IW And-Type State in IM Eridani

TL;DR

This paper reports observations of the IW And-type behavior in the dwarf nova IM Eridani, supporting the thermal-viscous disk instability model in a tilted disk, but without detecting the expected negative superhumps.

Contribution

It provides observational evidence of IW And-type cycles in IM Eridani consistent with the tilted disk instability model, highlighting both agreement and discrepancies with the theory.

Findings

Observed three IW And-type cycles in IM Eridani.

Detected characteristic oscillation and long cycle periods.

Did not detect negative superhumps, challenging some model predictions.

Abstract

IW And stars are a recently recognized group of dwarf novae which are characterized by a repeated sequence of brightening from a standstill-like phase with damping oscillations followed by a deep dip. Kimura et al. (2019) recently proposed a model based on thermal-viscous disk instability in a tilted disk to reproduce the IW And-type characteristics. IM Eri experienced the IW And-type phase in 2018 and we recorded three cycles of the (damping) oscillation phase terminated by brightening. We identified two periods during the IW And-type state: 4-5 d small-amplitude (often damping) oscillations and a 34-43 d long cycle. This behavior is typical for an IW And-type star. The object gradually brightened within the long cycle before the next brightening which terminated the (damping) oscillation phase. This observation agrees with the increasing disk mass during the long cycle predicted by a model of thermal-viscous disk instability in a tilted disk (Kimura et al. 2019). We, however, did not succeed in detecting negative superhumps, which are considered to be the signature of a tilted disk.