A Strong Variation of Accretion Disc Corona Size with Luminosity in LMXB

Abstract

AbstractThe controversy that has existed for many years over the nature of the continuum X-ray emission components in X-ray binaries is reviewed, in which workers have been polarized between the Eastern model with a small central Comptonizing region around the compact object and alternative models. We present measurements of the radial extent of the Comptonizing ADC in low mass X-ray binaries which rule out the Eastern model and show that, the ADC is extended (the Birmingham model). Dip ingress timing shows conclusively that the ADC radial extent varies from 7% of the accretion disc radius in faint sources, to 65% in bright sources. Remarkably, the size depends strongly on the source luminosity suggesting that the ADC is formed by irradiation of the disc by the neutron star and the hot inner disc. These results have fundamental implications for the correct description of Comptonization in X-ray binaries, and the spectral form is derived for the Comptonized emission of an extended ADC led by soft, seed photons from the underlying disc which differs substantially from that of the Eastern model. Measured ADC electron températures provide values of the Compton radius in broad agreement with measured values of the radial extent of the ADC. Finally, we show that the results are inconsistent with the non-thermal emission being produced in a jet, and so provide evidence against the recent suggestion that all LMXB have jets.