Abstract
Abstract 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.
Highlights
The power source of X-rays in cataclysmic variables (CVs) is known to be due to the accretion of matter onto a compact object: a white dwarf (WD)
CVs, mainly the nonmagnetic and IP types, share structural similarities with low-mass X-ray binaries (LMXBs); i.e., these systems contain an accretion disk, a transition layer (TL), and a compact object. Taking into account these geometric/structural similarities and the radiative processes used to describe the continuum of LMXBs—which has changed from bremsstrahlung to Comptonization—we tested whether the spectral continuum of nonmagnetic CVs (nmCVs) and IPs could be successfully described by the Comptonization of soft photons off the hot electrons of a Compton cloud around the compact object, as it is in LMXBs
Considering that nmCVs share similarities with LMXBs— that is, they have an accretion disk, a compact object, and a TL—we look for spectral similarities between these two types of X-ray binaries
Summary
The power source of X-rays in cataclysmic variables (CVs) is known to be due to the accretion of matter onto a compact object: a white dwarf (WD). CVs, mainly the nonmagnetic and IP types, share structural similarities with LMXBs; i.e., these systems contain an accretion disk (which appears either entirely in nmCVs or partially in IPs), a TL (corona), and a compact object Taking into account these geometric/structural similarities and the radiative processes used to describe the continuum of LMXBs—which has changed from bremsstrahlung to Comptonization—we tested whether the spectral continuum of nmCVs and IPs could be successfully described by the Comptonization (upscattering) of soft photons off the hot electrons of a Compton cloud around the compact object, as it is in LMXBs. In this paper, we present the results of this verification performed on nmCVs. We used four XMMNewton EPIC-pn, eight Chandra (LETG/HRC and HETG/ ACIS), and six RXTE (Proportional Counter Array (PCA) and HEXTE) public observations of four DNe: U Gem, SS Cyg, VW Hyi, and SS Aur. We used four XMMNewton EPIC-pn, eight Chandra (LETG/HRC and HETG/ ACIS), and six RXTE (Proportional Counter Array (PCA) and HEXTE) public observations of four DNe: U Gem, SS Cyg, VW Hyi, and SS Aur These sources were observed only in quiescence by XMM-Newton and in the quiescent and outburst states by Chandra and RXTE.
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