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  • 1
    In: Publications of the Astronomical Society of Japan, 2012, Vol. 64(5)
    Description: 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.
    Keywords: Accretion, Accretion Disks ; Methods: Numerical ; Stars: Dwarf Novae
    ISSN: 0004-6264
    E-ISSN: 2053-051X
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  • 2
    In: Publications of the Astronomical Society of Japan, 2014, Vol. 66(4)
    Description: 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.
    Keywords: Accretion, Accretion Disks ; Novae, Cataclysmic Variables ; Stars: Dwarf Novae ; Stars: Individual (Er Ursae Majoris)
    ISSN: 0004-6264
    E-ISSN: 2053-051X
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  • 3
    In: Publications of the Astronomical Society of Japan, 2012, Vol. 64(4)
    Description: 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.
    Keywords: Accretion, Accretion Disks ; Stars: Dwarf Novae ; Stars: Individual (Er Ursae Majoris) ; Novae, Cataclysmic Variables
    ISSN: 0004-6264
    E-ISSN: 2053-051X
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  • 4
    In: Publications of the Astronomical Society of Japan, 2014, Vol. 66(5)
    Description: We report on a superoutburst of the AM CVn-type object SDSS J090221.35+381941.9 [J0902; orbital period 0.03355(6) d] in 2014 March–April. The entire superoutburst consisted of a precursor outburst and the main superoutburst, followed by a short rebrightening. During the rising phase of the main superoutburst, we detected growing superhumps (stage A superhumps) with a period of 0.03409(1) d. During the plateau phase of the superoutburst, superhumps with a shorter period (stage B superhumps) were observed. Using the orbital period and the period of stage A superhumps, we were able to measure the dynamical precession rate of the accretion disk at the 3:1 resonance, and obtained a mass ratio ( q ) of 0.041(7). This is the first successful measurement of the mass ratio in an AM CVn-type object accomplished by the recently developed stage A superhump method. The value is generally in agreement with that based on the theoretical evolutionary model. The orbital period of J0902 is the longest among those of the outbursting AM CVn-type objects, and a period on the borderline between the outbursting system and the system with a stable cool disk appears to be longer than one supposed.
    Keywords: Accretion, Accretion Disks ; Novae, Cataclysmic Variables ; Stars: Dwarf Novae ; Stars: Individual (Sdss J090221. 35+381941. 9)
    ISSN: 0004-6264
    E-ISSN: 2053-051X
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  • 5
    In: Publications of the Astronomical Society of Japan, 2014, Vol. 66(6)
    Description: 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.
    Keywords: Accretion, Accretion Disks ; Novae, Cataclysmic Variables ; Stars: Dwarf Novae ; Stars: Individual (Ez Lyncis) ; Stars: Oscillations
    ISSN: 0004-6264
    E-ISSN: 2053-051X
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  • 6
    In: Publications of the Astronomical Society of Japan, 2019, Vol. 71(1)
    Description: ASASSN-14dx showed an extraordinary outburst whose features are a small outburst amplitude (∼2.3 mag) and long duration (〉4 yr). Because we found a long observational gap of 123 d before the outburst detection, we propose that the main outburst plateau was missed and that this outburst is just a “fading tail” often seen after the WZ Sge-type super-outbursts. In order to distinguish between WZ Sge and SU UMa-type dwarf novae (DNe), we investigated Gaia Data Release 2 (DR2) statistically. We applied a logistic regression model and succeeded in classifying by using absolute Gaia magnitudes M G and Gaia colors G BP – G RP . Our new classifier also suggests that ASASSN-14dx is the best candidate of a WZ Sge-type DN. We estimated distances from the Earth of known WZ Sge stars by using Gaia DR2 parallaxes. The result indicates that ASASSN-14dx is the third-nearest WZ Sge star (next to WZ Sge and V455 And), and hence the object can show the third-brightest WZ Sge-type super-outburst whose maximum is V = 8–9 mag.
    Keywords: Accretion, Accretion Disks ; Novae, Cataclysmic Variables ; Stars: Dwarf Novae ; Stars: Individual (Asassn - 14dx)
    ISSN: 0004-6264
    E-ISSN: 2053-051X
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  • 7
    In: Publications of the Astronomical Society of Japan, 2014, Vol. 66(6)
    Description: 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.
