Resolving the Effects of Resonant X-ray Line Scattering in Cen X-3 with Chandra

Abstract

The massive X-ray binary Cen X-3 was observed over approximately one quarter of the system's 2.08 day orbit, beginning before eclipse and ending slightly after eclipse center with the Chandra X-ray Observatory using its High-Energy Transmission Grating Spectrometer. The spectra show K shell emission lines from hydrogen- and helium-like ions of magnesium, silicon, sulfur, and iron as well as a K-alpha fluorescence emission feature from near-neutral iron. The helium-like n=2->1 triplet of silicon is fully resolved and the analogous triplet of iron is partially resolved. The helium-like triplet component flux ratios outside of eclipse are consistent with emission from recombination and subsequent cascades (recombination radiation) from a photoionized plasma. In eclipse, however, the w (resonance) lines of silicon and iron are stronger than that expected for recombination radiation, and are consistent with emission from a collisionally ionized plasma. The triplet line flux ratios at both phases can be explained more naturally, however, as emission from a photoionized plasma if the effects of resonant line scattering are included in addition to recombination radiation. We show that the emissivity due to resonant scattering depends sensitively on the line optical depth and, in the case of winds in X-ray binaries, this allows constraints on the wind velocity even when Doppler shifts cannot be resolved.