Anisotropic inverse Compton emission

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Abstract
Inverse Compton scattering of soft X-ray photons by electrons is the most likely mechanism for the generation of the hard X-ray spectrum in compact objects. Recent observations of the X-ray spectra of active galactic nuclei and galactic black-hole candidates suggest that the hard X-ray emission originates above and below a soft X-ray emitting disc. The radiation field within the hard X-ray emitting region will therefore be highly anisotropic. Here we consider the angular distribution of the power and spectrum of Compton-scattered radiation in the case of an anisotropic distribution of soft photons. It is found that both the intensity and spectrum of the scattered radiation is a function of the Lorentz factor of the electrons, the geometry of the soft X-ray source and the viewing angle. For an isotropic distribution of relativistic electrons at the centre of a hemispherical ‘bowl’ of soft X-ray emission, the intensity of the hard X-rays scattered back into the bowl can be significantly greater (by a factor of up to 5.5) than that scattered towards the observer. This factor is increased as the solid angle of the soft X-ray source subtended at the location of the electrons is reduced. Our findings have important implications for models invoking a further ‘reflection’ of the resultant hard X-rays from the surface of a disc.