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ASASSN-21au is an ultracompact accreting white dwarf binary (AM CVn) with aperiod of approx. 58 min. Using multiwavelength observations of the system, wediscovered a dichotomy in the behavior of outbursts in AM CVns. The binaryshowed an initial brightness increase which lasted for at least 82 days,followed by an additional increase which lasted 2 weeks. Afterwards ASASSN-21auwent into superoutburst with a total duration of 19 days, showing an amplitudewith respect to quiescence of approx. 7.5 mags in g, with a precursor and anecho outburst. A correlation between X-rays, UV and optical was identified forthe first time in an AM CVn during this stage. The color evolution ofASASSN-21au indicates that during the superoutburst the dominant component wasthe accretion disk. The short duration, large amplitude and color evolution ofthe superoutburst agree with expectations from the disk instability model.These characteristics are opposite to the ones observed in SDSS J080710+485259and SDSS J113732+405458, which have periods of approx. 53 min and 60 min,respectively. The initially slow brightness increase in the light curve ofASASSN-21au and the behavior after the superoutburst favors a scenario in whichchanges in the mass-transfer rate led to disk instabilities, while the outburstmechanism of SDSS J080710+485259 and SDSS J113732+405458 has been attributed toenhanced mass-transfer alone. Further observations are needed to understand theorigin of this dichotomy.

The Fast Evolving, Tremendous and Blue Superoutburst in ASASSN-21au Reveals a Dichotomy in the Outbursts of Long-period AM CVns