On the nature of quiescent light curves demonstrated by WZ Sge stars

Abstract

We present the results of simultaneous spectroscopic and photometric observations of the cataclysmic variable star (hereafter CVs) V455 Andromedae, belonging to the WZ Sge sub-class, in quiescence. Using the spectroscopic data we computed time-resolved Doppler tomograms of the system demonstrating its behavior at different orbital phases. In the tomograms one can see the periodic brightening of different regions within one orbital cycle. We interpret this brightening as being due to the interaction of four phase-locked shock waves in the disk with a specific internal precessing density wave that develops inside the disk, because of the tidal influence of the secondary star, and this density wave propagates up to the disk's outer regions. When the outer part of the density wave in its precessional motion reaches a shock wave the local density grows, which amplifies the shock (by increasing $\rho V^{2}/2$). This results in an additional energy release in the shock and can be observed as a brightening. Analysis of the tomographic results and the photometric data shows that two main sources contribute to the light curves of the system: the radiation of the "hot line" and the bow-shock gives us two major orbital humps, located approximately at the orbital phases $\phi=0.25$ and $\phi=0.75$; the amplification of the four shock waves may give us up to four "superhumps" shifting over the light curve with the precessional period. These two effects, when overlapping, change the shape of the light curves, shift the hump maxima, and they sometimes produce more than two humps in the light curve. We should emphasize that when saying "superhumps" we imply an effect that is observed in quiescent light curves of WZ Sge stars, as opposed to "classical" superhumps usually observed in outbursts.