Eclipse Mapping of Accretion Disks in Cataclysmic Binaries

Citation

Horne, Keith Douglas (1983) Eclipse Mapping of Accretion Disks in Cataclysmic Binaries. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5E49-4N30. https://resolver.caltech.edu/CaltechETD:etd-09082008-140704

Abstract

Surface brightness distributions for accretion disks in eclipsing cataclysmic binaries may be reconstructed from observed eclipse light curves by use maximum entropy imaging methods. Three-color eclipse photometry of six eclipsing cataclysmic binaries provides the basis for an observational study of the local and global structure of their accretion disks. Observations from several eclipse cycles are averaged to form mean eclipse profiles with diminished contamination by flickering. Flickering is shown to be widely distributed over the face of the accretion disks in RW Tri and LX Ser. Accretion disk maps are derived which give satisfying fits to the observed eclipse profiles and which are maximally symmetric about the center of the disk. Disk maps for nova DQ Her and for four nova-like systems show broad intensity distributions with bright spots, associated with the gas streams that feed the disks, in evidence at a radii comparable to the disk tidal radius. The HT Cas disk is smaller and is dominated by a compact source at its center. Analysis of disk intensity maps at U, B and R show that the brighter parts of the accretion disks are optically-thick thermal radiators with temperatures which decrease with radius. The angular scale of each disk is fixed by a method analogous to cluster main sequence fitting. Distance estimates are derived which are free of assumptions about the global structure of the disks, but which depend on currently uncertain dyamical parameters of the binaries. The temperature in the disk is observed to fall much more slowly with radius than the R^-3/4 law predicted by a steady-state mass-conserving viscous accretion disk model. Apparently, much of the visible light from the disk is produced by reprocessing of hard radiation from the center of the disk. Upper limits are found for the mass transfer rates which are one to two orders of magnitude smaller than the canonical mass transfer rate for novae and nova-like systems and which thus are in better agreement with mass transfer driven by gravitational radiation alone. Mass transfer rates have previously been systematically overestimated by the neglect of reprocessing. The accretion disk in HT Cas is shown to be optically thin in the Paschen continuum. The compact bright source at the center of its disk is a composite of light from the photosphere of the white dwarf and of free-free emission from hot gas in the accretion boundary layer.

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