X-ray variations in the inner accretion flow of Dwarf Novae

Abstract

We show for five DN systems, SS Cyg, VW Hyi, RU Peg, WW Cet and T Leo that the UV and X-ray power spectra of their time variable light curves are similar in quiescence. All of them show a break in their power spectra, which in the framework of the model of propagating fluctuations indicates inner disk truncation. We derive the inner disk radii for these systems in a range (10-3)$\times 10^9$ cm. We analyze the RXTE data of SS Cyg in outburst and compare it with the power spectra, obtained during the period of quiescence. We show that during the outburst the disk moves towards the white dwarf and recedes as the outburst declines. We calculate the correlation between the simultaneous UV and X-ray light curves of the five DN studied in this work, using the XMM-Newton data obtained in the quiescence and find X-ray time lags of 96-181 sec. This can be explained by the travel time of matter from a truncated inner disk to the white dwarf surface. We suggest that, in general, DN may have truncated accretion disks in quiescence which can also explain the UV and X-ray delays in the outburst stage and that the accretion may occur through coronal flows in the disk (e.g., rotating accretion disk coronae). Within a framework of the model of propagating fluctuations the comparison of the X-ray/UV time lags observed by us in the case of DN systems with those, detected for a magnetic Intermediate Polar allows us to make a rough estimate of the viscosity parameter $\alpha \sim 0.25$ in the innermost parts of the accretion flow of DN systems.

10 pages, 11 figures; accepted for publication in A&A as it stands

References in corpus (4)

• Modelling the behaviour of accretion flows in X-ray binaries
• The Universal Nature of Accretion-induced Variability: The RMS-Flux Relation in an Accreting White Dwarf
• XMM-Newton observations of the dwarf nova RU Peg in quiescence: Probe of the boundary layer
• On the Change of the Inner Boundary of an Optically Thick Accretion Disk around White Dwarfs Using the Dwarf Nova SS Cyg as an Example

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