Numerical Simulations of Shock-driven Accretion

We calculate how accretion in a mass transferring binary system takes place if shock waves are the only means of angular momentum transport and energy dissipation. Cooling by radiation from the disk is included. In the absence of a mass transferring stream, with shocks excited by the tidal force only, the disk quickly settles into a quasi-stationary shock pattern. The presence of a stream impacting on the disk has a profound effect by keeping the flow very nonsteady. From simulations covering several hundred binary orbits, we find the following sequence of events. After an initial transient (which lasts on the order of 20 orbits) most of the mass transferred accumulates in a ring while a lower level accretion takes place from the ring onto the central object. For disk temperatures of a few percent of the local virial temperature, the effective alpha-viscosity, as measured by the accretion rate, during this phase is of the order 10-3. The size of the disk and the shape of the brightness distribution across it agree well with observations of quiescent CV disks. The rotation profile in the ring approaches a constant angular momentum distribution and then becomes violently unstable by a process observed earlier by Blaes and Hawley. During the instability, the accretion rate onto the central object is enhanced. Storage of mass in a ring alternating with accreting phases due to instability of the torus is expected to take place in general at low disk viscosity, whatever the process responsible for the viscosity. This provides a new mechanism for soft X-ray transients and the superoutburst cycle in cataclysmic variables.