Outbursts of WZ Sagittae

Abstract

The outbursts of the dwarf nova WZ Sagittae occur on timescales which are much longer than expected from the standard thermal viscous disc instability model, which correctly reproduces the outburst behaviour of many other dwarf novae. In addition short `echo' outbursts have sometimes been observed after the main outburst. The increased timescales could be explained by a magnetic propeller, anchored to the primary star, which clears the inner disc . We argue that the propeller, combined with the resonances of the binary orbit, may also be the cause of the echo outbursts. We present numerical models which reproduce the echo outbursts.