A model for the Z-track sources based on spectral evolution along the Z-track

Abstract

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. On the Normal Branch we propose that the mass accretion rate Ṁ increases strongly as shown by the increasing luminosity of the ADC Comptonized emission. On the Horizontal Branch, this luminosity then falls suggesting a return of Ṁ to lower values. There are major changes in the neutron star blackbody emission with kT increasing to high values, while the blackbody radius decreases, these changes continuing monotonically on both Normal and Horizontal Branches. We propose that the NB and HB are dominated by radiation pressure of the blackbody, the emitted flux on the neutron star surface rising to twice the Eddington value, which disrupts the inner disc and we suggest a mechanism for how this produces the observed reduction of emitting area on the neutron star. A measured increase of column density on the NB and HB provides direct evidence for the disruption. We propose that the Flaring Branch comprises unstable thermonuclear burning since the constancy of the Comptonized emission luminosity rules out a change of Ṁ. Thus in our model, Ṁ does not increase monotonically along the Z‐track as previously thought. The large increase in blackbody radius on the FB is reminiscent of radius expansion in X‐ray bursts. Finally, we propose that the very strong radiation pressure on NB and HB is responsible for the launching of the jets detected in radio. Jets are not detected on the FB as the opening in the disc above the neutron star is blocked by its expanding atmosphere.