V1006 Cyg: SU UMa-type dwarf nova in the period gap that wobbles between subclasses

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Introduction
Non-magnetic cataclysmic variable stars (CVs) are close binary systems where a late-type component filling its Roche lobe loses matter on the white dwarf.CVs could be divided into three subclasses: SU UMa-type, Z Cam-type and U Gemtype (or SS Cyg-type).Originally this division was based on the morphology of the light curves (Warner, 1995).According to Warner, a distinctive feature of Z Cam-type stars is protracted standstills that are terminated by fading.There is a subgroup of "anomalous" Z Cam-type stars, IW And-type objects, displaying standstill terminated by brightening (Kato, 2019).SU UMa stars have two types of outbursts -superoutbursts lasting a couple of weeks and normal outbursts which are less bright and as short as 2-4 d; SS Cyg-type stars are neither Z Cam nor SU UMa stars.Osaki (2005) proposed that different outburst behaviour of non-magnetic CVs may be explained in a framework of disk instability model which uses two instabilities: thermal instability and tidal instability of accretion disk.There are two parameters characterizing accretion disks: mass transfer rate from the secondary component and the orbital period (or mass ratio).A period gap is a borderline region that determines the ability of CVs to undergo tidal instability (below the gap) or not (above the gap).Another borderline is a critical mass transfer rate Ṁcrit (Osaki, 1996).While CVs with mass-transfer rate higher than Ṁcrit are hot and exhibit "stable" disks, CVs with mass-transfer rate less than Ṁcrit , are in the region of thermal instability and show outbursts.Thus, accretion disks of U-Gem-type stars that have orbital periods longer than those in the "gap", are thermally unstable but tidally stable.SU UMa-type stars with orbital periods less than the gap have disks that are both thermally and tidally unstable.Z Cam-type stars are located close to the Ṁcrit and with periods above the gap.So the period gap is a region where these two instabilities intersect and therefore there is a possibility that some CVs in this region may have properties of neighboring subclasses.
As a dwarf nova V1006 Cyg is known since 1963 (Hoffmeister, 1963).Sheets et al. (2007) found that the orbital period of binary is 0.09903(9) d which classified it as a dwarf nova in the period gap.The 2006 outburst was suspected to be a superoutburst.Finally, V1006 Cyg was identified as the SU UMa-type dwarf nova in the period gap based on the results of studies during the 2015 superoutburst (Kato et al., 2016).V1006 Cyg got attention not only by its localization in the period gap but also by detection in 2007, 2009 and 2017 of long outbursts without superhumps but with orbital-related brightness variations (Pavlenko et al., 2014(Pavlenko et al., , 2018)).It was also found that this star showed three types of outbursts -normal, long normal and superoutbursts (Kato et al., 2016).A diversity of normal outbursts makes V1006 Cygnus look like the SS Cyg-type star.

Observations and data reduction
CCD photometry of V1006 Cyg has been carried out with eight telescopes at seven observatories during 72 nights in 2023.Most of the observations were made in unfiltered light corrected to R C .In selected nights of outburst and quiescence, V1006 Cyg was observed in the Johnson-Cousins U BV R C I C colour bands (see Tab.1).We used the U BV R C I C values of comparison star No 140 as in the paper by Pavlenko et al. (2018) and U = 15 m .58 for this star obtained relatively to the stars with known magnitudes in the vicinity of CH Cyg (Henden & Munari, 2006).Standard data reduction included flat-fielding, bias and dark signal removal.The MAXIM DL and Goranskij (http://www.vgoranskij.net/software/)WinFit packages were used.A periodogram analysis was done with the help of the Stellingwerf method implemented in the Pelt (1992) package.

2023 overall light curve
Our observations are shown in Fig 1 .They start from JD 2460076 capturing the decline of outburst superposed on the quiescent state at mean brightness about R C ∼ 16 m .4 that turned out to be 1 m − 1 m .5 magnitudes brighter compared to known previous observations in 2015-2017 (Pavlenko et al., 2018).This "bright" quiescence which lasted ∼ three weeks, was terminated by the next long outburst with a duration of 10 d and amplitude of ∼ 2 m .5.This behaviour resembles those in the IW And-type dwarf novae, where a quiescence terminates by outburst.After the end of the outburst, V1006 Cyg returned to its "usual" quiescent state, the return itself lasted about a couple of weeks.We did not detect any outburst during ∼ 3.5 months after the long outburst.Note that during the 2015-2017 quiescence there was a brief episode of increased brightness around normal outburst No 3 (see Fig. 1 in Pavlenko et al. (2018)).

