We present an explanation of the Z-track phenomenon based on spectral fitting results of Rossi-XTE observations of the source GX 340+0 using the emission model previously shown to describe the dipping Low Mass X-ray Binaries. In our Z-track model, the soft apex is a quiescent state of the source with lowest luminosity. Moving away from this point by ascending the normal branch the strongly increasing luminosity of the Accretion Disc Corona (ADC) Comptonized emission L ADC provides substantial evidence for a large increase of mass accretion rateṀ. There are major changes in the neutron star blackbody emission, kT increasing to high values, the blackbody radius R BB decreasing, these changes continuing monotonically on both normal and horizontal branches. The blackbody flux increases by a factor of ten to three times the Eddington flux so that the physics of the horizontal branch is dominated by the high radiation pressure of the neutron star, which we propose disrupts the inner disc, and an increase of column density is detected. We further propose that the very strong radiation pressure is responsible for the launching of the jets detected in radio on the horizontal branch. On the flaring branch, we find that L ADC is constant, suggesting no change inṀ so that flaring must consist of unstable nuclear burning. At the soft apex, the mass accretion rate per unit area on the neutron starṁ is minimum for the horizontal and normal branches and about equal to the theoretical upper limit for unstable burning. Thus it is possible that unstable burning begins as soon as the source arrives at this position, the onset of unstable burning being consistent with theory. The large increase in R BB in flaring is reminiscent of radius expansion in X-ray bursts. Finally, in our model,Ṁ does not increase monotonically along the Z-track as often previously thought.

The explanation of the strong physical changes clearly taking place in the Z-track class of Low Mass X-ray Binaries has so far not been obtained, and this remains a significant astrophysical problem, without which we cannot claim to understand accretion in LMXB. We have for the first time applied the Birmingham emission model (2,3) to this problem to attempt to obtain a solution from the spectral evolution along the Z-track in the source GX 340+0 observed with Rossi- ХTE. In this model, X-ray emission consists of blackbody from the neutron star, plus Comptonized emission from an extended ADC.