A Phenomenon Resembling Early Superhumps in a New SU UMa-Type Dwarf Nova with a 2-Hour Orbital Period

TL;DR

This paper reports the discovery of a new SU UMa-type dwarf nova with a 2-hour orbital period that exhibits early superhump-like phenomena, analyzed through Kepler light curves during superoutburst and quiescence.

Contribution

It presents the first detailed analysis of a dwarf nova showing early superhump-like oscillations at a short orbital period, expanding understanding of superhump phenomena.

Findings

Identification of a new SU UMa-type dwarf nova with a 2-hour orbital period.

Observation of double-wave oscillations resembling early superhumps.

Discussion on how this phenomenon affects classification of dwarf novae.

Abstract

We investigate K2BS5, an optical transient that we identified in Campaign 13 of the Kepler/K2 archives by the "K2 Background Survey", and classify it as a new SU UMa-type dwarf nova. Using the light curve generated from Kepler's long-cadence observation mode, we analyze the dwarf nova during quiescence and superoutburst. Following 20 days of quiescence at the start of the observation, the system entered a superoutburst lasting 12 days, after which it experienced at least one rebrightening. K2BS5 clearly meets the criteria for an SU UMa star, but at the peak of the superoutburst, it also shows double-wave oscillations consistent with the spectroscopic orbital period, a phenomenon that closely resembles early superhumps in WZ Sge stars. While we do not classify K2BS5 as a WZ Sge system, we discuss how this phenomenon could complicate efforts to use the suspected detection of early superhumps to distinguish SU UMa-type dwarf novae from the recently recognized class of long-orbital-period WZ Sge systems.