The Discovery of Broad P Cygni X-ray Lines from Circinus X-1 with the Chandra High Energy Transmission Grating Spectrometer

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 about 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 Hα 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 may arise in the moderate temperature (∼ 5×106 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. Subject headings: stars: individual (Cir X-1) — stars: neutron — X-rays: stars — binaries: close — accretion: accretion disks — techniques: spectroscopic