Mechanisms for Dwarf Nova Outbursts and Soft X-Ray Transients

Abstract

I review models trying to explain dwarf-nova outbursts and soft X-ray transients. The disc-instability model for dwarf-novae is still in its preliminary state of development: its predictions depend very strongly on the unknown viscosity mechanism. It is also doubtful that a {\sl pure} disc-instability phenomenon will be able to describe {\sl all} types of dwarf-nova outbursts, in particular superoutbursts. The disc-instability model for SXTs suffers from the same difficulties but in addition its predictions are contradicted by observations of transient sources in quiescence. The illuminated mass-transfer model cannot describe correctly the time-scales of SXT events for main-sequence secondaries with masses less than 1$M_{\odot}$. The existence of at least three systems with $P_{\rm orb} < 10$ hr seems to rule it out as an explanation of the SXT phenomenon.