On the nature of the Cygnus X-2 like Z-track sources

¹ School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK e-mail: mbc@star.sr.bham.ac.uk
² Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Cracow, Poland

Received: 28 August 2009
Accepted: 31 December 2009

Abstract

Based on the results of applying the extended ADC emission model for low mass X-ray binaries to three Z-track sources: GX 340+0, GX 5-1 and Cyg X-2, we propose an explanation of the Cygnus X-2 like Z-track sources. The Normal Branch is dominated by the increasing radiation pressure of the neutron star caused by a mass accretion rate that increases between the soft apex and the hard apex. The radiation pressure continues to increase on the Horizontal Branch becoming several times super-Eddington. We suggest that this disrupts the inner accretion disk and that part of the accretion flow is diverted vertically forming jets which are detected by their radio emission on this part of the Z-track. We thus propose that high radiation pressure is the necessary condition for the launching of jets. On the Flaring Branch there is a large increase in the neutron star blackbody luminosity at constant mass accretion rate indicating an additional energy source on the neutron star. We find that there is good agreement between the mass accretion rate per unit emitting area of the neutron star at the onset of flaring and the theoretical critical value at which burning becomes unstable. We thus propose that flaring in the Cygnus X-2 like sources consists of unstable nuclear burning. Correlation of measurements of kilohertz QPO frequencies in all three sources with spectral fitting results leads to the proposal that the upper kHz QPO is an oscillation always taking place at the inner accretion disk edge, the radius of which increases due to disruption of the disk by the high radiation pressure of the neutron star.