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PubMed | Mt Vernon Observatory, The Virtual Telescope Project, Las Campanas Observatory, Harvard - Smithsonian Center for Astrophysics and 14 more.
Type: Journal Article | Journal: Science (New York, N.Y.) | Year: 2016

We report the discovery of ASASSN-15lh (SN 2015L), which we interpret as the most luminous supernova yet found. At redshift z = 0.2326, ASASSN-15lh reached an absolute magnitude of Mu ,AB = -23.5 0.1 and bolometric luminosity Lbol = (2.2 0.2) 10(45) ergs s(-1), which is more than twice as luminous as any previously known supernova. It has several major features characteristic of the hydrogen-poor super-luminous supernovae (SLSNe-I), whose energy sources and progenitors are currently poorly understood. In contrast to most previously known SLSNe-I that reside in star-forming dwarf galaxies, ASASSN-15lh appears to be hosted by a luminous galaxy (MK -25.5) with little star formation. In the 4 months since first detection, ASASSN-15lh radiated (1.1 0.2) 10(52) ergs, challenging the magnetar model for its engine.

Ohshima T.,Kyoto University | Kato T.,Kyoto University | Pavlenko E.P.,Crimean Astrophysical Observatory | Itoh H.,Variable Star Observers League in Japan VSOLJ | And 19 more authors.
Publications of the Astronomical Society of Japan | Year: 2012

We report on a discovery of "negative" superhumps during the 2011 January superoutburst of ERUMa. During the superoutburst, which started on 2011 January 16, we detected negative superhumps having a period of 0.062242(9) d, shorter than its orbital period by 2.2%. No evidence of a positive superhump was detected during this observation. This finding indicates that the disk exhibited retrograde precession during this superoutburst, contrary to all other known cases of superoutbursts. The duration of this superoutburst was shorter than those of ordinary superoutbursts, and the interval of its normal outburstswas longer than those of ordinary normal outbursts of ER UMa. We suggest the possibility that such unusual outburst properties are likely to be a result of a disk tilt, which is supposed to be a cause of negative superhumps; the tilted disk could prevent the disk from being filled with materials in the outmost region, which is supposed to be responsible for long-duration superoutbursts in ERUMa-type dwarf novae. This discovery signifies the importance of the classical prograde precession in sustaining long-duration superoutbursts. Furthermore, the presence of pronounced negative superhumps in this system with a high mass-transfer rate supports the hypothesis that hydrodynamical lift is the cause of the disk tilt. © 2012. Astronomical Society of Japan.

Kato T.,Kyoto University | De Migue E.,Kyoto University | De Migue E.,University of Huelva | De Migue E.,Backyard | And 13 more authors.
Publications of the Astronomical Society of Japan | Year: 2016

We observed the first-ever recorded outburst of PM J03338+3320, the cataclysmic variable selected by proper-motion survey. The outburst was composed of a precursor and the main superoutburst. The precursor outburst occurred at least 5 d before the maximum of the main superoutburst. Despite this separation, long-period superhumps were continuously seen between the precursor and main superoutburst. The period of these superhumps is longer than its orbital period by 6.0(1)% and can be interpreted to reflect the dynamical precession rate at the 3 : 1 resonance for a mass ratio of 0.172(4). These superhumps smoothly evolved into those in the main superoutburst. These observations provide the clearest evidence that the 3 : 1 resonance is triggered by the precursor outburst, even if it is well separated, and the resonance eventually causes themain superoutburst as predicted by the thermaltidal instability model. The presence of superhumps well before the superoutburst cannot be explained by alternative models (the enhanced mass-transfer model and the pure thermal instability one) and the present observations clearly support the thermaltidal instability model. © The Author 2016.

Isogai K.,Kyoto University | Kato T.,Kyoto University | Ohshima T.,University of Hyogo | Kasai K.,Baselstrasse 133D | And 30 more authors.
Publications of the Astronomical Society of Japan | Year: 2016

We report on two superoutbursts of the AMCVn-type object CR Boo in 2014 April-March and 2015 May-June. A precursor outburst accompanied both of these superoutbursts. During the rising branch of the main superoutburst in 2014, we detected growing superhumps (stage A superhumps) whose period was 0.017669(24) d. Assuming that this period reflects the dynamical precession rate at the radius of the 3:1 resonance, we could estimate the mass ratio (q = M2/M1) of 0.101(4) by using the stage A superhump period and the orbital period of 0.0170290(6) d. This mass ratio is consistent with that expected from the theoretical evolutionary model of AMCVn-type objects. The detection of precursor outbursts and stage A superhumps is the second case in AMCVn-type objects. There are two interpretations of the outbursts of AMCVn-type objects. One is a dwarf nova (DN) outbursts analogy, which suggets that the outbursts are caused by thermal and tidal instabilities. Another is the VY Scl-type variation, which suggests that the outbursts are caused by the variation of the mass-transfer rate of the secondary. This detection of the superhump variations strongly supports the former interpretation. © The Author 2016.

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.

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.

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.

Kimura M.,Kyoto University | Isogai K.,Kyoto University | Kato T.,Kyoto University | Imada A.,Kyoto University | And 20 more authors.
Publications of the Astronomical Society of Japan | Year: 2016

We present optical photometry of aWZ Sge-type dwarf nova (DN), ASASSN-15jd. Its light curve showed a small dip in the middle of the superoutburst in 2015 for the first time among WZ Sge-type DNe. The unusual light curve implies a delay in the growth of the 3 : 1 resonance tidal instability. Also, the light curve is similar to those of two other WZ Sge-type stars, SSS J122221.7-311523 and OT J184228.1+483742, which are believed to be the best candidates for period bouncers on the basis of their small values of the mass ratio (q = M2/M1). Additionally, the small mean superhump amplitude (<0.1mag) and the long duration of no ordinary superhumps at the early stage of its superoutburst are common to the best candidates for period bouncers. Its average superhump period was Psh = 0.0649810(78) d and no early superhumps were detected. Although we could not estimate a mass ratio of ASASSN-15jd with high accuracy, this object is expected to be a candidate for a period bouncer-a binary accounting for the missing population of post-period minimum cataclysmic variables-based on the above characteristics. © The Author 2016.

Kimura M.,Kyoto University | Kato T.,Kyoto University | Imada A.,Kyoto University | Ikuta K.,Kyoto University | And 9 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.

Tanaka J.,Kyoto University | Nogami D.,Kyoto University | Fujii M.,Fujii Bisei Observatory | Ayani K.,Bisei Astronomical Observatory | And 4 more authors.
Publications of the Astronomical Society of Japan | Year: 2011

We report on the spectral evolution of the enigmatic, very slow nova V5558 Sgr, based on low-resolution spectra obtained at the Fujii-Bisei Observatory and the Bisei Astronomical Observatory, Japan during the period of 2007 April 6 to 2008 May 3. V5558 Sgr shows a pre-maximum halt, and then several flare-like rebrightenings, which is similar to another very slow nova, V723 Cas. In our observations, the spectral type of V5558 Sgr evolved from the He/N type toward the Fe II type during a pre-maximum halt, and then toward the He/N type again. This course of spectral transition was observed for the first time in the long history of nova research. In the rebrightening stage after the initial brightness maximum, we could identify many emission lines accompanied by a stronger absorption component of the P Cyg profile at the brightness maxima. We found that the velocity of the P Cyg absorption component, measured from the emission peak, decreased at the brightness maxima. Furthermore, we compared the spectra of V5558 Sgr with V723 Cas, and other novae that exhibited several rebrightenings during the early phase. © 2011. Astronomical Society of Japan.

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