A possible mechanism for the formation of humps in the orbital light curves of WZ Sge cataclysmic variable stars

A mechanism for the formation of humps in the light curves of WZ Sge cataclysmic variables is proposed, based on the results of three-dimensional simulations of the gas dynamics in the V455 And system. Analysis of the computed flow structure shows the formation of an accretion disk and four shocks in the system, located at the outer edge of the disk. Moreover, a precessional spiral density wave forms in the accretion disk, which is essentially at rest in the observer's frame. As a consequence, all four shocks (two arms of a tidal shock, a hot line, and an outgoing shock) pass through the location of the precessional density wave during each rotation of the disk. The interaction of the precessional spiral density wave with the shocks leads to an additional local density enhancement in the vicinity of the shocks. This strengthens the shocks and enhances the energy release at the shock fronts; this, in turn, should increase the brightness of the system, which could be observed as a hump in the light curve. The shift of the humps observed in the V455 And light curve in orbital phase as a function of the observing epoch is easy to explain in this picture, as a consequence of the difference in the velocities of the retrograde precession of the density wave and the orbital rotation of the binary. The variation in the observed number of humps can be explained by the presence of four, rather than two, strong shocks in the flow pattern and the conditions for their visibility at a given observing time.