Smoothed particle hydrodynamics simulations of apsidal and nodal superhumps

In recent years, a handful of systems have been observed to show "negative" (nodal) superhumps, with periods slightly shorter than the orbital period. It has been suggested that these modes are a consequence of the slow retro grade precession of the line of nodes in a disk tilted with respect to the orbital plane. Our simulations confirm and refine this model: they suggest a roughly axisymmetric, retrogradely precessing, tilted disk that is driven at a period slightly less than half the orbital period as the tidal field of the orbiting secondary encounters, in turn, the two halves of the disk a bove and below the midplane. Each of these passings leads to viscous dissip ation on one face of an optically thick disk - observers on opposite sides of the disk would each observe one brightening per orbit, but 180 degrees o ut of phase with each other.