V1006 Cygni: Dwarf nova showing three types of outbursts and simulating some features of the WZ Sge-type behavior

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). 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.

Long-period SU UMa dwarf nova V1006 Cygni outburst activity and variability at different brightness states in 2015-2017

“…During the short-term quiescence between the rebrightening and the start of the first normal outburst the mean period of positive superhumps was P 2 = 0 d .10406(17). These periods are in agreement with periods found by Kato et al (2016) superhumps evolution during JD 2457219 -2457240. A periodogram in quiescence after the first normal outburst points to the most significant peaks P 4 = 0 d .107594(85) and P 5 = 0 d .097142(74) that are not independent but are the day-aliased.…”

“…Our data are in good agreement with an empirical relation between ǫ + , ǫ − and the orbital period (Hellier, 2001). Moreover, we found that the ratio ǫ − /ǫ + = −0.037 corresponds to the mass ratio q ∼ 0.30 − 0.32 according to the Wood's et al (2009) model, that coincides with an independent estimate of Kato et al (2016), q ∼ 0.26 − 0.33. So we can conclude that our identification of the period 0 d .09714 as the period of negative superhumps is correct.…”

“…Recently, Kato et al (2009) showed that the evolution of superhumps of SU UMa stars has three stages. These stages were also found in the period of superhump variations of V1006 Cyg (Kato et al 2016). The authors estimated mass ratio for this binary as 0.26-0.33 solar masses.…”

“…Instead the orbital period was detected in both outbursts by Pavlenko et al (2014). Kato et al (2016) informed that the start of the first genuine superoutburst of V1006 Cyg was detected on July, 12 2015 by Shappee et al by the All-Sky Authomated Survey for Supernovae (ASAS-SN). The 2015 superoutburst was studied in detail by the VSNET campaign (Kato et al 2016).…”

“…Kato et al (2016) informed that the start of the first genuine superoutburst of V1006 Cyg was detected on July, 12 2015 by Shappee et al by the All-Sky Authomated Survey for Supernovae (ASAS-SN). The 2015 superoutburst was studied in detail by the VSNET campaign (Kato et al 2016). The 0 d .10541 superhump period was found that finally established this object to be an SU UMa-type dwarf nova in the period gap, i.e, a long-periodic SU UMa dwarf nova.…”