The X-ray emitting regions in LMXBs: implications of ADC size measurements

We present results of measurements of ADC radial extent by the technique of dip ingress timing proving the extended nature of the ADC in LMXB of size varying between 20,000 and 700,000 km, a substantial fraction of the accretion disk radius. The ADC radial size r(ADC) varies with the 1 - 30 keV source luminosity as r(ADC) similar to L0.88+/-0.16. These results rule out the Eastern model for LMXB in which the Comptonizing region is seen as a small, central region, having size similar to1,000 times smaller than we measure. We present the Comptonized spectrum of an extended ADC having seed photons produced on the extended part of the disk below the ADC, and show that this differs markedly from that of the Eastern model. The results have implications for models of ADC formation, and support models of illumination of the disk by the central source. However, there is some evidence that the dependence on L reflects an ADC size determined by the maximum radius for hydrostatic equilibrium: the Compton radius.