Modelling rebrightenings, reflares, and echoes in dwarf nova outbursts

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.

TACOS: TESS AM CVn Outbursts Survey

“…10, we note that they all have very similar (sharp) shapes, amplitudes and short duration (1.1 d). The similarity of that part of the light curve with the recently reported model by Hameury & Lasota (2021) for DNe of the type WZ Sge is remarkable. These authors explained such behaviour by invoking the EMT model.…”

“…It is important to note that in the LC of SDSS J1043+5632 there is no quiescent period between the several echoes. As occurs in the case of WZ Sge (Hameury & Lasota 2021), that behaviour suggests that the cooling front that propagates from the outer part of the disc towards the inner one is reflected by a heating front, thus making the disc hot. Some of the factors that would influence such behaviour are a hot and small WD (and hence massive) and a lack of truncation in the disc, in order to keep a small inner disc radius (Dubus, Hameury & Lasota 2001;Hameury & Lasota 2021).…”

“…The fact that the binary does not reach the original quiescent level after the SO, but instead it stays stuck at a higher flux, indicates changes in the mass transfer rate (see e.g. Hameury & Lasota 2021). All these characteristics are typical of the EMT model.…”

“…This could be explained due to an increase in the mass transfer rate in the DIM model (Hameury & Lasota 2021).…”

OGLE-BLG504.12.201843: A possible extreme dwarf nova

“…They could be much more short-lived and harder to observe in the case of normal outburst cycle time. However consecutive flares were observed after superoutbursts in WZ Sge systems (Kato 2015), which were successfully reproduced by Hameury & Lasota (2021) by adding several mechanisms to the DIM, such as variation of the mass transfer from the secondary, disc truncation and irradiation of the disc. In the case of O-201843, these flares could be small instabilities that fail to propagate far in the disc.…”