Abstract
Context. Although the disc instability model is widely accepted as the explanation for dwarf nova outbursts, it is still necessary to compare its predictions to observations because many of the constraints on angular momentum transport in accretion discs are derived from the application of this model to real systems. Aims. We test the predictions of the model concerning the multicolour time evolution of outbursts for two well-observed systems, SS Cyg and VW Hyi. Methods. We calculate the multicolour evolution of dwarf nova outbursts using the disc instability model and taking into account the contribution from the irradiated secondary, the white dwarf and the hot spot. Results. Observations definitely show the existence of a hysteresis in the optical colour–magnitude diagram during the evolution of dwarf nova outbursts. We find that the disc instability model naturally explains the existence and the orientation of this hysteresis. For the specific cases of SS Cyg and VW Hyi, the colour and magnitude ranges covered during the evolution of the system are in reasonable agreement with observations. However, the observed colours are bluer than observed near the peak of the outbursts, as in steady systems, and the amplitude of the hysteresis cycle is smaller than observed. The predicted colours significantly depend on the assumptions made for calculating the disc spectrum during rise, and on the magnitude of the secondary irradiation for the decaying part of the outburst. Conclusions. Improvements in the spectral disc models are strongly needed if the system evolution in the UV is to be addressed.
Highlights
Dwarf novae are a subclass of cataclysmic variables that undergo regular 4–6 mag outbursts lasting from one to a few days, with a typical recurrence time of a few weeks
The disc instability model (DIM) is based on the assumption that angular momentum transport in the disc can be described by an effective viscosity depending on a single parameter (Shakura & Sunyaev 1973), the DIM has been quite successful in explaining the overall properties of dwarf nova outbursts
Observations show that dwarf novae follow a characteristic loop in the (B − V, V) colour–magnitude diagram
Summary
Dwarf novae are a subclass of cataclysmic variables that undergo regular 4–6 mag outbursts lasting from one to a few days, with a typical recurrence time of a few weeks (see Warner 2003, for a review). Fitting the details of time-dependent spectra is unlikely to provide much information on dwarf novae; this conclusion is reinforced by the fact that our understanding of the spectra in the low state is very poor, despite some efforts in the past years (Idan et al 2010) For these reasons, and despite an early work by Cannizzo & Kenyon (1987), comparisons with observations have been limited to colour, and notably to the so-called UV delay at a time when it was believed that the time-lag between the rise in the UV and in the optical was an indicator of the inside-out (the outburst is triggered in the inner part of the disc) or outside-in nature of the dwarf nova eruption. The objective here is not to fit precisely the observational data, given the uncertainties and approximations inherent to the DIM, but rather to check if the general characteristics of the observations can be accounted for by the DIM
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