The X-ray spectral evolution of eta Carinae as seen by ASCA

Using data from the ASCA X-ray observatory, we examine the variations in th e X-ray spectrum of the supermassive star eta Carinae with an unprecedented combination of spatial and spectral resolution. We include data taken duri ng the recent X-ray eclipse in 1997-1998, after recovery from the eclipse, and during and after an X-ray flare. We show that the eclipse variation in the X-ray spectrum is apparently confined to a decrease in the emission mea sure of the source. We compare our results with a simple colliding-wind bin ary model and find that the observed spectral variations are consistent wit h the binary model only if there is significant high-temperature emission f ar from the star and/or a substantial change in the temperature distributio n of the hot plasma. If contamination in the 2-10 keV band is important, th e observed eclipse spectrum requires an absorbing column in excess of 10(24 ) cm(-2) for consistency with the binary model, which may indicate an incre ase in (M) over dot from eta Carinae near the time of periastron passage. T he flare spectra are consistent with the variability seen in nearly simulta neous RXTE observations and thus confirm that eta Carinae itself is the sou rce of the flare emission. The variation in the spectrum during the flare s eems confined to a change in the source emission measure. By comparing two observations obtained at the same phase in different X-ray cycles, we find that the current X-ray brightness of the source is slightly higher than the brightness of the source during the last cycle, perhaps indicative of a lo ng-term increase in (M) over dot not associated with the X-ray cycle.