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Register a new userCCD Photometry of a Newly Confirmed SU UMa-Type Dwarf Nova, NSV 4838
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We present time-resolved CCD photometry of a dwarf nova NSV 4838 (UMa 8, SDSS J102320.27+440509.8) during the 2005 June and 2007 February outburst. Both light curves showed superhumps with a mean period of 0.0699(1) days for the 2005 outburst and 0.069824(83) days for the 2007 outburst, respectively. Using its known orbital period of 0.0678 days, we estimated the mass ratio of the system to be $q$=0.13 based on an empirical relation. Although the majority of SU UMa-type dwarf novae having similar superhump periods show negative period derivatives, we found that the superhump period increased at $dot{P}$ / $P_{rm sh}$=+7(+3, -4)$times10^{-5}$ during the 2007 superoutburst. We also investigated long-term light curves of NSV 4838, from which we derived 340 days as a supercycle of this system.
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We found that the SU UMa-type dwarf nova NY Ser in the period gap [orbital period 0.097558(6) d] showed standstills twice in 2018. This is the first clear demonstration of a standstill occurring between superoutbursts of an SU UMa-type dwarf nova. There was no sign of superhumps during the standstill, and at least one superoutburst directly started from the standstill. This provides strong evidence that the 3:1 resonance was excited during standstills. This phenomenon indicates that the disk radius can grow during standstills. We also interpret that the condition close to the limit of the tidal instability caused early quenching of superoutbursts, which resulted substantial amount of matter left in the disk after the superoutburst. We interpret that the substantial matter in the disk in condition close to the limit of the tidal instability is responsible for standstills (as in the high mass-transfer system NY Ser) or multiple rebrightenings (as in the low mass-transfer system V1006 Cyg).
IW And stars are a recently recognized subgroup of dwarf novae which are characterized by (often repetitive) slowly rising standstills terminated by brightening, but the exact mechanism for this variation is not yet identified. We have identified BO Cet, which had been considered as a novalike cataclysmic variable, as a new member of IW And stars based on the behavior in 2019-2020. In addition to this, the object showed dwarf nova-type outbursts in 2020-2021, and superhumps having a period 7.8% longer than the orbital one developed at least during one long outburst. This object has been confirmed as an SU UMa-type dwarf nova with an exceptionally long orbital period (0.1398 d). BO Cet is thus the first cataclysmic variable showing both SU UMa-type and IW And-type features. We obtained a mass ratio (q) of 0.31-0.34 from the superhumps in the growing phase (stage A superhumps). At this q, the radius of the 3:1 resonance, responsible for tidal instability and superhumps, and the tidal truncation radius are very similar. We interpret that in some occasions this object showed IW And-type variation when the disk size was not large enough, but that the radius of the 3:1 resonance could be reached as the result of thermal instability. We also discuss that there are SU UMa-type dwarf novae above q=0.30, which is above the previously considered limit (q~0.25) derived from numerical simulations and that this is possible since the radius of the 3:1 resonance is inside the tidal truncation radius. We constrained the mass of the white dwarf larger than 1.0Msol, which may be responsible for the IW And-type behavior and the observed strength of the He II emission. The exact reason, however, why this object is unique in that it shows both SU UMa-type and IW And-type features is still unsolved.
We analyzed the Kepler long cadence data of KIC 7524178 (=KIS J192254.92+430905.4), and found that it is an SU UMa-type dwarf nova with frequent normal outbursts. The signal of the negative superhump was always the dominant one even during the superoutburst, in contrast to our common knowledge about superhumps in dwarf novae. The signal of the positive superhump was only transiently seen during the superoutburst, and it quickly decayed after the superoutburst. The frequency variation of the negative superhump was similar to the two previously studied dwarf novae in the Kepler field, V1504 Cyg and V344 Lyr. This is the first object in which the negative superhumps dominate throughout the supercycle. Nevertheless, the superoutburst was faithfully accompanied by the positive superhump, indicating that the tidal eccentric instability is essential for triggering a superoutburst. All the pieces of evidence strengthen the thermal-tidal instability as the origin of the superoutburst and supercycle, making this object the third such example in the Kepler field. This object had unusually small (~1.0 mag) outburst amplitude and we discussed that the object has a high mass-transfer rate close to the thermal stability limit of the accretion disk. The periods of the negative and positive superhumps, and that of the candidate orbital period were 0.07288 d (average, variable in the range 0.0723-0.0731 d), 0.0785 d (average, variable in the range 0.0772-0.0788 d) and 0.074606(1) d, respectively.
We observed the 2016 outburst of OT J002656.6+284933 (CSS101212:002657+284933) and found that it has the longest recorded [0.13225(1) d in average] superhumps among SU UMa-type dwarf novae. The object is the third known SU UMa-type dwarf nova above the period gap. The outburst, however, was unlike ordinary long-period SU UMa-type dwarf novae in that it showed two post-outburst rebrightenings. It showed superhump evolution similar to short-period SU UMa-type dwarf novae. We could constrain the mass ratio to less than 0.15 (most likely between 0.10 and 0.15) by using superhump periods in the early and post-superoutburst stages. These results suggest the possibility that OT J002656.6+284933 has an anomalously undermassive secondary and it should have passed a different evolutionary track from the standard one.
We photometrically observed the 2003 June superoutburst of GO Gom. The mean superhump period was 0.063059(13) d. The resultant data revealed that (1) the obtained light curve contained a precursor, (2) a plateau stage of the object lasted 8 days, which is remarkably shorter than that of ordinary SU UMa-type dwarf novae, and (3) the amplitude of the superoutburst was less than 5 mag, which is unpredictably small when taking into account the fact that the supercycle of GO Com is about 2800 days. In order to explain these anomalies, a mass elimination process from the accretion disk, such as evaporation, may play an important role.
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