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from Part II - Polarized emission in X-ray sources
Published online by Cambridge University Press: 06 July 2010
Active galactic nuclei (AGN) are definitely not spherical objects, but rather have an axisymmetric morphology. This, together with the fact that the primary X-ray emission is due to Comptonization, and that there are several circumnuclear regions which may scatter the primary radiation, implies that most, if not all, of the X-ray spectral components observed in radio-quiet AGN should be significantly polarized. Moreover, the polarization properties of the radiation emitted very close to the black hole are modified by Special and General Relativity effects, which therefore can be probed with X-ray polarimetry.
Introduction
AGN have a complex morphology (see Figure 18.1). The nucleus, where a super-massive (106–109M⊙) black hole accretes matter from an accretion disc, is surrounded by optically and geometrically thick matter, possibly clumpy and probably in a toroidal (or at least axisymmetric) configuration. If the line of sight intercepts this ‘torus’, the nucleus is not visible and the source is classified as of type 2. For type 1 sources, instead, the line of sight does not intercept the ‘torus’, and the nucleus can be directly seen. Outside the ‘torus’, ionized matter, responsible for scattering and polarizing the nuclear radiation in type 2 sources is present. A jet is also present in ‘radio-loud’ sources, which are not discussed in this contribution, devoted to ‘radio-quiet’ AGN.
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