Pulse-Phase Spectroscopy of SMC X-1 with Chandra and XMM-Newton: Reprocessing by a Precessing Disk?

We present pulse-phase X-ray spectroscopy of the high-mass X-ray pulsar SMC X-1 from five different epochs, using Chandra X-Ray Observatory ACIS-S and XMM-Newton EPIC pn data. The X-ray spectrum consistently shows two distinct components, a hard power law and a soft blackbody with kT_BB ~ 0.18 keV. For all five epochs the hard component shows a simple double-peaked pulse shape and also a variation in the power-law slope, which becomes harder at maximum flux and softer at minimum flux. For the soft component, the pulse profile changes between epochs in both shape and phase relative to the power-law pulses. The soft component is likely produced by reprocessing of the hard X-ray pulsar beam by the inner accretion disk. We use a model of a twisted inner disk, illuminated by the rotating X-ray pulsar beam, to simulate pulsations in the soft component due to this reprocessing. We find that for some disk and beam geometries, precession of an illuminated accretion disk can roughly reproduce the observed long-term changes in the soft-pulse profiles.