Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae. IX: The Ninth Year (2016–2017)

The data were obtained under campaigns led by the VSNET Collaboration (Kato et al., 2004). We also used the public data from the AAVSO International Database¹¹1 $<$http://www.aavso.org/data-download$>$. . Outburst detections of many new and known objects relied on the ASAS-SN CV patrol (Davis et al., 2015)²²2 $<$http://cv.asassn.astronomy.ohio-state.edu/$>$. , the MASTER network (Gorbovskoy et al., 2013), and Catalina Real-time Transient Survey (CRTS; Drake et al. (2009))³³3 $<$http://nesssi.cacr.caltech.edu/catalina/$>$. For the information of the individual Catalina CVs, see $<$http://nesssi.cacr.caltech.edu/catalina/AllCV.html$>$. in addition to outburst detections reported to VSNET, AAVSO⁴⁴4 $<$https://www.aavso.org/$>$. , BAAVSS alert⁵⁵5 $<$https://groups.yahoo.com/neo/groups/baavss-alert/$>$. and cvnet-outburst.⁶⁶6 $<$https://groups.yahoo.com/neo/groups/cvnet-outburst/$>$.

For objects detected in CRTS, we preferably used the names provided in Drake et al. (2014) and Coppejans et al. (2016). If these names are not yet available, we used the International Astronomical Union (IAU)-format names provided by the CRTS team in the public data release⁷⁷7 $<$http://nesssi.cacr.caltech.edu/DataRelease/$>$. Since Kato et al. (2009), we have used coordinate-based optical transient (OT) designations for some objects, such as apparent dwarf nova candidates reported in the Transient Objects Confirmation Page of the Central Bureau for Astronomical Telegrams⁸⁸8 $<$http://www.cbat.eps.harvard.edu/unconf/tocp.html$>$. and CRTS objects without registered designations in Drake et al. (2014) or in the CRTS public data release and listed the original identifiers in table 1.

We provided coordinates from astrometric catalogs for ASAS-SN (Shappee et al., 2014) CVs and two objects without precise coordinate-based names other than listed in the General Catalog of Variable Stars (Kholopov et al., 1985) in table 2. We mainly used Gaia DR1 (Gaia Collaboration, 2016), Sloan Digital Sky Survey (SDSS, Ahn et al. (2012)), the Initial Gaia Source List (IGSL, Smart (2013)) and Guide Star Catalog 2.3.2 (GSC 2.3.2, Lasker et al. (2007)). Some objects were detected as transients by Gaia⁹⁹9 $<$http://gsaweb.ast.cam.ac.uk/alerts/alertsindex$>$ and Gaia identifications supplied by the AAVSO VSX. and CRTS and we used their coordinates. The coordinates used in this paper are J2000.0. We also supplied SDSS $g$, Gaia $G$ and GALEX NUV magnitudes when counterparts are present.

The majority of the data were acquired by time-resolved CCD photometry by using 20–60cm telescopes located world-wide. The list of outbursts and observers is summarized in table 1. The data analysis was performed in the same way described in Kato et al. (2009) and Kato et al. (2014a) and we mainly used R software¹⁰¹⁰10 The R Foundation for Statistical Computing:
$<$http://cran.r-project.org/$>$. for data analysis.

In de-trending the data, we mainly used locally-weighted polynomial regression (LOWESS: Cleveland (1979)) and sometimes lower (1–3rd order) polynomial fitting when the observation baseline was short. The times of superhumps maxima were determined by the template fitting method as described in Kato et al. (2009). The times of all observations are expressed in barycentric Julian days (BJD).

In figures, the points are accompanied by 1$\sigma$ error bars whenever available, which are omitted when the error is smaller than the plot mark or the errors were not available (as in some raw light curves of superhumps).

The abbreviations used in this paper are the same as in Kato et al. (2014a): we used $ϵ\equiv P_{SH}/P_{orb}-1$ for the fractional superhump excess. We have used since Osaki and Kato (2013a) the alternative fractional superhump excess in the frequency unit ${ϵ}^{\ast }\equiv 1-P_{orb}/P_{SH}=ϵ/(1+ϵ)$ because this fractional superhump excess is a direct measure of the precession rate. We therefore used ${ϵ}^{\ast }$ in discussing the precession rate.

The $P_{SH}$, $P_{dot}$ and other parameters are listed in table 3 in same format as in Kato et al. (2009). The definitions of parameters $P_1,P_2,E_1,E_2$ and $P_{dot}$ are the same as in Kato et al. (2009): $P_1$ and $P_2$ represent periods in stage B and C, respectively ($P_1$ is averaged during the entire course of the observed segment of stage B), and $E_1$ and $E_2$ represent intervals (in cycle numbers) to determine $P_1$ and $P_2$, respectively.¹¹¹¹11 The intervals ($E_1$ and $E_2$) for the stages B and C given in the table sometimes overlap because there is sometimes observational ambiguity (usually due to the lack of observations and errors in determining the times of maxima) in determining the stages. Some superoutbursts are not listed in table 3 due to the lack of observations (e.g. single-night observations with less than two superhump maxima or poor observations for the object with already well measured $P_{SH}$).

We used the same terminology of superhumps summarized in Kato et al. (2012a). We especially call attention to the term “late superhumps”. We only used the concept of “traditional” late superhumps when there is an $\sim$0.5 phase shift [Vogt (1983); see also table 1 in Kato et al. (2012a) for various types of superhumps], since we suspect that many of the past claims of detections of “late superhumps” were likely stage C superhumps before it became evident that there are complex structures in the $O-C$ diagrams of superhumps (see discussion in Kato et al. (2009)).

