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BIB-VERSION:: AST-PP-v1.0
ID:: epreps.stsci//prep1207
ENTRY:: March 9, 1998
TITLE:: Recovery of 29-s Oscillations in HST/FOS Eclipse Observations of the Cataclysmic Variable UX Ursae Majoris
SUBTITLE::
AUTHOR:: Knigge, Christian (1)
AUTHOR:: Drake, Nick (2)
AUTHOR:: Long, Knox S. (1)
AUTHOR:: Wade, Richard A. (3)
AUTHOR:: Horne, Keith (4)
AUTHOR:: Baptista, Raymundo (5)
AFFIL:: (1) Space Telescope Science Institute 3700 San Martin Drive Baltimore, MD 21218 USA
AFFIL:: (2) The University of Southampton, Department of Physics & Astronomy, Southampton SO17 1BJ, United Kingdom
AFFIL:: (3) The Pennsylvania State University, Department of Astronomy and Astrophysics, 525 Davey Laboratory, University Park, PA 16802, USA
AFFIL:: (4) Department of Physics and Astronomy, The University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9SS, UK
AFFIL:: (5) Departamento de Fisica, Universidade de Santa Catarina Campus Universitario, Trindade, 88040 Florianopolis, Brasil
DATE:: December 1997
JOURNAL:: To appear in: The Astrophysical Journal
SUBMITTED:: 6 August 1997
ACCEPTED:: 25 November 1997
OTHER_ACCESS::
COPYRIGHT:: Copyright 1997 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
LANGUAGE:: English
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 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 -360 SIZE=-1>^o 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 HEIGHT=14> 95,000 K ( ALIGN=baseline WIDTH=11 HEIGHT=14> 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.

END:: epreps.stsci//prep1207