The emission regions in X-ray binaries: dipping as a diagnostic

Abstract

X-ray dipping in the black hole binary Cygnus X-1, the Galactic jet source GRO J1655-40 and in low mass X-ray binaries is discussed. It is shown that spectral analysis strongly constrains emission models. Measurement of dip ingress/egress times allows the sizes of extended emission regions to be determined, notably for the Accretion Disk Corona which is responsible for Comptonization in X-ray binaries. In LMXB, the radius of the ADC is shown to be between ~ 10^9 and ~ 5x10^10 cm, an appreciable fraction of the accretion disk radius. This is inconsistent with Comptonization models requiring a localized Comptonizing region, for example, in the immediate neighbourhood of the neutron star. Results from a survey of LMXB using ASCA and BeppoSAX reveal an approximate equality between the height of the blackbody emission region on the neutron star and the height of the inner radiatively-supported disk, suggesting either that there is a direct causal link, such as a radial accretion flow between the inner disk edge and the star, or an indirect link, as in the case of accretion flow creep on the surface of the neutron star as suggested by Inogamov & Sunyaev. Finally, the survey shows that the blackbody cannot originate on the accretion disk as the required inner radii in many sources are substantially less than the neutron star radius.