  Based on the results of applying the extended ADC emission model to three
Z-track sources: GX340+0, GX5-1 and CygX-2, we propose an explanation of the
Z-track sources in which the Normal and Horizontal Branches are dominated by
the increasing radiation pressure of the neutron star. The emitted flux becomes
several times super-Eddington at the Hard Apex and Horizontal Branch and we
suggest that the inner accretion disk is disrupted by this and that part of the
accretion flow is diverted vertically. This position on the Z-track is exactly
the position where radio emission is detected showing the presence of jets. We
thus propose that high radiation pressure is a necessary condition for the
launching of jets. We also show that flaring must consist of unstable nuclear
burning and that the mass accretion rate per unit emitting area of the neutron
star mdot at the onset of flaring agrees well with the critical theoretical
value at which burning becomes unstable.
