Mechanisms for dwarf nova outbursts and soft X-ray transients - A critical review

I review models that try to explain dwarf nova outbursts and soft X-ray transients. The disc instability model for dwarf novae is still in a preliminary state of development: its predictions depend very strongly on the unknown viscosity mechanism. It is also doubtful that a pure disc instability phenomenon will be able to describe 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 mass transfer model, incorporating illumination of the secondary, cannot describe correctly the time scales of SXT events for main-sequence secondaries with masses less than 1 M.. The existence of at least three systems with P-orb < 10 hr seems to rule it out as an explanation of the SXT phenomenon.