Variability of the X-ray P Cygni Line Profiles from Circinus X-1 Near Zero Phase

Abstract

The luminous X-ray binary Circinus X-1 has been observed twice near zero orbital phase using the High-Energy Transmission Grating Spectrometer (HETGS) onboard Chandra. The source was in a high-flux state during a flare for the first observation, and it was in a low-flux state during a dip for the second. Spectra from both flux states show clear P Cygni lines, predominantly from H-like and He-like ion species. These indicate the presence of a high-velocity outflow from the Cir X-1 system which we interpret as an equatorial accretion-disk wind, and from the blueshifted resonance absorption lines we determine outflow velocities of 200 - 1900 km/s with no clear velocity differences between the two flux states. The line strengths and profiles, however, are strongly variable both between the two observations as well as within the individual observations. We characterize this variability and suggest that it is due to both changes in the amount of absorbing material along the line of sight as well as changes in the ionization level of the wind. We also refine constraints on the accretion-disk wind model using improved plasma diagnostics such as the He-like Mg XI triplet, and we consider the possibility that the X-ray absorption features seen from superluminal jet sources can generally be explained via high-velocity outflows.