Modelling hystereses observed during dwarf nova outbursts

Abstract

Context. Although the disc instability model is widely accepted as the explanation for dwarf nova outbursts, it is still necessary to confront its predictions to observations because much 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 of the spectral disc models are strongly needed if one wishes to address the system evolution in the UV.

TACOS: TESS AM CVn Outbursts Survey

“…After the SO, the binary becomes redder and fainter reaching colours similar to those before the superoutburst. This behaviour is similar to dwarf novae outbursts and predicted by the DIM (Hameury et al 2020).…”

“…10. The initial colour behaviour of SDSS J1043+5632 is similar to that described by Hameury et al (2020) for DNe, where the binary initially turns redder and brighter, because despite the fact that the accretion disc is becoming hotter, the contribution of the hotspot is larger. Then, as the SO occurs the system becomes bluer and brighter.…”

Modelling rebrightenings, reflares, and echoes in dwarf nova outbursts

“…Truncation is required to avoid small outbursts (about 1 mag) and also to enable the system to be stable on the cold branch after 220 d. The truncation radius is of the order of 3 × 10 9 cm during quiescence, which is comparable to what has been observed in other systems. We include contributions from the accretion disc; the white dwarf, assuming a black-body spectrum; and from the hot spot, assuming a black-body spectrum with a temperature of 10 4 K, where the hot spot luminosity is determined as in Hameury et al (2020). For simplicity, we add this contribution even when the stream overflows the disc; in this case, the interaction between the stream and the disc still results in additional optical light, but the emitted spectrum is certainly different from that of the hot spot.…”

Evidence that short-period AM CVn systems are diverse in outburst behaviour

“…Normal outbursts are far shorter events lasting between 1 and 5 d, with a peak brightness typically 1 mag dimmer than their SO counterparts (Cannizzo et al 2012). SO generally have more complex profiles consisting of a sudden increase in brightness that is bluer in colour (when compared to the system in quiescence; Hameury et al 2020), which gradually decreases over the duration of the outburst. Regardless of the duration of the SO, they often exhibit a small dip in brightness soon after maximum brightness after which the brightness can increase again (Ramsay et al 2012).…”