Abstract Based on the results of applying the extended ADC emission model to three Z-track sources: GX 340+0, GX 5-1 and Cyg X-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 dot m at the onset of flaring agrees well with the critical theoretical value at which burning becomes unstable.

Keywords physical data and processes: acceleration of particles physical data and processes: accretion: accretion disks stars: neutron stars: individual: GX 340+0 GX 5-1 Cyg X-2 X-rays: bina ries

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