A review of the Z-track sources

TL;DR

This review synthesizes models of Z-track and Atoll low mass X-ray binary sources, linking their spectral states to accretion processes, nuclear burning, and corona heating, proposing a unified understanding of these systems.

Contribution

It presents a comprehensive physical model unifying Z-track and Atoll sources, explaining their spectral states through accretion rate variations and nuclear burning processes.

Findings

Normal Branch linked to increasing mass accretion rate and neutron star heating.

Flaring Branch caused by unstable nuclear burning on the neutron star.

Spectral hardening at low luminosity indicates corona heating by an unknown process.

Abstract

The brightest class of low mass X-ray binary source: the Z-track sources are reviewed specifically with regard to the nature of the three distinct states of the sources. A physical model is presented for the Cygnus X-2 sub-group in which increasing mass accretion rate takes place on the Normal Branch resulting in high neutron star temperature and radiation pressure responsible for inner disk disruption and launching of jets. The Flaring Branch consists of unstable nuclear burning on the neutron star. It is shown that the Sco X-1 like sub-group is dominated by almost non-stop flaring consisting of both unstable burning and increase of Mdot, causing higher neutron star temperatures. Finally, results of Atoll source surveys are presented and a model for the nature of the Banana and Island states in these sources is proposed. Motion along the Banana state is caused by variation of Mdot. Measurements of the high energy cut-off of the Comptonized emission E_CO provide the electron temperature T_e of the Comptonizing ADC; above a luminosity of 2x10^37 erg/s E_CO is a few keV and T_e equals the neutron star temperature. At lower luminosities, the cut-off energy rises towards 100 keV showing heating of the corona by an unknown process. This spectral hardening is the cause of the Island state of Atoll sources. The models for Z-track and Atoll sources thus constitute a unified model for low mass X-ray binary sources.