Observational study of double superoutbursts in long-orbital-period AM CVn stars

Abstract We present an observational study of superoutbursts in six AM CVn systems with orbital periods longer than 35 min. We focused on this range because such systems are expected to have very low mass ratios, making them particularly likely to exhibit double superoutbursts and fading-tail profiles. Using time-resolved photometry from the Variable Star Network and the American Association of Variable Star Observers campaigns, complemented by survey data from the Zwicky Transient Facility, the All-Sky Automated Survey for Supernovae, the Asteroid Terrestrial-impact Last Alert System, Gaia, and the Transiting Exoplanet Survey Satellite, we analyzed nine superoutbursts observed between 2014 and 2021. Among them, four events exhibited clear double superoutburst profiles, analogous to those seen in hydrogen-rich WZ Sge-type dwarf novae. We characterized the fading tail following the second superoutburst and found that it can be divided into three distinct phases (tails A, B, and C) with different power-law fading indices. The effective superoutburst durations were typically 7–13 d, while the dip duration is significantly different among systems. Notably, the effective durations of the second superoutbursts in the double superoutburst were consistently around 5 d. These results highlight the importance of thermal–tidal instabilities in explaining the outbursts of long-orbital-period AM CVn stars. They also suggested that the three-phase fading-tail structure may be a universal feature of the systems and further imply that in systems with short supercycles the effective viscosity during quiescence could be higher than expected from a true quiescent state.