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The All Sky Automated Survey for SuperNovae (ASAS-SN) reported a possible Galactic dwarf nova ASASSN-18fs on 2018 March 19 at $sim$13.2 mag in the V band, with a quiescent magnitude of V$>$17.6. Here we report on the follow-up photometry using the {it Neil Gehrels Swift Observatory}.
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The March 2011 outburst of the poorly-studied cataclysmic variable NSV 1436 offered an opportunity to decide between dwarf nova and recurrent nova classifications. We use seven daily observations in the X-ray and UV by the Swift satellite, together with AAVSO V photometry, to characterise the outburst and decline behaviour. The short optical outburst coincided with a faint and relatively soft X-ray state, whereas in decline to fainter optical magnitudes the X-ray source was harder and brighter. These attributes, and the modest optical outburst amplitude, indicate that this was a dwarf nova outburst and not a recurrent nova. The rapid optical fading suggests an orbital period below 2 hours.
The post-outburst rebrightening phenomenon in dwarf novae and X-ray novae is still one of the most challenging subjects for theories of accretion disks. It has been widely recognized that post-outburst rebrightenings are a key feature of WZ Sge-type dwarf novae, which predominantly have short ($lesssim$0.06 d) orbital periods. I found four post-outburst rebrightenings in ASASSN-14ho during its 2014 outburst, whose orbital period has recently measured to be exceptionally long [0.24315(10) d]. Using the formal solution of the radial velocity study in the literature, I discuss the possibility that this object can be an SU UMa-type dwarf nova near the stability border of the 3:1 resonance despite its exceptionally long orbital period. Such objects are considered to be produced if mass transfer occurs after the secondary has undergone significant nuclear evolution and they may be hidden in a significant number among dwarf novae showing multiple post-outburst rebrightenings.
We report on a superoutburst of a WZ Sge-type dwarf nova (DN), ASASSN-15po. The light curve showed the main superoutburst and multiple rebrightenings. In this outburst, we observed early superhumps and growing (stage A) superhumps with periods of 0.050454(2) and 0.051809(13) d, respectively. We estimated that the mass ratio of secondary to primary ($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.
V3890 Sgr is a recurrent nova which has been seen in outburst three times so far, with the most recent eruption occurring on 2019 August 27 UT. This latest outburst was followed in detail by the Neil Gehrels Swift Observatory, from less than a day after the eruption until the nova entered the Sun observing constraint, with a small number of additional observations after the constraint ended. The X-ray light-curve shows initial hard shock emission, followed by an early start of the super-soft source phase around day 8.5, with the soft emission ceasing by day 26. Together with the peak blackbody temperature of the super-soft spectrum being ~100 eV, these timings suggest the white dwarf mass to be high, ~1.3 M_sun. The UV photometric light-curve decays monotonically, with the decay rate changing a number of times, approximately simultaneously with variations in the X-ray emission. The UV grism spectra show both line and continuum emission, with emission lines of N, C, Mg and O being notable. These UV spectra are best dereddened using an SMC extinction law. Optical spectra from SMARTS show evidence of interaction between the nova ejecta and wind from the donor star, as well as the extended atmosphere of the red giant being flash-ionized by the super-soft X-ray photons. Data from NICER reveal a transient 83 s quasi-periodic oscillation, with a modulation amplitude of 5 per cent, adding to the sample of novae which show such short variabilities during their super-soft phase.
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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