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Kimura M.,Kyoto University | Kato T.,Kyoto University | Imada A.,Kyoto University | Ikuta K.,Kyoto University | And 7 more authors.
Publications of the Astronomical Society of Japan | Year: 2014

In 2015 March, the notable WZ Sge-type dwarf nova AL Com exhibited an unusual outburst with a recurrence time of ∼ 1.5 yr, which is the shortest interval of superoutbursts among WZ Sge-type dwarf novae. Early superhumps in the superoutburst light curve were absent, and a precursor was observed at the onset of the superoutburst for the first time in WZ Sge-type dwarf novae. The present superoutburst can be interpreted as a result of the condition that the disk radius barely reached the 3:1 resonance radius, but did not reach the 2:1 resonance one. Ordinary superhumps immediately grew following the precursor. The initial part of the outburst is indistinguishable from those of superoutbursts of ordinary SU UMa-type dwarf novae. This observation supports the interpretation that the 2:1 resonance suppresses a growth of ordinary superhumps. The estimated superhump period and superhump period derivative are Psh = 0.0573185(11) d and Pdot = +1.5(3.1) × 10-5, respectively. These values indicate that the evolution of ordinary superhumps is the same as that in past superoutbursts with much larger extent. Although the light curve during the plateau stage was typical for an SU UMa-type dwarf nova, this superoutburst showed a rebrightening, together with a regrowth of the superhumps. The overall light curve of the rebrightening was the almost the same as those observed in previous rebrightenings. This implies that the rebrightening type is inherent in the system. © 2016 The Author 2016. Published by Oxford University Press on behalf of the Astronomical Society of Japan. Source

Le Borgne J.F.,Toulouse 1 University Capitole | Le Borgne J.F.,French National Center for Scientific Research | Poretti E.,Toulouse 1 University Capitole | Poretti E.,French National Center for Scientific Research | And 24 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

RR Lyr is one of the most studied variable stars. Its light curve has been regularly monitored since the discovery of its periodic variability in 1899. The analysis of all observed maxima allows us to identify two primary pulsation states, defined as pulsation over a long (P0 longer than 0.56684 d) and a short (P0 shorter than 0.56682 d) primary pulsation period. These states alternate with intervals of 13-16 yr, and are well defined after 1943. The 40.8-d periodical modulations of the amplitude and the period (i.e. the Blazhko effect) were noticed in 1916.We provide homogeneous determinations of the Blazhko period in the different primary pulsation states. The Blazhko period does not follow the variations of P0 and suddenly diminished from 40.8 d to around 39.0 d in 1975. The monitoring of these periodicities deserved, and still deserves, a continuous and intensive observational effort. For this purpose, we have built dedicated, transportable and autonomous small instruments, Very Tiny Telescopes (VTTs), to observe the times of maximum brightnessof RR Lyr. As immediate results, the VTTs recorded the last change of the P0 state in mid-2009 and extended the time coverage of the Kepler observations, thus recording a maximumO-C amplitude of the Blazhko effect at the end of 2008, followed by the historically smallest O-C amplitude in late 2013. This decrease is still ongoing and the VTTs are ready to monitor the expected increase in the next few years. © 2014 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Pavlenko E.P.,Crimean Astrophysical Observatory | Kato T.,Virtual Telescope Project | Antonyuk O.I.,Crimean Astrophysical Observatory | Ohshima T.,Variable Star Observers League in Japan VSOLJ | And 15 more authors.
Publications of the Astronomical Society of Japan | Year: 2013

We present a photometric study of NYSer, an in-the-gap SUUMa-type nova, in 2002 and 2013. We determined the duration of its superoutburst and the mean period of its superhump are 18d and 0.10458d, respectively. We detected in 2013 that NYSer showed two distinct states separated by a superoutburst. A state of rather infrequent normal outbursts lasted at least 44d before the superoutburst, and a state of frequent outbursts started immediately after the superoutburst and lasted at least 34d. Unlike a typical SUUMa star with a bimodal distribution of outburst duration, NYSer displayed a diversity of normal outbursts. In the state of infrequent outbursts, we detected a wide ∼12d outburst accompanied by 0.098d orbital modulation but without superhumps ever established in NYSer. We classified this as a "wide normal outburst." The orbital period was dominant both in quiescence and during normal outbursts in this state. In the state of the most frequent normal outbursts, the 0.10465d positive superhump period was dominant and coexisted with the orbital modulation. In 2002 we detected the normal outburst of "intermediate" 5-6d duration that was also accompanied by orbital modulations. © 2014 © The Author 2014. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved. Source

Kato T.,Kyoto University | Pavlenko E.P.,Crimean Astrophysical Observatory | Shchurova A.V.,Taras Shevchenko National University | Sosnovskij A.A.,Crimean Astrophysical Observatory | And 17 more authors.
Publications of the Astronomical Society of Japan | Year: 2016

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 demonstrates that superhumps indeed slowly grow in systems near the stability limit, the idea first introduced by Kato et al. (2014, PASJ, 66, 90). 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 are 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. © 2016 The Author. Source

Kato T.,Kyoto University | Maehara H.,Kyoto University | Uemura M.,Hiroshima University | Henden A.,American Association of Variable Star Observers AAVSO | And 57 more authors.
Publications of the Astronomical Society of Japan | Year: 2010

Continued from Kato et al. (2009, PASJ, 61, S395), we collected the times of superhump maxima for 68 SU UMa-type dwarf novae, mainly observed during the 2009-2010 season. The newly obtained data confirmed the basic findings reported in Kato et al. (ibid.): the presence of stages A-C and the predominance of positive period derivatives during stage B in systems with superhump periods shorter than 0.07 d. There was a systematic difference in the period derivatives for the systems with superhump periods longer than 0.075 d between this study and Kato et al. (ibid.). We suggest that this difference was possibly caused by a relative lack of frequently outbursting SU UMa-type dwarf novae in this period regime in the present study. We recorded a strong beat phenomenon during the 2009 superoutburst of IY UMa. A close correlation between the beat period and the superhump period suggests that the changing angular velocity of the apsidal motion of the elliptical disk is responsible for the variation of the superhump periods. We also described three new WZ Sge-type objects with established early superhumps and one with likely early superhumps. We suggest that two systems, VX For and EL UMa, are WZ Sge-type dwarf novae with multiple rebrightenings. The O -C variation in OT J213806.6+261957 suggests that the frequent absence of rebrightenings in very short-Porb objects can be the result of a sustained superoutburst plateau at the epoch when usual SU UMa-type dwarf novae return to quiescence, preceding a rebrightening. We also present a formulation for a variety of Bayesian extensions to traditional period analyses. © 2010. Astronomical Society of Japan. Source

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