV X-ray emission from the supermassive star η Carinae from 1996 through late 2003. These data cover more than one of the stellar variability cycles in temporal detail and include especially detailed monitoring through two X-ray minima. We compare the current X-ray minimum, which began on 2003 June 29, with the previous X-ray minimum, which began on 1997 December 15, and refine the X-ray period to 2024 days. We examine the variations in the X-ray spectrum with phase and with time, and also refine our understanding of the X-ray ``peaks,'' which have a quasi period of 84 days, with significant variation. Cycle-to-cycle differences are seen in the level of X-ray intensity and in the detailed variations of the X-ray flux on the rise to maximum just prior to the X-ray minimum. Despite these differences, the similarities between the decline to minimum, the duration of the minimum, and correlated variations of the X-ray flux and other measures throughout the electromagnetic spectrum leave little doubt that the X-ray variation is strictly periodic and produced by orbital motion as the wind from η Carinae collides with the wind of an otherwise unseen companion.