An outburst of the magnetic cataclysmic variable XY Arietis observed with RXTE

We report the first observed outburst of the magnetic cataclysmic variable XYAri. X-ray observations show a flux increase by an order of magnitude the dayafter the first signs of outburst. During the 5-d duration the X-ray spin pulseis greatly enhanced and the X-ray spectrum far more absorbed. We suggest thatthe inner disc pushes inwards during outburst, blocking the view to the loweraccreting pole, breaking the symmetry present in quiescence, and so producing alarge pulsation. The observations are consistent with a disc instability as thecause of the outburst, although we can't rule out alternatives. We drawparallels between our data and the UV delay and dwarf nova oscillations seen innon-magnetic dwarf novae.