Abstract
Context. The disc instability model (DIM) 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. Aims. We determine the additional ingredients that must be added to the DIM to account for the observed rebrightenings. Methods. We analyse in detail a recently discovered system, TCP J21040470+4631129, which has shown very peculiar rebrightenings. We also model the light curve of this system 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. Results. We show that the luminosity in quiescence is dominated by a hot white dwarf that cools down on timescales of months. For a reason that remains elusive, the mass transfer rate from the secondary has to increase by several orders of magnitudes during the initial superoutburst. The mass transfer rate slowly returns to its secular average and causes the observed succession of outbursts with increasing quiescence durations until the disc can be steady, cold, and neutral; its inner parts are 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 also produced light curves close to those observed in two WZ Sge stars, the prototype and EG Cnc. Conclusions. 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.
Highlights
Dwarf novae (DNe) are cataclysmic variables undergoing regular outbursts, that are well explained by the disc instability model (DIM)
We have shown that the very unusual light curve of TCP J2104 can be explained by the DIM provided that its standard version is enriched by the inclusion of several ingredients related to timevariations of the parameters entering the DIM
The mass transfer rate from the secondary is increased during the main outburst to values comparable to the mass accretion rate and exponentially decreases with time, remaining high for a few months before returning to its low quiescence value
Summary
Dwarf novae (DNe) are cataclysmic variables undergoing regular outbursts (see Warner 2003, for a full review of these systems), that are well explained by the disc instability model (DIM). Several suggestions have been made in the past to explain rebrightenings, a detailed list of which can be found in Kato (2015) These include a temporary increase of the mass transfer rate as a consequence of irradiation of the secondary (Augusteijn et al 1993; Hameury et al 2000); an exponential decay of the viscosity in the disc between rebrightening accompanied by a partial revival of the viscous processes during these rebrightenings (Osaki et al 2001); the existence of a mass reservoir beyond the 3:1 resonance radius (Uemura et al 2008); and the decoupling of the tidal and thermal instability that might occur in systems with a low mass ratio (Hellier 2001). This indicates that the velocity of the cooling front is the same for a given magnitude, or, equivalently for a given position of the front. Since different physical phenomena dominate or characterize each phase, we first discuss separately the models of each phase and in Sect. 3.4 we present the synthesis modelling the full outburst light curve
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