We used phase dispersion minimization (PDM; Stellingwerf (1978)) for period analysis and 1$\sigma$ errors for the PDM analysis was estimated by the methods of Fernie (1989) and Kato et al. (2010). We have used a variety of bootstrapping in estimating the robustness of the result of the PDM analysis since Kato et al. (2012a). We analyzed 100 samples which randomly contain 50% of observations, and performed PDM analysis for these samples. The bootstrap result is shown as a form of 90% confidence intervals in the resultant PDM $\theta$ statistics. If this paper provides the first solid presentation of a new SU UMa-type classification, we provide the result of PDM period analysis and averaged superhump profile.

Comparisons of $O-C$ diagrams between different superoutbursts are also presented whenever available. This figure not only provides information about the difference of $O-C$ diagrams between different superoutbursts but also helps identifying superhump stages especially when observations were insufficient or the start of the outburst was missed. In drawing combined $O-C$ diagrams, we usually used $E=$0 for the start of the superoutburst, which usually refers to the first positive detection of the outburst. This epoch usually has an accuracy of $\sim$1 d for well-observed objects, and if the outburst was not sufficiently observed, we mentioned in the figure caption how to estimate $E$ in such an outburst. In some cases, this $E=$0 is defined as the appearance of superhumps. This treatment is necessary since some objects have a long waiting time before appearance of superhumps. We also note that there is sometimes an ambiguity in selecting the true period among aliases. In some cases, this can be resolved by the help of the $O-C$ analysis. The procedure and example are shown in subsection 2.2 in Kato et al. (2015a).

V1047 Aql was discovered as a dwarf nova (S 8191) by Hoffmeister (1964). Hoffmeister (1964) reported a blue color in contrast to the nearby stars. Mason and Howell (2003) obtained a spectrum typical for a quiescent dwarf nova. According to R. Stubbings, the observation by Greg Bolt during the 2005 August outburst detected superhumps, and the superhump period was about 0.074 d (see Kato et al. (2012b)). The object shows rather frequent outbursts (approximately once in 50 d), and a number of outbursts have been detected mainly by R. Stubbings visually since 2004.

The 2016 superoutburst was detected by R. Stubbings at a visual magnitude of 15.0 on July 8. Subsequent observations detected superhumps (vsnet-alert 19974; figure 2). Using the 2005 period, we could identify two maxima on two nights: $E$=0, BJD 2457581.3853(7) ($N$=74) and $E$=14, BJD 2457582.4190(11) ($N$=72). The period given in table 3 is determined by the PDM method.

Although observations are not sufficient, visual observations by R. Stubbings suggest a supercycle of $\sim$90 d, which would make V1047 Aql one of ordinary SU UMa-type dwarf novae with shortest supercycles.

This object was discovered as a variable star (Ross 182, NSV 907) on a plate on 1926 November 26 (Ross, 1927). The dwarf nova-type nature was suspected by the association with an ROSAT source (Kato, vsnet-chat 3317). The SU UMa-type nature was confirmed during the 2004 superoutburst. For more information, see Kato et al. (2014a).

The 2016 superoutburst was detected by P. Schmeer at a visual magnitude of 13.2 on October 30 (vsnet-alert 20273). Thanks to the early detection (this visual detection was 1 d earlier than the ASAS-SN detection), stage A growing superhumps were detected (vsnet-alert 20292). At the time of the initial observation, the object was fading from a precursor outburst. Further observations recorded development of superhumps clearly (vsnet-alert 20312, 20321). The times of superhump maxima are listed in table 4. There were clear stages A–C (figure 3). The 2013 superoutburst had a separate precursor outburst and a comparison of the $O-C$ diagrams suggests a difference of 44 cycle count from that used in Kato et al. (2014a). The value suggests that superhumps during the 2013 superoutburst evolved 3 d after the precursor outburst.

This object (=1RXS J053234.9$+$624755) was discovered as a dwarf nova by Bernhard et al. (2005). Kapusta and Thorstensen (2006) provided a radial-velocity study and yielded an orbital period of 0.05620(4) d. The SU UMa-type nature was established during the 2005 superoutburst (Imada et al., 2009). See Kato et al. (2009) for more history. The 2009 superoutburst was also studied in Kato et al. (2010).

The 2017 superoutburst was detected by P. Schmeer at a visual magnitude of 11.4 and also by the ASAS-SN team at $V$=11.82 on March 15. Single superhump was recorded at BJD 2457829.3171(2) ($N$=236). Although there were observations on three nights immediately after the superoutburst, we could neither detect superhump nor orbital periods.

See Kato et al. (2015a) for the history of this well-known eclipsing SU UMa-type dwarf nova. The 2016 superoutburst was detected by R. Stubbings at a visual magnitude of 11.6 on April 2 (vsnet-alert 19676). Due to an accidental delay in the start of observations, the earliest time-resolved CCD observations were obtained on April 3 (vsnet-alert 19706). On that night, superhumps (likely in the growing phase) unfortunately overlapped with eclipses (figure 4, upper panel). Distinct superhumps were recorded on April 4 (vsnet-alert 19692; figure 4, middle panel). A further analysis suggested that stage A superhumps escaped detection before April 4 (due to the lack of observations and overlapping eclipses). At the time of April 4, the superhumps were already likely stage B (table 5, maxima outside eclipses). We could, however, confirmed a positive $P_{dot}$ for stage B superhumps (cf. figure 5), whose confirmation had been still awaited (cf. Kato et al. (2015a)).

The combined data used in Kato et al. (2015a) and new observations, we have obtained the eclipse ephemeris for the use of defining the orbital phases in this paper using the MCMC analysis (Kato et al., 2013):

The epoch corresponds to the center of the entire observation. The mean period, however, did not show a secular decrease (e.g. Han et al. (2015); Kato et al. (2015a)). It may be that period changes in this system are sporadic and do not reflect the secular CV evolution.