Abstract
We present a broad-band X-ray study of the effect of superorbital periods on
X-ray spectra and pulse profiles in the neutron star X-ray binaries LMC X-4 and
SMC X-1. These two sources display periodic or quasi-periodic variations in
luminosity on the order of tens of days which are known to be superorbital, and
are attributed to warped, precessing accretion disks. Using joint
NuSTAR and XMM-Newton observations that span a complete
superorbital cycle, we examine the broad-band spectra of these sources and find
the shape to be well described by an absorbed power law with a soft blackbody
component. Changes in spectral shape and pulse profile shape are periodic with
superorbital period, as expected from a precessing disk. We perform X-ray
tomography using the changes in pulse profiles to model the geometry and
kinematics of the inner accretion disk. Our simple beam and inner disk
geometric model indicates that the long term changes in soft pulse shape and
phase are consistent with reprocessed emission from a precessing inner disk.
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