  X-ray spectra of dwarf novae in quiescence observed by Chandra and XMM-Newton
provide new information on the boundary layers of their accreting white dwarfs.
Comparison of observations and models allows us to extract estimates for the
thermal conductivity in the accretion layer and reach conclusions on the
relevant physical processes. We calculate the structure of the dense thermal
boundary layer that forms under gravity and cooling at the white dwarf surface
on accretion of gas from a hot tenuous ADAF-type coronal inflow. The
distribution of density and temperature obtained allows us to calculate the
strength and spectrum of the emitted X-ray radiation. They depend strongly on
the values of thermal conductivity and mass accretion rate. We apply our model
to the dwarf nova system VW Hyi and compare the spectra predicted for different
values of the thermal conductivity with the observed spectrum. We find a
significant deviation for all values of thermal conductivity that are a sizable
fraction of the Spitzer conductivity. A good fit arises however for a
conductivity of about 1% of the Spitzer value. This also seems to hold for
other dwarf nova systems in quiescence. We compare this result with thermal
conduction in other astrophysical situations. The highly reduced thermal
conductivity in the boundary layer requires magnetic fields perpendicular to
the temperature gradient. Locating their origin in the accretion of magnetic
fields from the hot ADAF-type coronal flow we find that dynamical effects of
these fields will lead to a spatially intermittent, localized accretion
geometry at the white dwarf surface.
