The Nature of the Hard X-Ray-Emitting Symbiotic Star RT Cru

We describe Chandra High Energy Transmission Grating Spectrometer observations of RT Cru, the first of a new subclass 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 subclass (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.