  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.
