A model for the Z-track phenomenon, jet formation and the kilohertz QPO based on Rossi-XTE observations of the Z-track sources

Abstract

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.