Abstract
SS Cyg has long been recognized as the prototype of a group of dwarf novae that show only outbursts. However, this object has entered a quite anomalous event in 2021, which at first appeared ...to be standstill, i.e., an almost constant luminosity state observed in Z Cam-type dwarf novae. This unexpected event gives us a great opportunity to reconsider the nature of standstill in cataclysmic variables. We have observed this anomalous event and its forerunner, a gradual and simultaneous increase in the optical and X-ray flux during quiescence, through many optical telescopes and the X-ray telescopes NICER and NuSTAR. We have not found any amplification of the orbital hump during quiescence before the anomalous event, which suggests that the mass transfer rate did not significantly fluctuate on average. The estimated X-ray flux was not enough to explain the increment of the optical flux during quiescence via X-ray irradiation of the disk and the secondary star. It would be natural to consider that viscosity in the quiescent disk was enhanced before the anomalous event, which increased mass accretion rates in the disk and raised not only the optical flux but also the X-ray flux. We suggest that enhanced viscosity also triggered the standstill-like phenomenon in SS Cyg, which is considered to be a series of small outbursts. The inner part of the disk would always stay in the outburst state and only its outer part would be unstable against the thermal–viscous instability during this phenomenon, which is consistent with the observed optical color variations. This scenario is in line with our X-ray spectral analyses which imply that the X-ray-emitting inner accretion flow became hotter than usual and vertically expanded, and that it became denser and was cooled down after the onset of the standstill-like state.
Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 102 SU UMa-type dwarf novae, observed mainly during the 2014–2015 season, and ...characterized these objects. Our project has greatly improved the statistics of the distribution of orbital periods, which is a good approximation of the distribution of cataclysmic variables at the terminal evolutionary stage, and has confirmed the presence of a period minimum at a period of 0.053 d and a period spike just above this period. The number density monotonically decreased toward the longer period and there was no strong indication of a period gap. We detected possible negative superhumps in Z Cha. It is possible that normal outbursts are also suppressed by the presence of a disk tilt in this system. There was no indication of enhanced orbital humps just preceding the superoutburst, and this result favors the thermal–tidal disk instability as the origin of superoutbursts. We detected superhumps in three AM CVn-type dwarf novae. Our observations and recent other detections suggest that 8% of objects showing dwarf nova-type outbursts are AM CVn-type objects. AM CVn-type objects and EI Psc-type objects may be more abundant than previously recognized. OT J213806, a WZ Sge-type object, exhibited remarkably different features between the 2010 and 2014 superoutbursts. Although the 2014 superoutburst was much fainter, the plateau phase was shorter than the 2010 one, and the course of the rebrightening phase was similar. This object indicates that the O − C diagrams of superhumps can indeed be variable, at least in WZ Sge-type objects. Four deeply eclipsing SU UMa-type dwarf novae (ASASSN-13cx, ASASSN-14ag, ASASSN-15bu, and NSV 4618) were identified. We studied long-term trends in supercycles in MM Hya and CY UMa and found systematic variations of supercycles of ∼20%.
Abstract
We present spectroscopic and photometric observations of 17 dwarf-nova superoutbursts obtained by KOOLS-IFU mounted on the 3.8 m telescope Seimei at the Okayama Observatory of Kyoto ...University and through the Variable Star Network collaboration (VSNET). Our spectroscopic observations for six outbursts were performed within 1 d of their optical peak. 11 objects (TCP J00590972+3438357, ASASSN-19ado, TCP J06073081−0101501, ZTF20aavnpug, ASASSN-19ady, MASTER OT J061642.05+435617.9, TCP J20034647+1335125, ASASSN-20kv, ASASSN-20kw, MASTER OT J213908.79+161240.2, and ASASSN-20mf) were previously unknown systems, and our observations enabled quick classification of their transient type. These results illustrate that the Seimei telescope has the capability to conduct quick follow-up observations of unknown transients. Our photometric observations yielded that 11 of the objects are WZ Sge-type dwarf novae and their candidates, and the other six are SU UMa-type dwarf novae and their candidates. The He ii 4686 Å emission line was clearly detected among ASASSN-19ado, TCP J06073081−0101501 and MASTER OT J213908.79+161240.2, the association of which with a spiral arm structure in an accretion disk has been suggested in previous studies. Our result suggests that a higher-inclination system shows a stronger emission line of He ii 4686 Å, as well as larger-amplitude early superhumps.
Abstract
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 its behavior in 2019–2020. In addition to this, the object showed dwarf nova-type outbursts in 2020–2021, and superhumps that had periods $7.8\%$ longer than the orbital one developed during at least 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 a 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 (∼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 1.0 M⊙, 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.
Abstract
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 deduce that the condition close to the limit of the tidal instability caused early quenching of superoutbursts, which resulted in a substantial amount of matter left in the disk after the superoutburst. We think that substantial matter in the disk in a 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).
