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P. S. Wojdowski (Massachusetts Institute of Technology)
We have observed SMC X-1 over an eclipse with the XMM observatory. The resolution of the Reflection Grating Spectrometer is sufficient to resolve the emission line triplets of ions from nitrogen to magnesium. For all of these triplets we detect flux only in the middle component, the intercombination line. We show that this pattern of line emission is consistent with the pre-existing paradigm: line emission by recombination and resonant line scattering in the photoionized wind of the massive star. The lack of observed flux in the component with the longest wavelength, the forbidden line, can be explained by pumping of the forbidden line into the intercombination line by UV photons from the high mass star. The lack of observed flux in the component with with the shortest wavelength, the resonance line, is consistent with line emission from recombination but not resonant line scattering in the photoionized wind. From this, we infer that the optical depths of the resonance lines of the helium-like triplets are large. From this and the observed fluxes of the intercombination lines, we derive upper limits of approximately 4\Pi steradians km/s on the product of the solid angle subtended at the neutron star by the region containing the helium-like ions and the velocity dispersion of the helium-like ion along lines of sight from the neutron star. As the expected terminal velocity of the wind in SMC X-1 is expected to be of order 1000 km/s, this represents a significant constraint on the structure and kinematics of the wind.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.