Simultaneous Chandra and Hubble Space Telescope Observations of SMC X-1

Abstract

We present first results from simultaneous ultraviolet (Hubble Space Telescope/STIS) and X-ray (Chandra/ACIS) observations of the SMC X-1/SK 160 eclipsing binary system. Observations covering four orbital phases during each of the X-ray high and X-ray low states were taken in 2000 October-November and 2001 April. The ultraviolet P Cygni lines show strong broad absorption near X-ray eclipse and narrow absorption when the X-ray source is in the line of sight. The effect is visible during both the X-ray high and X-ray low states; the UV continuum flux remains roughly constant in spite of more than an order-of-magnitude reduction in X-ray flux, as expected if the X-ray flux reduction is due to occultation of the X-ray source by a precessing disk rather than an intrinsic change in X-ray luminosity. The X-ray spectra are dominated by continuum emission in the X-ray high state. Occultation of the neutron star by the disk during the low state also implies that X-ray emission from the disk surface should be present, and the low-state spectra do show strong emission lines. During eclipse and during the X-ray low state, the continuum emission largely disappears, and we see line emission from O, Ne, Mg, and Fe and possibly from Si and S. The emission lines are consistent with recombination lines from mostly hydrogenic and helium-like species, which could be produced by photoionization in an extended stellar wind. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.