KIC 7524178 -- an SU UMa-Type Dwarf Nova Showing Predominantly Negative Superhumps throughout Supercycle

TL;DR

This study reveals that KIC 7524178, an SU UMa-type dwarf nova, exhibits predominantly negative superhumps throughout its supercycle, providing new insights into superhump behavior and the thermal-tidal instability mechanism.

Contribution

It is the first known dwarf nova where negative superhumps dominate throughout the supercycle, supporting the thermal-tidal instability model for superoutbursts.

Findings

Negative superhumps dominate during the supercycle.

Superoutbursts are accompanied by transient positive superhumps.

The superhump periods are 0.07288 d (negative) and 0.0785 d (positive).

Abstract

We analyzed the Kepler long cadence data of KIC 7524178 (=KIS J192254.92+430905.4), and found that it is an SU UMa-type dwarf nova with frequent normal outbursts. The signal of the negative superhump was always the dominant one even during the superoutburst, in contrast to our common knowledge about superhumps in dwarf novae. The signal of the positive superhump was only transiently seen during the superoutburst, and it quickly decayed after the superoutburst. The frequency variation of the negative superhump was similar to the two previously studied dwarf novae in the Kepler field, V1504 Cyg and V344 Lyr. This is the first object in which the negative superhumps dominate throughout the supercycle. Nevertheless, the superoutburst was faithfully accompanied by the positive superhump, indicating that the tidal eccentric instability is essential for triggering a superoutburst. All the pieces of evidence strengthen the thermal-tidal instability as the origin of the superoutburst and supercycle, making this object the third such example in the Kepler field. This object had unusually small (~1.0 mag) outburst amplitude and we discussed that the object has a high mass-transfer rate close to the thermal stability limit of the accretion disk. The periods of the negative and positive superhumps, and that of the candidate orbital period were 0.07288 d (average, variable in the range 0.0723-0.0731 d), 0.0785 d (average, variable in the range 0.0772-0.0788 d) and 0.074606(1) d, respectively.