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 Kα 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. We measure fluxes, shifts, and widths of the observed emission lines. 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 with a temperature of ~100 eV. 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 with a temperature of ~1 keV. 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, a process that has generally been neglected in X-ray spectroscopy, are included in addition to recombination radiation. We show that resonant line scattering in photoionized plasmas may increase the emissivity of n = 2 → 1 line emission in hydrogen- and helium-like ions by a factor as large as 4 relative to that of pure recombination, and so previous studies, in which resonant scattering has been neglected, may contain significant errors in the derived plasma parameters. The emissivity due to resonance 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.
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