The X-ray light curve of η Carinae:: Refinement of the orbit and evidence for phase-dependent mass loss

We solve the Rossi X-Ray Timing Explorer X-ray light curve of the extremely luminous and massive star eta Carinae with a colliding-wind emission model to refine the ground-based orbital elements. The sharp decline to X-ray minimum at the end of 1997 fixes the date of the last periastron passage at 1997.95 +/- 0.05, not 1998.13 as derived from ground-based radial velocities. This helps resolve a discrepancy between the ground-based radial velocities and spatially resolved velocity measures obtained by Space Telescope Imaging Spectograph (STIS). The X-ray data are consistent with a mass function f (M) approximate to 1.5, lower than the value f (M) approximate to 7.5 previously reported, so that the masses of eta Carinae and the companion are M-eta greater than or equal to 80 M. and M-c similar to 30 M-., respectively. In addition, the X-ray data suggest that the mass-loss rate from eta Carinae is generally less than 3 x 10(-4) M-. yr(-1), about a factor of 5 lower than that derived from some observations in other wave bands. We could not match the duration of the X-ray minimum with any standard colliding-wind model in which the wind is spherically symmetric and the mass-loss rate is constant. However, we show that we can match the variations around X-ray minimum if we include an increase of a factor of similar to 20 in the mass-loss rate from eta Carinae for approximately 80 days following periastron. If real, this excess in (M) over dot would be the first evidence of enhanced mass flow off the primary when the two stars are close (presumably driven by tidal interactions). Our interpretation of the X-ray data suggests that the ASCA and RXTE X-ray spectra near the X-ray minimum are significantly contaminated by unresolved hard emission (E greater than or equal to 2 keV) from some other nearby source, probably associated with scattering of the colliding-wind emission by circumstellar dust. Based on the X-ray fluxes, the distance to eta Carinae is 2300 pc with formal uncertainties of only similar to 10%.