  We present results of a combined investigation of the spectral and kHz QPO
evolution around the Z-track in GX 5-1 based on high-quality RXTE data. The
Extended ADC emission model provides very good fits to the spectra, the results
pointing clearly to a model for the nature of the Z-track, in agreement with
previous results for the similar source GX 340+0. In this model, at the soft
apex of the Z-track, the mass accretion rate Mdot is minimum and the neutron
star has its lowest temperature; but as the source moves along the normal
branch, the luminosity of the Comptonized emission increases, indicating that
Mdot increases and the neutron star gets hotter. The measured flux f of the
neutron star emission increases by a factor of ten becoming super-Eddington,
and we propose that this disrupts the inner disk so forming jets. In flaring,
the luminosity of the dominant Comptonized emission from the ADC is constant,
while the neutron star emission increases, and we propose for the first time
that flaring consists of unstable nuclear burning on the neutron star, and the
measured mass accretion rate per unit area mdot at the onset of flaring agrees
well with the theoretical critical value at which burning becomes unstable.
There is a striking correlation between the frequencies of the kHz QPO and the
ratio of the flux to the Eddington value: f/f_Edd, suggesting an explanation of
the higher frequency QPO and of its variation along the Z-track. It is well
known that a Keplerian orbit in the disk at this frequency corresponds to a
position some distance from the neutron star; we propose that the oscillation
always occurs at the inner disk edge, which moves radially outwards on the
upper normal and horizontal branches as the measured increasing radiation
pressure increasingly disrupts the inner disk.
