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.Comment: solicited review paper presented at COSPAR 2000 "X-ray and Gamma-ray
Signatures of Black Holes and Weakly Magnetized Neutron Stars"; accepted for
publication in Advances in Space Research (2nd affiliation added
