We present results of spectral investigations of the Sco X-1 like Z-track sources Sco X-1, GX 349+2 and GX 17+2 based on Rossi-XTE observations using an extended accretion disk corona model. The results are compared with previous results for the Cyg
X-2 like group: Cyg X-2, GX 340+0 and GX 5-1 and a general model for the Z-track sources proposed. On the normal branch, the Sco-like and Cyg-like sources are similar, the results indicating an increase of mass accretion rate Mdot between soft and hard apex, not as in the standard view that this increases around the Z. In the Cyg-like sources, increasing Mdot causes the neutron star temperature kT to increase from ~1 to ~2 keV. At the lower kT, the radiation pressure is small, but at the higher kT, the emitted flux of the neutron star is several times super-Eddington and the high radiation pressure disrupts the inner disk launching the relativistic jets observed on the upper normal and horizontal branches. In the Sco-like sources, the main physical difference is the high kT of more than 2 keV on all parts of the Z-track suggesting that jets are always possible, even on the flaring branch. The flaring branch in the Cyg-like sources is associated with release of energy on the neutron star consistent with unstable nuclear burning. The Sco-like sources are very different as flaring appears to be a combination of unstable burning and an increase of Mdot which makes flaring much stronger. Analysis of 15 years or RXTE ASM data on all 6 classic Z-track sources shows the high rate and strength of flaring in the Sco-like sources suggesting that continual release of energy heats the neutron star causing the high kT. A Sco X-1 observation with unusually little flaring supports this. GX 17+2 appears to be transitional between the Cyg and Sco-like types. Our results do not support the suggestion that Cyg or Sco-like nature is determined by luminosity.
We review the longterm confusion which has existed over the nature of flaring in the brightest class of low mass X-ray binary: the Z-track sources, specifically in the Cygnus X-2 sub-group. Intensity reductions in the lightcurve produce a branch in c
olour -colour diagrams similar to that of real flares in the Sco X-1 like group, and the nature of this branch was not clear. However, based on observations of Cygnus X-2 in which this dipping/flaring occurred it was proposed that the mass accretion rate in Z-track sources in general increases monotonically along the Z-track towards the Flaring Branch, a standard assumption widely held. It was also suggested that the Cygnus X-2 group have high inclination. Based on recent multi-wavelength observations of Cygnus X-2 we resolve these issues, showing by spectral analysis that the Dipping Branch consists of absorption events in the outer disk, unrelated to the occasional real flaring in the source. Thus motivation for Mdot increasing along the Z from Horizontal - Normal to Flaring Branch is removed, as is the idea that high inclination distinguishes the Cygnus X-2 group. Finally, the observations provide further evidence for the extended nature of the Accretion Disk Corona (ADC), and the correct modelling of the ADC Comptonized emission is crucial to the interpretation of low mass X-ray binary data.
We present results of the Suzaku observation of the dipping, periodically bursting low mass X-ray binary XB 1323-619 in which we concentrate of the spectral evolution in dipping in the energy range 0.8 - 70 keV. It is shown that spectral evolution in
dipping is well-described by absorption on the bulge in the outer accretion disk of two continuum components: emission of the neutron star plus the dominant, extended Comptonized emission of the accretion disk corona (ADC). This model is further supported by detection of a relatively small, energy-independent decrease of flux above 20 keV due to Thomson scattering. It is shown that this is consistent with the electron scattering expected of the bulge plasma. We address the recent proposal that the dip sources may be explained by an ionized absorber model giving a number of physical arguments against this model. In particular, that model is inconsistent with the extended nature of the ADC for which the evidence is now overwhelming.
