Abstract
Diversity of the X-ray observations of dwarf nova are still not fully understood. I review the X-ray spectral characteristics of dwarf novae during the quiescence in general explained by cooling flow models and the outburst spectra that show hard X-ray emission dominantly with few sources that reveal soft X-ray/EUV blackbody emission. The nature of aperiodic time variability of brightness of dwarf novae shows band limited noise, which can be adequately described in the framework of the model of propagating fluctuations. The frequency of the break (1-6 mHz) indicates inner disk truncation of the optically thick disk with a range of radii (3.0-10.0)×109 cm. The RXTE and optical (RTT150) data of SS Cyg in outburst and quiescence reveal that the inner disk radius moves towards the white dwarf and receeds as the outburst declines to quiescence. A preliminary analysis of SU UMa indicates a similar behaviour. In addition, I find that the outburst spectra of WZ Sge shows two component spectrum of only hard X-ray emission, one of which may be fitted with a power law suggesting thermal Comptonization occuring in the system. Cross-correlations between the simultaneous UV and X-ray light curves (XMM −Newton) of five DNe in quiescence show time lags in the X-rays of 96-181 sec consistent with travel time of matter from a truncated inner disk to the white dwarf surface. All this suggests that dwarf novae and other plausible nonmagnetic systems have truncated accretion disks indicating that the disks may be partially evaporated and the accretion may occur through hot (coronal) flows in the disk.
Highlights
Dwarf novae (DNe) are a class of cataclysmic variables (CVs) which are interacting compact binaries in which a white dwarf (WD, the primary star) accretes matter and angular momentum from a main sequence star filling its Roche-lobe
The material in the inner disk of nonmagnetic CVs initially moving with Keplerian velocity dissipates its kinetic energy in order to accrete onto the slowly rotating WD creating a boundary layer (BL)
Standard accretion disk theory predicts half of the accretion luminosity to originate from the disk in the optical and ultraviolet (UV) wavelengths and the other half to emerge from the boundary layer as X-ray and/or extreme UV (EUV)/soft X-ray emission which may be summerized as LBL∼Ldisk=GMW DMacc/2RW D=Lacc/2 (Lynden-Bell & Pringle 1974, Godon et al 1995)
Summary
Dwarf novae (DNe) are a class of cataclysmic variables (CVs) which are interacting compact binaries in which a white dwarf (WD, the primary star) accretes matter and angular momentum from a main (or post-main) sequence star (the secondary) filling its Roche-lobe. The material in the inner disk of nonmagnetic CVs initially moving with Keplerian velocity dissipates its kinetic energy in order to accrete onto the slowly rotating WD creating a boundary layer (BL) (see Mauche 1997, Kuulkers et al 2006 for an overview). Standard accretion disk theory predicts half of the accretion luminosity to originate from the disk in the optical and ultraviolet (UV) wavelengths and the other half to emerge from the boundary layer as X-ray and/or extreme UV (EUV)/soft X-ray emission which may be summerized as LBL∼Ldisk=GMW DMacc/2RW D=Lacc/2 During low-mass accretion rates, Macc
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