We propose a new method to reconstruct the structure of accretion disks in dwarf novae using multi-band light curves of early superhumps. Our model assumes that early superhumps are caused by the ...rotation effect of non-axisymmetrically flaring disks. We have developed a Bayesian model for this reconstruction, in which a smoother disk-structure tends to have a higher prior probability. We analyzed simultaneous optical and near-infrared photometric data of early superhumps of the dwarf nova, V455 And using this technique. The reconstructed disk has two flaring parts in the outermost region of the disk. These parts are responsible for the primary and secondary maxima of the light curves. The height-to-radius ratio is
$ h/r$
$ =$
0.20–0.25 in the outermost region. In addition to the outermost flaring structures, flaring arm-like patterns can be seen in an inner region of the reconstructed disk. The overall profile of the reconstructed disk is reminiscent of the disk structure that is deformed by the tidal effect. However, an inner arm-like pattern, which is responsible for the secondary minimum in the light curve, cannot be reproduced only by the tidal effect. It implies the presence of another mechanism that deforms the disk structure. Alternatively, the temperature distribution of the disk could be non-axisymmetric. We demonstrate that the disk structure with weaker arm-like patterns is optimal in the model including the irradiation effect. However, the strongly irradiated disk gives quite blue colors, which may conflict with the observation. Our results suggest that the amplitude of early superhumps depends mainly on the height of the outermost flaring regions of the disk. We predict that early superhumps can be detected with an amplitude of
$ \gt$
0.02 mag in about 90% of WZ Sge stars.
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-
$P_{\rm orb}$
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.
Abstract Continuing the project undertaken by Kato et al. (2009), we collected times of superhump maxima for 56 SU UMa-type dwarf novae mainly observed during the 2013–2014 season and characterized ...these objects. We detected negative superhumps in VW Hyi and indicated that the low number of normal outbursts in some supercycles can be interpreted as a result of disk tilt. This finding, combined with the Kepler observation of V1504 Cyg and V344 Lyr, suggests that disk tilt is responsible for modulating the outburst pattern in SU UMa-type dwarf novae. We also studied the deeply eclipsing WZ Sge-type dwarf nova MASTER OT J005740.99+443101.5 and found evidence of a sharp eclipse during the phase of early superhumps. The profile can be reproduced by a combination of the eclipse of the axisymmetric disk and the uneclipsed light source of early superhumps. This finding shows the lack of evidence for a greatly enhanced hot spot during the early stage of WZ Sge-type outburst. We detected growing (stage A) superhumps in MN Dra and give a suggestion that some of SU UMa-type dwarf novae situated near the critical condition of tidal instability may show long-lasting stage A superhumps. The large negative period derivatives reported in such systems can be understood as a result of the combination of stage A and B superhumps. Two WZ Sge-type dwarf novae, AL Com and ASASSN-13ck, showed a long-lasting (plateau-type) rebrightening. In the early phase of their rebrightenings, both objects showed a precursor-like outburst, suggesting that the long-lasting rebrightening is triggered by a precursor outburst.
The secular variation in the interval of outbursts in the following six Z Cam-type dwarf novae (including thesubtype IW And-type) is investigated: Z Cam, RX And, AH Her, HL CMa, SY Cnc, andWWCet. An ...analysis usingthe O − C diagram shows that the interval of outbursts is not steady in one system. The outburst properties beforestandstill are the decrease in outburst interval, enhancement of the magnitude in quiescence, and disappearance ofthe long outburst. Meanwhile, several objects have at least two typical intervals of outbursts. These characteristicsare difficult to be explained only by the variation in mass transfer from the secondary.
Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected the times of superhump maxima for 51 SU UMa-type dwarf novae, mainly observed during the 2010–2011 season. ...Although most of the new data for systems with short superhump periods basically confirmed the findings by Kato et al. (ibid.) and Kato et al. (2010, PASJ, 62, 1525), the long-period system GX Cas showed an exceptionally large positive-period derivative. An analysis of public Kepler data of V344 Lyr and V1504 Cyg yielded less-striking stage transitions. In V344 Lyr, there was a prominent secondary component growing during the late stage of superoutbursts, and this component persisted for at least two more cycles of successive normal outbursts. We also investigated the superoutburst of two conspicuous eclipsing objects: HT Cas and the WZ Sge-type object SDSS J080434.20
$+$
510349.2. Strong beat phenomena were detected in both objects, and late-stage superhumps in the latter object had an almost constant luminosity during repeated rebrightenings. The WZ Sge-type object SDSS J133941.11
$+$
484727.5 showed a phase reversal around the rapid fading from the superoutburst. The object showed a prominent beat phenomenon, even after the end of the superoutburst. A pilot study of superhump amplitudes indicated that the amplitude of superhumps is strongly correlated with the orbital period, and the dependence on the inclination is weak in systems with inclinations smaller than 80
$^{\circ}$
.