Brightness variations in outburst and quiescence
During the long outburst short-term periodical brightness variations were detected.The outburst and nightly light curves are shown in Fig 2 .It is seen that these variations existed on the two nights at the top of the outburst and one night at the outburst decline (HJD 2460108-2460110).Variations at the JD 2460111 were not detected.A periodogram calculated for data of these three nights after the removal of the trend corresponding to the outburst profile, is shown in Fig 3, a.The most significant peak on the periodogram corresponds to the 0.09837(22) d period which, within the limits of error, coincides with known orbital period estimates.The phased light curve was calculated using the zero-epoch HJD=24060108.A mean R C light curve has a symmetric profile and amplitude of 0 m .05.Note that 2007 and 2009 orbital light curves displayed similar profiles (Pavlenko et al., 2014).For HJD 2460108 and 2460109 we calculated phase-resolved colour indices after removing the trend corresponding to the outburst profile.We have done this procedure for the closest colour bands U − B and for the most distant wavelengths U − I C (see Fig 3, c and d, respectively).The mean amplitude in V is 0 m .016.The U − B and U − I C light curves displayed dependence on the phase of the orbital period with amplitudes 0 m .012 in U − B and 0 m .028 in U − I C .The blue peak of both U − B and especially U − I C curves coincides with a minimum of the V light curve.We interpret these light curves as being caused by a hot spot on the disk.However, this behaviour is opposite to that observed in light curves, where the main contribution to the emission comes from the hot spot.We suppose that the ultraviolet excess at minimum is caused by a contribution from the innermost part of the accretion disk that is hotter than the hot spot.
Characteristic of the quiescence is the presence of quasi-periodic oscillations (QPOs) in a range of minutes-hours.An example of short-term QPOs is shown in Fig 4,a.These QPOs were coherent on a scale of at least about an hour (see Fig 4, b).As during 2016 quiescence, no evidence of an orbital period was found in 2023 quiescence (Pavlenko et al., 2018).

Colour variations
Colour-colour diagrams including changes in colour indices of V1006 Cyg during the transition from outburst to quiescence are shown in Fig 5 .One could see that in U-B the object has a higher temperature at quiescence than at outburst.The opposite behaviour is observed in the region of longer wavelengths: radiation of the object is hotter at outburst compared to quiescence.This may mean that the radiation can come from the parts of the disk with different temperatures.In quiescence, the disk returns to a cold state, and its size decreases significantly.
In this case, it is natural that the contribution to the total radiation of the hottest innermost layers of the disk increases.This can explain the ultraviolet excess in a quiet state in the U − B, B − V diagram.Similar behaviour of colour indices was noted by Golysheva & Shugarov (2014) for the dwarf nova PNV J19150199+071947.

Conclusion
We presented the results of multi-longitudinal 71-d photometric monitoring of the SU UMa-type dwarf nova in the period gap, V1006 Cyg in 2023.This dwarf nova showed that being a star of the SU UMa type, it can have properties of various subclasses -SS Cyg-type, Z Cam-type and IW And-type.Thus the dwarf nova V1006 Cyg shows that a division between CV subclasses may not be so sharp and deviations from standard behavior may not be so rare.

Figure 2 .
Figure 2. Short-term periodic variations in the R C band (left) during the long outburst (right).

Figure 3 .
Figure 3. Orbital period of V1006 Cyg during the long outburst.Periodogram (a), R C data from HJD 2460108, 2460109, 2460110, and mean U −B and U −I C data from HJD 2460108, 2460109 folded on the orbital period 0.09837 d (b, c, d respectively).

Figure 4 .
Figure 4. Example of nightly light curve in quiescence displaying short-term QPOs (a); periodogram and data folded on the 14-min period (b).

Figure 5 .
Figure5.Two-colour diagrams.The dotted lines denote the black body sequence with a Kelvin scale marked along it.The Main sequence is designated by a solid line, the spectral classes are marked.The system state is indicated: outburst, fading or quiescence (Min).Data for the 2023 are indicated by filled circled and for the 2015(Pavlenko et al., 2018) -by open circles.The positions of surrounding stars (shown by filled stars) are also plotted.

Table 1 .
Journal of observations

Table 1 .
Journal of observations (continued)

Table 1 .
Journal of observations (continued) CCD: CCD camera type.N: number of observations.