Discovery of Fourier-dependent time lags in cataclysmic variables

We report the first study of Fourier-frequency-dependent coherence and phase/time lags at optical wavelengths of cataclysmic variables (MV Lyr and LU Cam) displaying typical flickering variability in white light. Observations were performed on the William Herschel Telescope using ULTRACAM. Light curves for both systems have been obtained with the SDSS filters u′, g′ and r′ simultaneously with cadences in the range ≈0.5-2 s, and allow us to probe temporal frequencies between ≈10−3 and ≈1 Hz. We find high levels of coherence between the u′, g′ and r′ light curves up to at least ≈10−2 Hz. Furthermore, we detect red/negative lags where the redder bands lag the bluer ones at the lowest observed frequencies. For MV Lyr time lags up to ≈3 s are observed, whilst LU Cam displays larger time lags of ≈10 s. Mechanisms which seek to explain red/negative lags observed in X-ray binaries and active galactic nuclei involve reflection of photons generated close to the compact object on to the surface layers of the accretion disc, where the lag delay is simply the light travel time from the emitting source to the reflecting accretion disc area. Although this could be a viable explanation for the lags observed in MV Lyr, the lags observed in LU Cam are too large to be explained by reflection from the disc and/or the donor star. We suggest reprocessing on the thermal time-scale of boundary layer photons on to the accretion disc as a possible mechanism to explain the lags observed in accreting white dwarfs, or reverse (inside-out) shocks within the disc travelling through cooler disc regions as they move outwards.