Polarization modeling

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 polarimetric studies have a great potential to reveal physics and geometry of accretion discs in strong-gravity regime.

Accretion discs in central regions of active galactic nuclei are subject to strong external illumination originating from some kind coronal flares, giving rise to specific spectral features in the X-ray band. In particular, the K-shell lines of iron are found to be prominent around 6–7 keV. It has been shown that the shape of the intrinsic spectra must be further modified by the strong gravitational field of the central mass, and so X-ray spectroscopy could allow us to explore the innermost regions of accretion flows near supermassive black holes. Similar mechanisms operate also in 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.

References

  • Dovciak M., Karas V., Matt G. (2004), "Polarization signatures of strong gravity in active galactic nuclei accretion discs", Monthly Notices of the Royal Astronomical Society, 355, 1005-1009 [ads]
  • Goosmann R. W., Gaskell C. M. (2007), "Modeling optical and UV polarization of AGNs. I. Imprints of individual scattering regions", Astronomy and Astrophysics, 465, 129-145 [ads]
  • Horák J., Karas V. (2006), "On the role of strong gravity in polarization from scattering of light in relativistic flows", Monthly Notices of the Royal Astronomical Society, 365, 813-826 [ads]
  • Meyer L., Schoedel R., Eckart A., Karas V., Dovciak M., Duschl W. J. (2006), "K-band polarimetry of an Sgr A* flare with a clear sub-flare structure", Astronomy and Astrophysics, 458, L25-L28 [ads]