SPECTRAL ECLIPSE MAPPING OF THE ACCRETION DISK IN THE NOVA-LIKE VARIABLE UX URSAE-MAJORIS

We analyse narrow-band eclipse light curves of the nova-like cataclysm ic variable UX UMa, obtained from low-resolution spectra spanning lamb dalambda3600-9800 angstrom. The light curves for narrow bands in the c ontinuum as well as those for individual spectral lines are treated in dependently, and are used to construct images of the accretion disk's brightness distribution using the maximum-entropy eclipse-mapping tech nique. Particular attention is paid to the propagation of statistical uncertainties in the data and to how the analysis may introduce system atic errors in the final result. From the many narrow band images we h ave reconstructed the spectra from isolated parts of the accretion dis k. These spectra reveal that the inner disk radiates a continuum spect rum which peaks in the near UV and has the hydrogen Balmer lines in ab sorption (with the exception of Halpha), whereas the outer disk is muc h fainter, has a much redder spectrum, and has Balmer emission lines. The bright spot spectrum suggests optically thick material with Balmer absorption lines, in contrast with other parts of the disk at the sam e radial distance from the white dwarf. Our analysis also reveals the presence of an uneclipsed component of the total light, whose spectrum is very red and has Balmer lines in emission. We show that the second ary star is the most likely source of this light. This unexpected feat ure of the eclipse mapping technique offers a new tool for an independ ent assessment of the secondary star's spectrum in eclipsing cataclysm ic variables.