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W.N. Brandt (Penn State), N.S. Schulz (MIT)
We present the first grating-resolution X-ray spectra of the X-ray binary Cir X-1, obtained with the High Energy Transmission Grating Spectrometer on the Chandra X-ray Observatory. These reveal a rich set of highly significant lines from H-like and/or He-like Ne, Mg, Si, S and Fe. Remarkably, the lines are broad (±2000 km s-1) and show P Cygni profiles. The absorption components of the lines extend to low velocity, and they have almost the same widths and strengths as the corresponding emission components. The widths of the X-ray P Cygni lines are comparable to that of the broad component of the strong, asymmetric H-alpha line that has been intensively studied over the past 20 years, suggesting that the two phenomena are related. We discuss outflow models and propose that the P Cygni profiles arise in the moderate temperature (~ 5\times 106 K) region of the wind from a flared and X-ray heated accretion disk. This basic picture strengthens the idea that the accretion disk in Cir X-1 is viewed in a relatively edge-on manner, and it suggests that Cir X-1 is the X-ray binary analog of a Broad Absorption Line quasar.
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