The 2022 active state of the AM CVn star NSV 1440

TL;DR

This paper reports the 2022 active state of the AM CVn star NSV 1440, characterized by increased brightness and superoutbursts, providing insights into the mechanisms behind such phenomena and their relation to similar events in other star types.

Contribution

It presents the first detailed observation of an active state in NSV 1440, linking it to the broader understanding of fading tails and outburst mechanisms in AM CVn stars.

Findings

Active state lasted ~200 days with brightness 2.0-2.5 mag above quiescence.

Brightness evolution resembled post-superoutburst fading tails from 2021.

Faint long superoutbursts in similar stars are likely not true disk instability outbursts.

Abstract

We found an active state lasting for ~200 d in the AM CVn star NSV 1440 in 2022. During this state, the object reached a magnitude of 16.5, 2.0-2.5 mag above quiescence, and showed a number of superposed normal outbursts. Such an active state was probably brought either by an enhanced mass-transfer from the secondary or increased quiescent viscosity of the accretion disk. These possibilities are expected to be distinguished by an observation of the interval to the next superoutburst. We also found that the brightness and the course toward the end of the event were similar to the post-superoutburst fading tail in 2021. The mechanism producing the 2022 active state and post-superoutburst fading tails in AM CVn stars may be the same, and the present finding is expected to clarify the nature of these still poorly understood fading tails in AM CVn stars, and potentially of the corresponding phenomenon in hydrogen-rich WZ Sge stars. We also note that the faint, long "superoutbursts" in long-period AM CVn stars claimed in the past were not true outbursts powered by disk instability, but were more likely phenomena similar to the 2022 active state in NSV 1440.