Recovery of 29-s Oscillations in HST/FOS Eclipse Observations of the Cataclysmic Variable UX Ursae Majoris

Abstract

[abridged abstract] Low amplitude (~=0.5%) 29-s oscillations have been detected in HST/FOS eclipse observations of the nova-like cataclysmic variable UX UMa. These are the same dwarf nova-type oscillations that were originally discovered in this system by Warner & Nather in 1972. The oscillations are sinusoidal to within the small observational errors and undergo an approximately -360 degree phase shift during eclipses. Their amplitudes are highest at pre-eclipse orbital phases and exhibit a rather gradual eclipse whose shape is roughly similar to UX~UMa's overall light curve. Oscillation spectra derived from pre-eclipse data segments are extremely blue, whereas mid-eclipse oscillation spectra are much redder. We suggest that the ultimate source of the oscillations is a hot, compact region near disk center, but that a significant fraction of the observed, modulated flux is due to reprocessing of the light emitted by this source in the accretion disk atmosphere. The compact source is occulted at orbital phases near mid-eclipse, leaving only part of the more extended reprocessing region(s) to produce the weak oscillations that persist even at conjunction. The highly sinusoidal oscillation pulse shape does not permit the identification of the compact component in this model with emission produced by a rotating disturbance in the inner disk or in an equatorial boundary layer, and a standard intermediate polar model can also be ruled out. A model invoking magnetically controlled accretion onto differentially rotating WD surface layers may be viable, but needs more theoretical work.