We present an explanation of the Z-track phenomenon based on spectral fitting results of Rossi-XTE observations of the source GX 340+0 using the emission model previously shown to describe the dipping Low Mass X-ray Binaries. In our Z-track model, the soft apex is a quiescent state of the source with lowest luminosity. Moving away from this point by ascending the normal branch the strongly increasing luminosity of the Accretion Disc Corona (ADC) Comptonized emission L ADC provides substantial evidence for a large increase of mass accretion rateṀ. There are major changes in the neutron star blackbody emission, kT increasing to high values, the blackbody radius R BB decreasing, these changes continuing monotonically on both normal and horizontal branches. The blackbody flux increases by a factor of ten to three times the Eddington flux so that the physics of the horizontal branch is dominated by the high radiation pressure of the neutron star, which we propose disrupts the inner disc, and an increase of column density is detected. We further propose that the very strong radiation pressure is responsible for the launching of the jets detected in radio on the horizontal branch. On the flaring branch, we find that L ADC is constant, suggesting no change inṀ so that flaring must consist of unstable nuclear burning. At the soft apex, the mass accretion rate per unit area on the neutron starṁ is minimum for the horizontal and normal branches and about equal to the theoretical upper limit for unstable burning. Thus it is possible that unstable burning begins as soon as the source arrives at this position, the onset of unstable burning being consistent with theory. The large increase in R BB in flaring is reminiscent of radius expansion in X-ray bursts. Finally, in our model,Ṁ does not increase monotonically along the Z-track as often previously thought.

We report results of analysis of the ASCA observation of the Low Mass X-ray Binary dipping source XB 1916-053 made on 1993, May 2nd, during which dipping was very deep such that in the deepest parts of dips, the X-ray intensity in the band 0.5 -12.0 keV fell to zero, demonstrating that all emission components were completely removed. The best-fit orbital period of the binary system determined from the X-ray data was found to be 3005 ± 10 s. The high quality ASCA data allowed spectral evolution in dipping to be systematically investigated by spectral analysis in intensity bands covering the full range of dipping from intensities close to zero to non-dip values. We have shown that the spectra can be well fitted by the same two-component model previously used to give good explanations of the very different dip sources X 1755-338 and X 1624-490, consisting of point-source blackbody emission from the neutron star plus extended Comptonised emission probably from the accretion disk corona. In the case of XB 1916-053 we show that all levels of dipping can be fitted using kT bb = 2.14 ± 0.28 keV and power law photon index = 2.42 ± 0.21 which are the best-fit values for non-dip data, together with the corresponding non-dip normalisations. Dipping is shown to be due to large increases of column density for the point-like blackbody, combined with the extended power law component being progressively covered by absorber until in the deepest parts of dips, the partial covering fraction approaches unity. This approach differs radically from the "absorbed plus unabsorbed" approach previously used in spectral modelling of XB 1916-053 and similar sources, in which the normalisation of the unabsorbed component is allowed to decrease markedly in dipping, behavior generally attributed to the effects of electron scattering. Thus we have shown that spectral evolution in XB 1916-053 can be explained simply in terms of photoelectric absorption without the need for substantial electron scattering. This explanation is supported by calculation of the relative importance of photoelectric absorption and electron scattering in the absorbing region which shows that little electron scattering is expected in the ASCA energy band.

We present results of measurements of the radial extent of the accretion disc corona in low mass X-ray binaries, i.e. of the radial extent of the thin, hot ADC above the accretion disc. These results prove conclusively the extended nature of the ADC, with radial extent varying from 20,000 km in the faintest sources to 700,000 km in the brightest, a substantial fraction of the accretion disc radius, typically 15 per cent. This result rules out the Eastern model for LMXB which is extensively used, in which the Comptonizing region is a small central region. The ADC size depends strongly on the 1 -30 keV source luminosity via a simple relationship r ADC = L 0.88±0.16 at 99 per cent confidence, which is close to a simple dependence r ADC ∝ L. We also present limited evidence that the ADC size agrees with the Compton radius r C , or maximum radius for hydrostatic equilibrium. Thus, the results are consistent with models in which an extended ADC is formed by illumination of the disc by the central source. However, the dependence on luminosity may reflect the known decrease of coronal temperature as the source luminosity increases leading to an increase of r C . The extended nature of the ADC means that the seed photons for Comptonization must consist of emission from the disc to the same radial extent as the corona, providing copious supplies of soft seed photons. We thus demonstrate the importance of the size of the ADC to the correct description of Comptonization, and we derive the Comptonized spectrum of a LMXB based on the thermal Comptonization of these seed photons and show that this differs fundamentally from that of the Eastern model, which assumes a cut-off in the spectrum below 1 keV. Finally, we argue that our results are inconsistent with the assumption often made that the X-ray emission of accreting Black Holes and Neutron Stars has a common mechanism depending on the properties of the accretion flow only.

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