HEAD Division Meeting 1999, April 1999
Session 15. X-Ray Binaries
Poster, Tuesday, April 13, 1999, 8:30am-6:02pm, Gold Room

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[15.21] Variability of the X-ray Spectra of LMC~X-4 and SMC~X-1 During Their Long-Period Variability

W.A. Heindl, D.E. Gruber, R.E. Rothschild (UCSD/CASS), C.J. Crannell (NASA/GSFC), F.L. Lang (CUA and NASA/GSFC), Leah Kaplan (Wellesley College)

LMC~X-4 and SMC~X-1 are accreting X-ray pulsars, which, like Her~X-1, have approximately periodic modulations of their X-ray flux on timescales much longer than the neutron star spin period and the binary orbital period. In Her~X-1, it is well established that these long-period cycles are due to occultation by a twisted, precessing accretion disk. We present results from a Rossi X-ray Timing Explorer ({\em RXTE}) long-term monitoring campaign on LMC~X-4 and SMC~X-1. By measuring the X-ray spectrum at fixed orbital phase but varying long-term phase, we hoped to test the hypothesis that these systems have precessing accretion disks. In SMC~X-1, the neutral column to the source was highest during the ``off'' part of the cycle, which is consistent with absorption in a cool accretion disk. In LMC~X-4, however, no neutral column was significantly detected at any time. This means that if the flux modulation is caused by a disk, it must occur at the hot, ionized inner disk region where Thomson scattering can remove photons from the beam without affecting the overall spectral shape.

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