Advection-Dominated Accretion Onto Weakly-Magnetized White Dwarfs

Abstract

The boundary layers of weakly-magnetized white dwarfs (WDs) accreting at rates <=10^16 g/s are radially extended, hot, optically-thin, and they advect some of their internally-dissipated energy (Narayan & Popham 1993). Motivated by this, I construct here idealized spectral models of an Advection-Dominated Accretion Flow (ADAF) around a WD, for application to quiescent Dwarf Novae (DN). The Bremsstrahlung cooling of the gas in the ADAF, with temperatures ranging from a few keV to a few tens of keV, can account for the X-ray emission properties of quiescent DN. If the energy advected by the flow is thermalized in the WD atmosphere, the resulting emission from the entire stellar surface (blackbody of temperature T_eff ~ 5 eV) outshines the X-ray luminosity substantially. This extreme-UV component provides a flux in the 0.055-0.28 keV band which is sufficient to power the strong HeII lambda4686 emission lines of quiescent DN by photoionization of the disk material. Reprocessing of the ADAF X-ray emission by a cold outer thin disk could also lead to an observable iron Kalpha fluorescence emission line, which can be used to probe the geometry of the accretion flow. Existing observational data indicate that the presence of ADAFs in quiescent DN is not ubiquitous, while future observations, in particular with the X-ray satellites Chandra and XMM-Newton, have the potential to detect signatures of the hot flow in promising candidates.