    Keywords: Accretion, Accretion Disks ; Novae, Cataclysmic Variables ; Stars: Dwarf Novae ; Stars: Individual (Ot J075418. 7+381225, Ot J230425. 8+062546)
    ISSN: 0004-6264
    E-ISSN: 2053-051X
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  • 8
    In: Publications of the Astronomical Society of Japan, 2013, Vol. 65(6)
    Description: We report on photometric observations of two WZ Sge-type dwarf novae, MASTER OT J211258.65+242145.4 and MASTER OT J203749.39+552210.3, which underwent outbursts in 2012. Early superhumps were recorded in both systems. During the superoutburst plateau, ordinary superhumps with a period of 0.060291(4) d (MASTER J211258) and with 0.061368(11) d (MASTER J203749) on average were observed. MASTERJ211258 and MASTERJ203749 exhibited eight post-superoutburst rebrightenings and more than four, respectively. In the final part of the superoutburst, an increase in superhump period was seen in both systems. We made a survey of WZSge-type dwarf novae with multiple rebrightenings, and confirmed that the superhump periods of WZSge-type dwarf novae with multiple rebrightenings were longer than those of WZSge-type dwarf novae without a rebrightening. Although WZSge-type dwarf novae with multiple rebrightenings have been thought to be likely candidates for period bouncers based on their low mass ratio ( q ), inferred from the period of fully grown (stage B) superhumps, our new method of using the period of growing superhumps (stage A superhumps), however, implies higher q 's than those expected from stage B superhumps. These q values appear to be consistent with the duration of the stage A superoutbursts, which likely reflects the growth time of the 3W1 resonance. We present a working hypothesis that the small fractional superhump excesses for stage B superhumps in these systems may be explained by a gas pressure effect that works more efficiently in these systems than in ordinary SU UMa-type dwarf novae. This result forms a new picture that WZSge-type dwarf novae with multiple rebrightenings and SU UMa-type ones without a rebrightening (they are not period bouncers) are located in the same place on the evolutionary track.
    Keywords: Accretion, Accretion Disks ; Stars: Dwarf Novae ; Stars: Individual(Master Ot J211258. 65+242145. 4, Master Ot J203749. 39+552210. 3) ; Novae, Cataclysmic Variables
    ISSN: 0004-6264
    E-ISSN: 2053-051X
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  • 9
    In: Publications of the Astronomical Society of Japan, 2014, Vol. 66(6)
    Description: We present a photometric study of NY Ser, an in-the-gap SU UMa-type nova, in 2002 and 2013. We determined the duration of its superoutburst and the mean period of its superhump are 18 d and 0.10458 d, respectively. We detected in 2013 that NY Ser showed two distinct states separated by a superoutburst. A state of rather infrequent normal outbursts lasted at least 44 d before the superoutburst, and a state of frequent outbursts started immediately after the superoutburst and lasted at least 34 d. Unlike a typical SU UMa star with a bimodal distribution of outburst duration, NY Ser displayed a diversity of normal outbursts. In the state of infrequent outbursts, we detected a wide ∼ 12 d outburst accompanied by 0.098 d orbital modulation but without superhumps ever established in NY Ser. 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 0.10465 d positive superhump period was dominant and coexisted with the orbital modulation. In 2002 we detected the normal outburst of “intermediate” 5–6 d duration that was also accompanied by orbital modulations.
    Keywords: Accretion, Accretion Disks ; Novae, Cataclysmic Variables ; Stars: Dwarf Novae ; Stars: Individual (Ny Serpentis) ; Stars: Individual (V1006 Cygni)
    ISSN: 0004-6264
    E-ISSN: 2053-051X
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  • 10
    In: Publications of the Astronomical Society of Japan, 2014, Vol. 66(6)
    Description: We present results of optical polarimetric and multi-band photometric observations for γ-ray loud, narrow-line Seyfert 1 galaxy 1H 0323+342. This object has been monitored by the 1.5-m Kanata telescope since 2012 September, but following a γ-ray flux enhancement detected by Fermi Large Area Telescope (Fermi-LAT) on MJD 56483 (2013 July 10), dense follow-up was performed by 10 0.5–2.0-m telescopes in Japan over one week. The 2-yr R C -band light curve showed clear brightening corresponding to the γ-ray flux increase, and then decayed gradually. The high state as a whole lasted for ∼ 20 d, during which we clearly detected optical polarization from this object. The polarization degree (PD) of the source increased from 0%–1% in quiescence to ∼ 3% at maximum and then declined to the quiescent level, with the duration of the enhancement being less than 10 d. The moderate PD around the peak allowed us to precisely measure the daily polarization angle (PA). As a result, we found that the daily PAs were almost constant and aligned to the jet axis, suggesting that the magnetic field direction at the emission region is transverse to the jet. This implies either a presence of helical/toroidal magnetic field or transverse magnetic field compressed by shock(s). We also found small-amplitude intra-night variability during the 2-hr continuous exposure on a single night. We discuss these findings based on the turbulent multi-zone model recently advocated by Marscher ( 2014 , ApJ, 780, 87). Optical to ultraviolet (UV) spectrum showed a rising shape in the higher frequency and the UV magnitude measured by Swift/UVOT (UV and Optical Telescope) was steady even during the flaring state, suggesting that thermal emission from accretion disk is dominant in that band.
    Keywords: Galaxies: Active ; Galaxies: Jets ; Galaxies: Individual (1h 0323+342) ; Galaxies: Seyfert ; Radiation Mechanisms: Non - Thermal
    ISSN: 0004-6264
    E-ISSN: 2053-051X
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