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Register a new userASASSN-14ho: Longest-period Dwarf Nova with Multiple Rebrightenings
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Authors
Taichi Kato
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The post-outburst rebrightening phenomenon in dwarf novae and X-ray novae is still one of the most challenging subjects for theories of accretion disks. It has been widely recognized that post-outburst rebrightenings are a key feature of WZ Sge-type dwarf novae, which predominantly have short ($lesssim$0.06 d) orbital periods. I found four post-outburst rebrightenings in ASASSN-14ho during its 2014 outburst, whose orbital period has recently measured to be exceptionally long [0.24315(10) d]. Using the formal solution of the radial velocity study in the literature, I discuss the possibility that this object can be an SU UMa-type dwarf nova near the stability border of the 3:1 resonance despite its exceptionally long orbital period. Such objects are considered to be produced if mass transfer occurs after the secondary has undergone significant nuclear evolution and they may be hidden in a significant number among dwarf novae showing multiple post-outburst rebrightenings.
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The disc instability model accounts well for most of the observed properties of dwarf novae and soft X-ray transients, but the rebrightenings, reflares, and echoes occurring at the end of outbursts or shortly after in WZ Sge stars or soft X-ray transients have not yet been convincingly explained by any model. We determine the additional ingredients that must be added to the DIM to account for the observed rebrightenings. We analyse in detail a recently discovered system, TCP J21040470+4631129, which has shown very peculiar rebrightenings, model its light curve using our numerical code including mass transfer variations from the secondary, inner-disc truncation, disc irradiation by a hot white dwarf and, in some cases, the mass-transfer stream over(under)flow. We show that the luminosity in quiescence is dominated by a hot white dwarf that cools down on time scales of months. The mass transfer rate from the secondary has to increase by several orders of magnitudes during the initial superoutburst for a reason that remains elusive, slowly returning to its secular average, causing the observed succession of outbursts with increasing quiescence durations, until the disc can be steady, cold, and neutral; its inner parts being truncated either by the white dwarf magnetic field or by evaporation. The very short, quiescence phases between reflares are reproduced when the mass-transfer stream overflows the disc. Using similar additions to the DIM, we have also produced light curves close to those observed in two WZ Sge stars, the prototype and EG Cnc. Our model successfully explains the reflares observed in WZ Sge systems. It requires, however, the inner disc truncation in dwarf novae to be due not (only) to the white dwarf magnetic field but, as in X-ray binaries, rather to evaporation of the inner disc. A similar model could also explain reflares observed in soft X-ray transients.
We report on a superoutburst of a WZ Sge-type dwarf nova (DN), ASASSN-15po. The light curve showed the main superoutburst and multiple rebrightenings. In this outburst, we observed early superhumps and growing (stage A) superhumps with periods of 0.050454(2) and 0.051809(13) d, respectively. We estimated that the mass ratio of secondary to primary ($q$) is 0.0699(8) by using $P_{rm orb}$ and a superhump period $P_{rm SH}$ of stage A. ASASSN-15po [$P_{rm orb} sim$ 72.6 min] is the first DN with the orbital period between 67--76 min. Although the theoretical predicted period minimum $P_{rm min}$ of hydrogen-rich cataclysmic variables (CVs) is about 65--70 min, the observational cut-off of the orbital period distribution at 80 min implies that the period minimum is about 82 min, and the value is widely accepted. We suggest the following four possibilities: the object is (1) a theoretical period minimum object (2) a binary with a evolved secondary (3) a binary with a metal-poor (Popullation II) seconday (4) a binary which was born with a brown-dwarf donor below the period minimum.
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.
We report spectroscopy and photometry of the cataclysmic variable stars ASASSN-14ho and V1062 Cyg. Both are dwarf novae with spectra dominated by their secondary stars, which we classify as approxomately K4 and M0.5, respectively. Their orbital periods, determined mostly from the secondary stars radial velociites, proved to be nearly identical, respectively 350.14 +- 0.15 and 348.25 +- 0.60 min. The H-alpha emission line in V1062 Cyg displays a relatively sharp emission component that tracks the secondarys motion, which may arise on the irradiated face of the secondary; tihs is not often seen and may indicate an unusually strong flux of ionizing radiation. Both systems exhibit double-peaked orbital modulation consistent with ellipsoidal variation from the changing aspect of the secondary. We model these variations to constrain the orbital inclination i, and estimate approximate component masses based oni and the secondary velocity amplitude K2.
We report on our photometric observations of the 2016 superoutburst of ASASSN-16eg. This object showed a WZ Sge-type superoutburst with prominent early superhumps with a period of 0.075478(8) d and a post-superoutburst rebrightening. During the superoutburst plateau, it showed ordinary superhumps with a period of 0.077880(3) d and a period derivative of 10.6(1.1) $times$ 10$^{-5}$ in stage B. The orbital period ($P_{rm orb}$), which is almost identical with the period of early superhumps, is exceptionally long for a WZ Sge-type dwarf nova. The mass ratio ($q$ = $M_2/M_1$) estimated from the period of developing (stage A) superhumps is 0.166(2), which is also very large for a WZ Sge-type dwarf nova. This suggests that the 2:1 resonance can be reached in such high-$q$ systems, contrary to our expectation. Such conditions are considered to be achieved if the mass-transfer rate is much lower than those in typical SU UMa-type dwarf novae that have comparable orbital periods to ASASSN-16eg and a resultant accumulation of a large amount of matter on the disk is realized at the onset of an outburst. We examined other candidates of long-period WZ Sge-type dwarf novae for their supercycles, which are considered to reflect the mass-transfer rate, and found that V1251 Cyg and RZ Leo have longer supercycles than those of other WZ Sge-type dwarf novae. This result indicates that these long-period objects including ASASSN-16eg have a low mass-transfer rate in comparison to other WZ Sge-type dwarf novae.
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