Spiral Structure in WZ Sagittae around the 2001 Outburst Maximum

Intermediate-resolution phase-resolved spectra of WZ Sge were obtained on five consecutive nights (July 23–27) covering the initial stage of the 2001 superoutburst. Double-peaked emission lines of He II at 4686 u A, which were absent on July 23, emerged on July 24 together with emission lines of C III/N III Bowen blend. Analyses of the He II emission lines using the Doppler tomography revealed an asymmetric spiral structure on the accretion disk. This finding demonstrates that spiral shocks with a very short orbital period can arise during the initial stage of an outburst and may be present in all SU UMa stars.