  We describe Chandra High-Energy Transmission Grating Spectrometer
observations of RT Cru, the first of a new sub-class of symbiotic stars that
appear to contain white dwarfs (WDs) capable of producing hard X-ray emission
out to greater than 50 keV. The production of such hard X-ray emission from the
objects in this sub-class (which also includes CD -57 3057, T CrB, and CH Cyg)
challenges our understanding of accreting WDs. We find that the 0.3 -- 8.0 keV
X-ray spectrum of RT Cru emanates from an isobaric cooling flow, as in the
optically thin accretion-disk boundary layers of some dwarf novae. The
parameters of the spectral fit confirm that the compact accretor is a WD, and
they are consistent with the WD being massive. We detect rapid, stochastic
variability from the X-ray emission below 4 keV. The combination of flickering
variability and a cooling-flow spectrum indicates that RT Cru is likely powered
by accretion through a disk. Whereas the cataclysmic variable stars with the
hardest X-ray emission are typically magnetic accretors with X-ray flux
modulated at the WD spin period, we find that the X-ray emission from RT Cru is
not pulsed. RT Cru therefore shows no evidence for magnetically channeled
accretion, consistent with our interpretation that the Chandra spectrum arises
from an accretion-disk boundary layer.
