Fourier time lags in the dwarf nova SS Cygni

To understand the physical processes governing accretion discs we can study active galactic nuclei (AGN), X-ray binary systems (XRBs) and cataclysmic variables (CVs). It has been shown that XRBs and CVs show similar observational properties such as recurrent outbursts and aperiodic variability. The latter has been extensively studied for XRBs, but only recently have direct phenomenological analogies been found between XRBs and CVs, including the discovery of the rms--flux relation and the optical detection of Fourier-dependent time-lags. We present a Fourier analysis of the well-known CV SS Cyg in quiescence based on data collected at the 4.2--m William Herschel Telescope using ULTRACAM. Light curves in SDSS filters $u'$, $g'$ and $r'$ were taken simultaneously with sub-second cadence. The high cadence and sensitivity of the camera allow us to study the broad-band noise component of the source in the time range $\sim 10000 - 0.5$ s ($10^{-4}-2$ Hz). Soft/negative lags with an amplitude ~ 5 s at a time-scale of ~ 250 s were observed, indicating that the emission in the redder bands lags the emission in the bluer bands. This effect could be explained by thermal reprocessing of hard photons in the innermost region of the accretion disc, assuming a high viscosity parameter $\alpha > 0.3$, and high irradiation of the disc. Alternatively, it could be associated with the recombination time-scale on the upper layer of the accretions disc.