The ASCA X-Ray Spectrum of η Carinae

We have obtained high signal-to-noise ratio 0.5-10 keV band X-ray spectra of the peculiar, extremely luminous star η Carinae with the Advanced Satellite for Cosmology and Astrophysics (ASCA) X-ray observatory during Cycle 4 observations in mid-1996. These data comprise the best X-ray spectra to date of the cool source (kT ~ 0.3 keV) surrounding the homunculus and of the hot source (kT ~ 5 keV) associated with η Car itself. We identify line emission from ions of N, Mg, Si, and S and numerous strong transitions of Fe in a variety of ionization stages, including the first clear identification of a fluorescent Fe line produced by photoionization of cool material by the X-ray continuum from the hot source. The line strengths are consistent with thermal equilibrium models, though abundances of some important elements are nonsolar. Our analysis suggests that Fe is slightly underabundant and S and Si somewhat overabundant. Most importantly, we confirm the high N enrichment derived by Tsuboi et al. from their analysis of a shorter ASCA observation obtained during the performance verification (PV) phase in 1993 August. The O/N abundance ratio derived from the ASCA spectra is consistent with an upper limit from optical/UV spectral analysis (Davidson et al.) and with predictions of evolutionary models for extremely massive stars. Comparison of the Cycle 4 and PV-phase spectra shows that the X-ray luminosity increased by ~50% during this 3 year interval. Using the ASCA spectral model as a template, we reevaluate the spectrum of η Car obtained by the Röntgen Satelite (ROSAT) in late 1992 and construct an X-ray light curve for the 1992.4-1996.6 interval. We present spectra from the International Ultraviolet Explorer (IUE) satellite obtained at nearly the same time as the Cycle 4 ASCA spectra and show that the observed X-ray variability is reflected in changes of some important UV spectral features. Our data suggest that the X-ray emission and state of the stellar wind are intimately connected, though the exact mechanism of coupling is not known. We suggest two alternatives: an underlying photospheric change of undetermined origin in η Car itself, or a collision between a dense stellar wind from η Car and the wind or photosphere of a companion.