Abstract
Superoutbursts in WZ Sge-type dwarf novae (DNe) are characterized by both early superhumps and ordinary superhumps originating from the 2 : 1 and 3 : 1 resonances, respectively. However, ...some WZ Sge-type DNe show a superoutburst lacking early superhumps; it is not well established how these differ from superoutbursts with an early superhump phase. We report time-resolved photometric observations of the WZ Sge-type DN V627 Peg during its 2021 superoutburst. The detection of ordinary superhumps before the superoutburst peak highlights that this 2021 superoutburst of V627 Peg, like that in 2014, did not feature an early superhump phase. The duration of stage B superhumps was slightly longer in the 2010 superoutburst accompanied by early superhumps than that in the 2014 and 2021 superoutbursts, which lacked early superhumps. This result suggests that an accretion disk experiencing the 2 : 1 resonance may have a larger mass at the inner part of the disk and hence needs more time for the inner disk to become eccentric. The presence of a precursor outburst in the 2021 superoutburst suggests that the maximum disk radius should be smaller than that of the 2014 superoutburst, even though the duration of quiescence was longer than that before the 2021 superoutburst. This could be accomplished if the 2021 superoutburst was triggered as an inside-out outburst or if the mass transfer rate in quiescence changes by a factor of two, suggesting that the outburst mechanism and quiescence state of WZ Sge-type DNe may have more variety than ever thought.
Abstract
Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 127 SU UMa-type dwarf novae observed mainly during the 2016–2017 season and ...characterized these objects. We provide updated statistics of the relation between the orbital period and the variation of superhumps, the relation between period variations and the rebrightening type in WZ Sge-type objects. We obtained the period minimum of 0.05290(2) d and confirmed the presence of the period gap above the orbital period ∼0.09 d. We note that four objects (NY Her, 1RXS J161659.5+620014, CRTS J033349.8−282244, and SDSS J153015.04+094946.3) have supercycles shorter than 100 d but show infrequent normal outbursts. We consider that these objects are similar to V503 Cyg, whose normal outbursts are likely suppressed by a disk tilt. These four objects are excellent candidates to search for negative superhumps. DDE 48 appears to be a member of ER UMa-type dwarf novae. We identified a new eclipsing SU UMa-type object, MASTER OT J220559.40−341434.9. We observed 21 WZ Sge-type dwarf novae during this interval and report 18 of them in this paper. Among them, ASASSN-16js is a good candidate for a period bouncer. ASASSN-16ia showed a precursor outburst for the first time in a WZ Sge-type superoutburst. ASASSN-16kg, CRTS J000130.5+050624, and SDSS J113551.09+532246.2 are located in the period gap. We have newly obtained 15 orbital periods, including periods from early superhumps.
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.
Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 128 SU UMa-type dwarf novae observed mainly during the 2015–2016 season and ...characterized these objects. The data have improved the distribution of orbital periods, the relation between the orbital period and the variation of superhumps, and the relation between period variations and the rebrightening type in WZ Sge-type objects. Coupled with new measurements of mass ratios using growing stages of superhumps, we now have a clearer and statistically greatly improved evolutionary path near the terminal stage of evolution of cataclysmic variables. Three objects (V452 Cas, KK Tel, and ASASSN-15cl) appear to have slowly growing superhumps, which is proposed to reflect the slow growth of the 3 : 1 resonance near the stability border. ASASSN-15sl, ASASSN-15ux, SDSS J074859.55+312512.6, and CRTS J200331.3−284941 are newly identified eclipsing SU UMa-type (or WZ Sge-type) dwarf novae. ASASSN-15cy has a short (∼0.050 d) superhump period and appears to belong to EI Psc-type objects with compact secondaries having an evolved core. ASASSN-15gn, ASASSN-15hn, ASASSN-15kh, and ASASSN-16bu are candidate period bouncers with superhump periods longer than 0.06 d. We have newly obtained superhump periods for 79 objects and 13 orbital periods, including periods from early superhumps. In order that future observations will be more astrophysically beneficial and rewarding to observers, we propose guidelines on how to organize observations of various superoutbursts.
Abstract
Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 102 SU UMa-type dwarf novae observed mainly during the 2017 season, and ...characterized these objects. WZ Sge-type stars identified in this study are PT And, ASASSN-17ei, ASASSN-17el, ASASSN-17es, ASASSN-17fn, ASASSN-17fz, ASASSN-17hw, ASASSN-17kd, ASASSN-17la, PNV J20205397$+$2508145, and TCP J00332502$-$3518565. We obtained new mass ratios for seven objects using growing superhumps (stage A). ASASSN-17gf is an EI Psc-type object below the period minimum. CRTS J080941.3$+$171528 and DDE 51 are objects in the period gap, and both showed a long-lasting phase of stage A superhumps. We also summarize the recent advances in understanding of SU UMa-type and WZ Sge-type dwarf novae.
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