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Register a new userWZ Sge-type dwarf novae with multiple rebrightenings: MASTER OT J211258.65+242145.4 and MASTER OT J203749.39+552210.3
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We report on photometric observations of 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 superoutburst plateau, ordinary superhumps with a period of 0.060291(4) d (MASTER J211258) and of 0.061307(9) d (MASTER J203749) in average were observed. MASTER J211258 and MASTER J203749 exhibited eight and more than four post-superoutburst rebrightenings, respectively. In the final part of the superoutburst, an increase in the superhump periods was seen in both systems. We have made a survey of WZ Sge-type dwarf novae with multiple rebrightenings, and confirmed that the superhump periods of WZ Sge-type dwarf novae with multiple rebrightenings were longer than those of WZ Sge-type dwarf novae without a rebrightening. Although WZ Sge-type dwarf novae with multiple rebrightenings have been thought to be the good candidates for period bouncers based on their low mass ratio (q) from inferred from the period of fully grown (stage B) superhumps, our new method using the period of growing superhumps (stage A superhumps), however, implies higher q 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 3:1 resonance. We present a working hypothesis that the small fractional superhump excesses for stage B superhumps in these systems may be explained as a result that a higher gas pressure effect works in these systems than in ordinary SU UMa-type dwarf novae. This result leads to a new picture that WZ Sge-type dwarf novae with multiple rebrightenings and SU UMa-type dwarf novae without a rebrightening (they are not period bouncers) are located in the same place on the evolutionary track.
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We present our photometric studies of the newly discovered optical transient, OT J012059.6+325545, which underwent a large outburst between 2010 November and 2011 January. The amplitude of the outburst was about 8 mag. We performed simultaneous multi-color photometry by using g, Rc, and i-band filters from the early stage of the outburst. The time resolved photometry during the early stage revealed periodic variations with double-peaked profiles, which are referred to as early superhumps, with amplitudes of about 0.08 mag. After the rapid fading from the main outburst, we found rebrightening phenomena, which occurred at least nine times. The large amplitude of the outburst, early superhumps, and rebrightening phenomena are typical features of WZ Sge-type dwarf novae. We detected color variations within the early superhump modulations making this only the second system, after V445 And, for which this has been established. We carried out numerical calculations of the accretion disk to explain both of the modulations and the color variations of the early superhump. This modeling of the disk height supports the idea that height variations within the outer disk can produce the early superhump modulations, though we cannot rule out that temperature asymmetries may also play a role.
We present time-resolved photometry and spectroscopy of the recently classified polar MASTER OT J132104.04+560957.8. The spectrum shows a smooth, non-thermal continuum at the time of maximum light, without any individually discernible cyclotron harmonics. Using homogeneous cyclotron modeling, we interpret this as cyclotron radiation whose individual harmonics have blended together, and on this basis, we loosely constrain the magnetic field strength to be less than ~30 MG. In addition, for about one-tenth of the orbital period, the Balmer and He I emission lines transition into absorption features, with He II developing an absorption core. We use our observations of this phenomenon to test theoretical models of the accretion curtain and conclude that the H and He I lines are produced throughout the curtain, in contravention of theoretical predictions of separate H and He I line-forming regions. Moreover, a significant amount of He II emission originates within the accretion curtain, implying that the curtain is significantly hotter than expected from theory. Finally, we comment on the objects long-term photometry, including evidence that it recently transitioned into a prolonged, exceptionally stable high state following a potentially decades-long low state.
We present the result of a multi-longitude campaign on the photometric study of the dwarf nova ASASSN-18fk during its superoutburst in 2018. It was observed with 18 telescopes at 15 sites during ~70 nights within a three-month interval. Observations covered the main outburst, six rebrightenings and 50-d decline to a near-quiescent state. We identify ASASSN-18fk as WZ Sge-type dwarf nova with multiple rebrightenings and show the evolution of the 0.06-d superhump period over all stages of the superoutburst. A strong 22-min brightness modulation that superimposed on superhumps is found during rebrightenings and decline. Some evidence of this modulation in a form of a sideband signal is detected during the very onset of the outburst. We interpret the 22-min modulation as a spin period of the white dwarf and suggest that ASASSN-18fk is a good candidate for a superhumping intermediate polar.
A CCD photometry of the dwarf nova MASTER OT J172758.09 +380021.5 was carried out in 2019 during 134 nights. Observations covered three superoutbursts, five normal outbursts and quiescence between them. The available ASASSN and ZTF data for 2014-2020 were also examined. Spectral observations were done in 2020 when the object was in quiescence. Spectra and photometry revealed that the star is an H-rich active ER UMa-type dwarf nova with a highly variable supercycle of ~50-100 d that implies a high and variable mass-transfer rate. This object demonstrated peculiar behaviour: short-lasted superoutbursts (a week); a slow superoutburst decline and cases of rebrightenings; low frequency (from none to a few) of the normal outbursts during the supercycle. In 2019 a mean period of positive superhumps was found to be 0.05829 d during the superoutbursts. Late superhumps with a mean period of 0.057915 d which lasted about ~20 d after the end of superoutburst and were replaced by an orbital period of 0.057026 d or its orbital-negative superhump beat period were detected. An absence of eclipse in the orbital light curve and its moderate amplitude are consistent with the orbital inclination of about 40 degr found from spectroscopy. The blue peaks of the V-Ic and B-Rc of superhumps during the superoutburst coincided with minima of the light curves, while B-Rc of the late superhumps coincided with a rising branch of the light curves. We found that a low mass ratio q=0.08 could explain most of the peculiarities of this dwarf nova. The mass-transfer rate should be accordingly higher than what is expected from gravitational radiation only, this assumes the object is in a post-nova state and underwent a nova eruption relatively recently -- hundreds of years ago. This object would provide probably the first observational evidence that a nova eruption can occur even in CVs near the period minimum.
We report the discovery of a new eclipsing polar, MASTER OT J061451.70-272535.5, detected as an optical transient by MASTER auto-detection software at the recently commissioned MASTER-SAAO telescope. Time resolved (10-20 s) photometry with the SAAO 1.9-m, and 1.0-m telescopes, utilizing the SHOC EM-CCD cameras, revealed that the source eclipses, with a period of 2.08 hours (7482.9$pm$3.5$,$s). The eclipse light curve has a peculiar morphology, comprising an initial dip, where the source brightness drops to ${sim}$50% of the pre-eclipse level before gradually increasing again in brightness. A second rapid ingress follows, where the brightness drops by ${sim}$60-80%, followed by a more gradual decrease to zero flux. We interpret the eclipse profile as the result of an initial obscuration of the accretion hot-spot on the magnetic white dwarf by the accretion stream, followed by an eclipse of both the hot-spot and the partially illuminated stream by the red dwarf donor star. This is similar to what has been observed in other eclipsing polars such as HU Aqr, but here the stream absorption is more pronounced. The object was subsequently observed with South African Large Telescope (SALT) using the Robert Stobie Spectrograph (RSS). This revealed a spectrum with all of the Balmer lines in emission, a strong HeII 4686AA{} line with a peak flux greater than that of H$beta$, as well as weaker HeI lines. The spectral features, along with the structure of the light curve, suggest that MASTER OT J061451.70-272535.5 is a new magnetic cataclysmic variable, most likely of the synchronised Polar subclass.
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