AN OUTBURST OF THE MAGNETIC CATACLYSMIC VARIABLE XY ARIETIS OBSERVED WITH RXTE

We report the first observed outburst of the magnetic cataclysmic vari able XY Ari. X-ray observations show a flux increase by an order of ma gnitude the day after the first signs of outburst. For the 5-d duratio n the X-ray spin pulse is greatly enhanced and the X-ray spectrum far more absorbed. We suggest that the inner disc pushes inwards during ou tburst, blocking the view to the lower accreting pole, breaking the sy mmetry present in quiescence, and so producing a large pulsation. The observations are consistent with a disc instability as the cause of th e outburst, although we cannot rule out alternatives. We draw parallel s between our data and the UV delay and dwarf nova oscillations seen i n non-magnetic dwarf novae.