We carried out photometric observations of the SU UMa-type dwarf nova ER UMa during 2011 and 2012, which showed the existence of persistent negative superhumps even during the superoutburst. We ...performed a two-dimensional period analysis of its light curves by using a method called “least absolute shrinkage and selection operator” (Lasso) and the “phase dispersion minimization” (PDM) analysis, and found that the period of negative superhumps systematically changed between a superoutburst and the next superoutburst. The trend of the period change can be interpreted as a reflection of the change of the disk radius. This change is in agreement with the one predicted by the thermal tidal instability model. The normal outburst during a supercycle showed a general trend that the rising rate to its maximum becomes slower as the next superoutburst is approaching. The change can be interpreted as the consequence of the increased gas-stream flow into the inner region of the disk as a result of the tilted disk. Some of superoutbursts were found to be triggered by a precursor normal outburst when the positive superhump appeared to develop. The positive and negative superhumps coexisted during the superoutburst. Positive superhumps were prominent only for four or five days after the supermaximum, while the signal of negative superhumps became stronger after the middle phase of the superoutburst plateau. A simple combination of the positive and negative superhumps was found to be insufficient for reproduction of the complex profile variation. We were able to detect the developing phase of positive superhumps (stage A superhumps) for the first time in ER UMa-type dwarf novae. Using the period of stage A superhumps, we obtained a mass ratio of 0.100(15), which indicates that ER UMa is on the ordinary evolutional track of cataclysmic variable stars.
We report on a discovery of “negative” superhumps during the 2011 January superoutburst of ER UMa. 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 outbursts was 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 ER UMa-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.
Abstract
We report on photometric observations of two dwarf novae, OT J075418.7+381225 and OT J230425.8+062546, which showed superoutbursts in 2013 (OT J075418) and in 2011 (OT J230425). Their mean ...periods of the superhump were 0.0722403(26) d (OT J075418) and 0.067317(35) d (OT J230425). These objects showed a very long growth stage of the superhump (stage A) and a large period decrease in the stage A–B transition. The long stage A suggests slow evolution of the superhump due to the very small mass ratio of these objects. The declining rates during the plateau phase in the superoutburst of these objects were lower than those of SU UMa-type dwarf novae (DNe) with a similar superhump period. These properties were similar to those of SSS J122221.7−311523, the most likely candidate for the period bouncer. Therefore, these two DNe are regarded as likely candidates for the period bouncer. We estimated the number density of period bouncers roughly from our observations for the last five years. There is a possibility that these WZ Sge-type DNe with unusual outburst properties might account for the missing population of the period bouncer suggested by the evolutionary scenario.
Abstract
We present monitoring campaign observations at optical and near-infrared (NIR) wavelengths for a radio-loud active galactic nucleus (AGN) at z = 0.840, SDSS J110006.07+442144.3 (hereafter, ...J1100+4421), which was identified during a flare phase in late 2014 February. The campaigns consist of three intensive observing runs from the discovery to 2015 March, mostly within the scheme of the OISTER collaboration. Optical–NIR light curves and simultaneous spectral energy distributions (SEDs) are obtained. Our measurements show the strongest brightening in 2015 March. We found that the optical–NIR SEDs of J1100+4421 show an almost steady shape despite the large and rapid intranight variability. This constant SED shape is confirmed to extend to ∼5 μm in the observed frame using the archival WISE data. Given the lack of absorption lines and the steep power-law spectrum of αν ∼ −1.4, where $f_{\nu }\propto \nu ^{\alpha _{\nu }}$, synchrotron radiation by a relativistic jet with no or small contributions from the host galaxy and the accretion disk seems most plausible as an optical–NIR emission mechanism. The steep optical–NIR spectral shape and the large amplitude of variability are consistent with this object being a low νpeak jet-dominated AGN. In addition, sub-arcsecond resolution optical imaging data taken with Subaru Hyper Suprime-Cam does not show a clear extended component and the spatial scales are significantly smaller than the large extensions detected at radio wavelengths. The optical spectrum of a possible faint companion galaxy does not show any emission lines at the same redshift, and hence a merging hypothesis for this AGN-related activity is not supported by our observations.
Abstract
The analysis of 14 periodograms of EZ Lyn for data spaced over 565 d in 2012–2014 (2–3.5 yr after the 2010 outburst) yielded the existence of a stable signal around 100 cycles/day (c/d) and ...three signals around 310 c/d, 338 c/d, and 368 c/d (the corresponding periods are 864 s, 279 s, 256 s, and 235 s). We interpret them as independent nonradial pulsations of the white dwarf in EZ Lyn, but the possibility that a linear combination of a frequency of 100 c/d and a harmonic of the orbital period might produce a frequency of 368 c/d also cannot be excluded. The signal at 100 c/d was detected as a transient during the first stay in the instability strip. The period at 338 c/d has been a known nonradial pulsation since EZ Lyn entered the instability strip after the 2010 outburst. We detected the signals around 310 c/d and 368 c/d for the first time. We applied the two-dimensional least absolute shrinkage and selection operator (Lasso) analysis for the first time to explore the behavior of these signals on a time scale of hours in nightly runs of observations having a duration of 6–12 hr. The Lasso analysis revealed the simultaneous existence of all three frequencies (310 c/d, 338 c/d, and 368 c/d) on a majority of nights of observations, but with variable amplitudes and variable drifts of frequencies by 2%–6% on a time scale of ∼ 5–7 hr. The largest drift we detected corresponded to 17.5 s in period in ∼ 5 hr.
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