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K. Pottschmidt (CASS-UCSD), M.A. Nowak (CXC-MIT), J. Wilms (Univ. Warwick)
We present results of a ~50 ks Chandra High Energy Transmission Grating Spectrometer (HETGS) observation of the black hole high mass X-ray binary Cygnus X-1 performed in 2003 April during superior conjunction of the black hole. The data set contains several transient dips in the soft X-ray intensity. These dips are known to occur around this phase and have been attributed to absorption of the X-rays by ``blobs'' in the focused stellar wind from the O-star companion passing our line of sight to the black hole. Our analysis constitutes the first dedicated high-quality spectral study of the properties of of this material as well as the undisturbed accretion disk emission during superior conjunction. We divided the data set into three different levels of absorption, according to count rate and hardness, and present high-resolution spectra for those selections in several orders for all HETGS gratings. Numerous emission and absorption lines are present in each case, e.g., from Si, Fe, or Mg. Clear trends can be identified like stronger absorption by hydrogen-like Fe with increased depth of the dips. We present line identifications for the different absorption levels, as well as an initial interpretation. Also, further dip properties are investigated, e.g., there is residual soft X-ray emission below ~0.8 keV during the dips, likely from an extended atmosphere. The state of Cyg X-1 during the Chandra observation can be well characterized in the context of our multi-year RXTE monitoring of the source.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.