Studies of long-period variability in the HMXB systems LMC X-4 and SMC X-1

We present results from our Rossi X-Ray Timing Explorer (RXTE) long-term monitoring campaign of the X-ray pulsars LMC X-4 and SMC X-1. We have analyzed spectra spanning a little more than five similar to 30-day cycles in LMC X-4 and nearly three similar to 60-day cycles in SMC X-1. While the results obtained for SMC X-l are consistent with a simple model in which the modulation is due to absorption in a tilted accretion disk, the LMC X-4 spectra are not. If the long-term modulation were due solely to variable absorption, the column density should be highest when the flux is lowest. The hydrogen column density obtained for LMC X-4 during the low portion of its long-period cycle, however, is consistent with zero, with a 1 sigma upper limit of 1.5 x 10(21) cm(-2).