  We present a new model for the Z-track phenomenon, based on analysis of the
spectral evolution around the Z-track in several Z-track sources, in which
radiation pressure plays a major role. Increasing mass accretion rate on the
normal branch causes heating of the neutron star with the emissive flux from
the surface increasing by an order of magnitude to become super-Eddington at
the horizontal branch where radio detection shows the presence of jets. We
propose that the radiation pressure disrupts the inner disk leading to the
launching of the jets. Secondly, by timing analysis of the same data we find a
correlation of the frequency of kHz QPO with the emissive flux and propose that
the higher frequency QPO is an oscillation at the inner disk edge which
progressively moves to larger radial positions as the disk is disrupted by
radiation pressure.
