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.


If the author provided an email address or URL for general inquiries, it is as follows:

wheindl@ucsd.edu


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