Quasi-periodic Occultation by a Precessing Accretion Disk and Other Variabilities of SMC X-1

We have investigated the variability of the binary X-ray pulsar SMC X-1 in data from several X-ray observatories. We confirm the ~60 day cyclic variation of the X-ray flux in the long-term monitoring data from the RXTE and CGRO observatories. X-ray light curves and spectra from the ROSAT, Ginga, and ASCA observatories show that the uneclipsed flux varies by as much as a factor of 20 between a high-flux state when 0.71 s pulses are present and a low-flux state when pulses are absent. In contrast, during eclipses when the X-rays consist of radiation scattered from circumsource matter, the fluxes and spectra in the high and low states are approximately the same. These observations prove that the low state of SMC X-1 is not caused by a reduction in the intrinsic luminosity of the source or a spectral redistribution thereof, but rather by a quasi-periodic blockage of the line of sight, most likely by a precessing tilted accretion disk. In each of two observations in the midst of low states a brief increase in the X-ray flux and reappearance of 0.71 s pulses occurred near orbital phase 0.2. A similar brief flux increase near orbital phase 0.2 was observed during a low-state observation that did not have sufficient time resolution to detect 0.71 s pulses. These brief increases result from an opening of the line of sight to the pulsar that may be caused by wobble in the precessing accretion disk. The records of spin-up of the neutron star and decay of the binary orbit are extended during 1991-1996 by pulse-timing analysis of ROSAT, ASCA, and RXTE PCA data. The pulse profiles in various energy ranges from 0.1 to greater than 21 keV are well represented as a combination of a pencil beam and a fan beam. Finally, there is a marked difference between the power spectra of random fluctuations in the high-state data from the RXTE PCA below and above 3.4 keV. In the higher energy range the spectrum has a sharp break at 3.3 Hz, with fitted power-law indices of 0.45 and 1.76 below and above the break. No break is evident in the power spectrum below 3.4 keV, and the fitted power-law index is 0.51. In both spectra there is a positive deviation from the fitted power law around 0.06 Hz that may be quasi-periodic oscillation.