Superhump-like variation during the anomalous state of SU UMa

We observed an anomalously outbursting state of SU UMa which occurred in 1992. Time-resolved photometry revealed the presence of signals with a period of 0.0832 +/- 0.0019 d, which is 3.6 sigma longer than the orbital period (0.07635 d) of this system. We attributed this signal to superhumps, based on its deviation from the orbital period and its characteristic profile. During this anomalous state of SU UMa, normal outbursts were almost suppressed, in spite of relatively regular occurrences of superoutbursts. We consider that an ensuing tidally unstable state following the preceding superoutburst can be a viable mechanism to effectively suppress normal outbursts, resulting in an anomalously outbursting state.Comment: 3 pages, 4 figures, accepted for publication in Astronomy and Astrophysic

Changing Supercycle of the ER UMa-Type Star V1159 Ori

We examined the VSNET light curve of the ER UMa-type star V1159 Ori. We detected a large variation of the supercycle (the interval between successive superoutbursts) between extremes of 44.6 and 53.3 d. The outburst activity was also found to decrease when the supercycle was long. The observed variation of the supercycle corresponds to a variation of ~40 % of the mass-transfer rate from the secondary star, totally unexpected for this class of objects. We also detected a hint of ~1800 d periodicity in the variation, whose period is close to what has been suggested for solar-type cycles for cataclysmic variables (CVs). If this periodicity is caused by the magnetic activity of the secondary star, this detection constitutes the first clear evidence of continuing magnetic activity in CV evolution, even after crossing the period gap. This activity may partly explain still poorly understood origins of the high mass-transfer rates in ER UMa-type stars.Comment: 4 pages, 3 figures, using a non-standard style fil

Period Analysis using the Least Absolute Shrinkage and Selection Operator (Lasso)

We introduced least absolute shrinkage and selection operator (lasso) in obtaining periodic signals in unevenly spaced time-series data. A very simple formulation with a combination of a large set of sine and cosine functions has been shown to yield a very robust estimate, and the peaks in the resultant power spectra were very sharp. We studied the response of lasso to low signal-to-noise data, asymmetric signals and very closely separated multiple signals. When the length of the observation is sufficiently long, all of them were not serious obstacles to lasso. We analyzed the 100-year visual observations of delta Cep, and obtained a very accurate period of 5.366326(16) d. The error in period estimation was several times smaller than in Phase Dispersion Minimization. We also modeled the historical data of R Sct, and obtained a reasonable fit to the data. The model, however, lost its predictive ability after the end of the interval used for modeling, which is probably a result of chaotic nature of the pulsations of this star. We also provide a sample R code for making this analysis.Comment: 9 pages, 13 figures, accepted for publication in PAS

The Cause of the Superoutburst in SU UMa Stars is Finally Revealed by Kepler Light Curve of V1504 Cygni

We have studied the SC (short cadence) Kepler light curve of an SU UMa star, V1504 Cyg, which extends for a period of about 630 d. All superoutbursts in V1504 Cyg have turned out to be of the precursor-main type and the superhump first appears near the maximum of the precursor. The superhumps grow smoothly from the precursor to the main superoutburst showing that the superoutburst is initiated by the tidal instability (as evidenced by growing superhump) as envisioned in the thermal-tidal instability (TTI) model proposed by Osaki (1989). We have performed power spectral analysis of the light curve of V1504 Cyg. One of outstanding features is an appearance of a negative superhump extending for around 300 d, well over a supercycle. We have found that an appearance of the negative superhump tends to reduce the frequency of occurrence of normal outbursts. Two types of supercycles are recognized in V1504 Cyg, which are similar to those of the Type L and S supercycles in the light curve of VW Hyi, a prototype SU UMa star, introduced by Smak (1985). It is found that the Type L supercycle is the one accompanied with the negative superhump and the Type S is that without the negative superhump. If we adopt a tilted disk as an origin of the negative superhump, two types of the supercycles are understood to be due to a difference in outburst intervals, which is in turn caused by a difference in mass supply from the secondary to different parts of the disk. The frequency of the negative superhump varies systematically during a supercycle in V1504 Cyg. This variation can be used as an indicator of the disk radius variation and we have found that observed disk radius variation in V1504 Cyg fits very well with a prediction of the TTI model.Comment: 17 pages, 8 figures, accepted for publication in PASJ, minor correction