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

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