Discovery of a new IW And-type dwarf nova with both tilted disc and tidal instability

Abstract

ABSTRACT IW And-type dwarf novae are anomalous Z Cam stars featured with outbursts happening during standstill states, which are not expected in the standard disc instability model. The physical mechanisms for these variations remain unclear. In this study, we report the discovery of a new candidate IW And-type dwarf nova J0652+2436, identified with its frequent outbursts from the slowly rising standstill states. Luckily, the Transiting Exoplanet Survey Satellite observations during a long standstill state and the earlier K2 observations give a chance to find the orbital and negative superhump period in the light curve of J0652+2436, allowing the measurement of its mass ratio of 0.366. This mass ratio is marginally possible for the tidal instability to set in according to previous smoothed particle hydrodynamics simulations. Thus, we propose that the outbursts in J0652+2436 are likely to be caused by the growing accretion disc during standstills, in favour of the previous hypothesis of the mechanisms lying in all IW And stars. We conclude that J0652+2436 might be the first IW And star with both a precessing tilted disc and tidal instability, which will be an important laboratory for studying the accretion disc dynamics and help understand the IW And phenomenon.