STScI Preprint #1207
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., Under NASA Contract NAS 5-2655
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 29-s oscillations are seen in one pair of eclipse sequences obtained with the FOS/PRISM in November of 1994, but not in a similar pair obtained with the FOS/G160L grating in August of the same year. The oscillations in the PRISM data are sinusoidal to within the small observational errors and undergo an approximately -360o phase shift during eclipses (i.e. one cycle is lost). Their amplitudes are highest at pre-eclipse orbital phases and exhibit a rather gradual eclipse whose shape is roughly similar to, though perhaps slightly narrower than, UX UMa's overall light curve in the PRISM bandpass (2000 Å - 8000 Å).
Spectra of the oscillations have been constructed from pre-, mid- and post-eclipse data segments of the November observations. The spectra obtained from the out-of-eclipse segments are extremely blue, and only lower limits can be placed on the temperature of the source which dominates the modulated flux at these orbital phases. Lower limits derived from blackbody (stellar atmosphere) model fits to these data are 95,000 K ( 85,000 K); the corresponding upper limits on the projected area of this source are all < 2% of the WD surface area. By contrast, oscillation spectra derived from mid-eclipse data segments are much redder. Fits to these spectra yield temperature estimates in the range 20,000 K T 30,000 K for both BB and SA models and corresponding projected areas of a few percent of the WD surface area. These estimates are subject to revision if the modulated emission is optically thin.
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 a classical, equatorial boundary layer. Instead, this component could arise in a bright spot on the surface of the WD, possibly associated with a magnetic pole. However, a standard intermediate polar model can also be ruled out, since UX UMa's oscillation period has been seen to change on time-scales much shorter than the minimum time-scale required to spin up the WD by accretion torques. A model invoking magnetically controlled accretion onto differentially rotating WD surface layers may be viable, but needs more theoretical work.