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The X-ray emission from most accreting white dwarfs (WDs) in symbiotic binarystars is quite soft. Several symbiotic WDs, however, produce strong X-rayemission at energies greater than ~20 keV. The Swift BAT instrument hasdetected hard X-ray emission from 4 such accreting WDs in symbiotic stars: RTCru, T CrB, CD -57 3057, and CH Cyg. In one case (RT Cru), Swift detectedX-rays out to greater than 50 keV at a > 5 sigma confidence level. Combiningdata from the XRT and BAT detectors, we find that the 0.3-150 keV spectra of RTCru, T CrB, and CD -57 3057 are well described by emission from asingle-temperature, optically thin thermal plasma, plus an unresolved 6.4-6.9keV Fe line complex. The X-ray spectrum of CH Cyg contains an additional brightsoft component. For all 4 systems, the spectra suffer high levels of absorptionfrom material that both fully and partially covers the source of hard X-rays.The XRT data did not show any of the rapid, periodic variations that one wouldexpect if the X-ray emission were due to accretion onto a rotating, highlymagnetized WD. The X-rays were thus more likely from the accretion-diskboundary layer around a massive, non-magnetic WD in each binary. The X-rayemission from RT Cru varied on timescales of a few days. This variability isconsistent with being due to changes in the absorber that partially covers thesource, suggesting localized absorption from a clumpy medium moving into theline of sight. The X-ray emission from CD -57 3057 and T CrB also varied duringthe 9 months of Swift observations, in a manner that was also consistent withvariable absorption.

Swift Observations of Hard X-ray Emitting White Dwarfs in Symbiotic Stars