V1006 Cygni: Dwarf Nova Showing Three Types of Outbursts and Simulating Some Features of the WZ Sge-Type Behavior

TL;DR

This study of V1006 Cygni, an SU UMa-type dwarf nova, reveals it exhibits three outburst types, shows stage A superhumps with a near-stability-limit mass ratio, and models features of WZ Sge-type behavior through weak tidal effects.

Contribution

First detailed observation of V1006 Cygni's three outburst types and superhump development near the tidal instability limit, linking outburst behavior to tidal effects.

Findings

V1006 Cygni shows three types of outbursts.

Stage A superhumps grow near the tidal stability limit.

Weak tidal effects explain features of extreme dwarf novae.

Abstract

We observed the 2015 July-August long outburst of V1006 Cyg and established this object to be an SU UMa-type dwarf nova in the period gap. Our observations have confirmed that V1006 Cyg is the second established object showing three types of outbursts (normal, long normal and superoutbursts) after TU Men. We have succeeded in recording the growing stage of superhumps (stage A superhumps) and obtained a mass ratio of 0.26-0.33, which is close to the stability limit of tidal instability. This identification of stage A superhumps demonstrated that superhumps indeed slowly grow in systems near the stability limit, the idea first introduced by Kato et al. 2014, arXiv:1406.6428). The superoutburst showed a temporary dip followed by a rebrightening. The moment of the dip coincided with the stage transition of superhumps, and we suggest that stage C superhumps is related to the start of the cooling wave in the accretion disk. We interpret that the tidal instability was not strong enough to maintain the disk in the hot state when the cooling wave started. We propose that the properties commonly seen in the extreme ends of mass ratios (WZ Sge-type objects and long-period systems) can be understood as a result of weak tidal effect.