The Chandra HETGS X‐Ray Grating Spectrum of η Carinae

Abstract

{η} Carinae may be the most massive and luminous star in the Galaxyand is suspected to be a massive colliding-wind binary system. TheChandra X-ray observatory has obtained a calibrated high-resolutionX-ray spectrum of the star, uncontaminated by the nearby extended softX-ray emission. Our 89 ks Chandra observation with the High-EnergyTransmission Grating Spectrometer (HETGS) shows that the hot gas nearthe star is nonisothermal. The temperature distribution may representthe emission on either side of the colliding-wind bow shock, effectively``resolving'' the shock. If so, the preshock wind velocities are ~700and ~1800 km s^{-1} in our analysis, and these velocities may beinterpreted as the terminal velocities of the winds from {η} Carinaeand from the hidden companion star. The forbidden-to-intercombination(f/i) line ratios for the He-like ions of S, Si, and Fe are large,indicating that the line-forming region lies far from the stellarphotosphere. The iron fluorescent line at 1.93 {Å}, first detectedby ASCA, is clearly resolved from the thermal iron line in the Chandragrating spectrum. The Fe fluorescent line is weaker in our Chandraobservation than in any of the ASCA spectra. The Chandra observationalso provides an uninterrupted, high time resolution light curve of thestellar X-ray emission from {η} Carinae and suggests that there was nosignificant coherent variability during the Chandra observation. The{η} Carinae Chandra grating spectrum is unlike recently publishedX-ray grating spectra of single massive stars in significant ways and isgenerally consistent with colliding-wind emission in a massive binary.