2020ApJ...900..153M

The X-ray spectra of nonmagnetic cataclysmic variables (nmCVs) in the ∼0.3-15 keV energy band have been described by either one or several optically thin thermal plasma components or by cooling flow models. We tested whether the spectral continuum in nmCVs could be successfully described by Comptonization of soft photons off hot electrons presented in a cloud surrounding the source (the transition layer (TL)). We used public XMM-Newton EPIC-pn, Chandra HETG/ACIS and LETG/HRC, and RXTE PCA and HEXTE observations of four dwarf novae (U Gem, SS Cyg, VW Hyi, and SS Aur) observed in the quiescent and outburst states. In total, we analyzed 18 observations, including a simultaneous 0.4-150 keV Chandra/RXTE spectrum of SS Cyg in quiescence. We fitted the spectral continuum with up to two thermal Comptonization components (the COMPTT or COMPTB models in XSPEC) using only one thermal plasma temperature and one optical depth. In this framework, the two seed photon components are presumably coming from the innermost and outer parts of the TL (or innermost part of the disk). We obtained that the thermal Comptonization can successfully describe the spectral continuum of these nmCVs in the ∼0.4-150 keV energy band. Moreover, we present the first principal radiative transfer model that explains the quasi-constancy of the spectral photon index observed around 1.8, which strongly supports the Comptonization framework in nmCVs.