2005MNRAS.357...69R

We exploit rare observations covering the time before and during a normal outburst in the deeply eclipsing SUUMa system IY UMa to study the dramatic changes in the accretion flow and emission at the onset of outburst. Through Doppler tomography we study the emission distribution, revealing classic accretion flow behaviour in quiescence, with the stream-disc impact ionizing the nearby accretion disc. We observe a delay of hours to a couple of days between the rise in continuum and the rise in the emission lines at the onset of the outburst. From line profiles and Doppler maps during normal and superoutburst, we conclude that reprocessing of boundary layer (BL) radiation is the dominant emission line mechanism in outburst and that the normal outburst began in the outer disc. The stream-disc impact feature (the orbital hump) in the Hα line flux light curve disappears before the onset of the normal outburst and may be an observable signal heralding an impending outburst.