Accretion and Outflow in Interacting Binary Systems: Far Ultraviolet Spectroscopic Explorer Observations of the Nova‐like Cataclysmic Variable UX Ursae Majoris

Abstract

We present far-ultraviolet (905-1182 Å), time-series spectroscopy of the eclipsing, nova-like cataclysmic variable UX UMa acquired with Far Ultraviolet Spectroscopic Explorer (FUSE). The time-averaged spectrum is complex and is dominated by overlapping spectral features. The most prominent features are emission lines of C III, N III, N IV, and O VI. They are broad (FWHM>=1800 km s-1) and double-peaked, with a central absorption at zero velocity. During eclipse, the spectrum is simpler: the emission lines remain bright, but the absorption components of the lines and the weaker features between the emission lines disappear entirely, leaving a flat continuum. This behavior is also evident in Goddard High-Resolution Spectrograph (1149-1660 Å) spectra that we retrieved from the Hubble Space Telescope (HST) archive. The FUV spectra show flickering on timescales of several minutes. The flickering is seen primarily in the continuum and/or the weaker lines rather than in the prominent emission lines. The orbital light curve has a dip in the FUV flux between orbital phases 0.45-0.65, similar to a preeclipse dip detected in HST observations. The equivalent widths of the line absorption features decrease during the dip. We have detected a systematic wavelength shift of spectral features on the orbital period, but with a phase lag of ~=20°, a phenomenon that has been reported at optical wavelengths. We discuss the implications of our results in the context of models of an accretion disk with a chromosphere between the disk and the extended wind. Finally, we note that the observed FUV flux is too low to be consistent with the temperature and radius of the WD derived in 1995 by Baptista et al., suggesting that their remaining binary parameters, including a mass ratio of 1, ought to be viewed with skepticism. Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS 5-32985.