The Chandra HETGS X-ray grating spectrum of eta Carinae

eta Carinae may be the most massive and luminous star in the Galaxy and is suspected to be a massive colliding-wind binary system. The Chandra X-ray o bservatory has obtained a calibrated high-resolution X-ray spectrum of the star, uncontaminated by the nearby extended soft X-ray emission. Our 89 ks Chandra observation with the High-Energy Transmission Grating Spectrometer (HETGS) shows that the hot gas near the star is nonisothermal. The temperat ure distribution may represent the emission on either side of the colliding -wind bow shock, effectively "resolving" the shock. If so, the preshock win d velocities are similar to 700 and similar to 1800 km s(-1) in our analysi s, and these velocities may be interpreted as the terminal velocities of th e winds from eta Carinae and from the hidden companion star. The forbidden- to-intercombination ( f/i) line ratios for the He-like ions of S, Si, and F e are large, indicating that the line-forming region lies far from the stel lar photosphere. The iron fluorescent line at 1.93 Angstrom first detected by ASCA, is clearly resolved from the thermal iron line in the Chandra grat ing spectrum. The Fe fluorescent line is weaker in our Chandra observation than in any of the ASCA spectra. The Chandra observation also provides an u ninterrupted, high time resolution light curve of the stellar X-ray emissio n from eta Carinae and suggests that there was no significant coherent vari ability during the Chandra observation. The eta Carinae Chandra grating spe ctrum is unlike recently published X-ray grating spectra of single massive stars in significant ways and is generally consistent with colliding-wind e mission in a massive binary.