We develop models for the polarization signature of the radiation coming from the vicinity of accreting black holes. In the optical/UV range, polarimetry observations and modeling have already proven to be a very useful tool to investigate active galaxies by tracing geometrical and dynamical properties of structures surrounding their nuclei.In future, time-resolved X-ray polarimetricstudies have a great potential to reveal physics and geometry of accretion discs in strong-gravity regime.
Accretion discs in central regions of active galacticnuclei are subject to strong external illumination originatingfrom some kind coronal flares, giving rise to specificspectral features in the X-ray band. In particular,the K-shell lines of iron are found to be prominentaround 6–7 keV. It has been shown that the shape ofthe intrinsic spectra must be further modified by thestrong gravitational field of the central mass, and soX-ray spectroscopy could allow us to explore the innermostregions of accretion flows near supermassiveblack holes. Similar mechanisms operate alsoin some Galactic black-hole candidates.
In our own Galactic Centre, recent polarimetric measurements showed frequent flares with a degree of linear polarization of up to 20%. These flares come along with a swing in the polarization angle. The new information provided by the polarization state is extremely useful because it breaks the degeneracy of various model parameters. We demonstrate that the available data are consistent with an orbiting spot model and that they offer a new avenue to constrain the black hole parameters in the future.
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