An explanation of the Z-track sources

Abstract

We present an explanation of the Z-track phenomenon based on spectral fitting of RXTE observations of GX340+0 using the emission model previously shown to describe the dipping LMXB. 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 rate Mdot. 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 in Mdot so that flaring must consist of unstable nuclear burning. At the soft apex, the mass accretion rate per unit area on the neutron star m_dot 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 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, Mdot does not increase monotonically along the Z-track as often previously thought.