Outburst period–energy relations in cataclysmic novae

Abstract

SS Cygni is by far the best studied dwarf nova; since its discovery its brightness variations have been monitored almost continuously, mainly by members of the American Association of Variable Star Observers (AAVSO), the Variable Star Section of the British Astronomical Association and L'Association Francais D'Observateurs D'Etoiles Variables (see Fig. 1). More than 600 outbursts have been observed to date. The eruption cycle of the dwarf nova SS Cygni behaves as a relaxation oscillator, with outburst energy correlated with the following quiescent waiting period. Two distinct, wide and narrow outbursts exist. The outburst behaviour is closely related to outburst rise-time, and to the quiescent brightness level. There is a strong tendency for low-amplitude, slow rise-time outbursts to be clustered in periods associated with bright quiescent levels. We show here that the behaviour of SS Cygni is in close agreement with theoretical predictions of dynamically unstable states of the red star when it fills the Roche-lobe, and is in conflict with the assumptions of early disk instability models.