Anisotropic radiation from accretion disc-coronae in active galactic nuclei

TL;DR

This paper models anisotropic X-ray radiation from accretion disc-coronae in active galactic nuclei to explain observed differences in Fe K extalpha line luminosity between Seyfert 1 and Seyfert 2 galaxies based on viewing angle.

Contribution

It introduces a numerical accretion disc-corona model that accounts for anisotropic radiation transfer and explains observational differences in Seyfert galaxy emission lines.

Findings

X-ray intensity varies little at small angles, sensitive at large angles.

Disc emission follows cos heta dependence up to 40 degrees.

Model reproduces observed Fe K extalpha EW differences with inclination angles.

Abstract

In the unification scheme of active galactic nuclei (AGN), Seyfert 1s and Seyfert 2s are intrinsically same, but they are viewed at different angles. However, the Fe K\alpha emission line luminosity of Seyfert 1s was found in average to be about twice of that of Seyfert 2s at given X-ray continuum luminosity in the previous work (Ricci et al. 2014). We construct an accretion disc-corona model, in which a fraction of energy dissipated in the disc is extracted to heat the corona above the disc. The radiation transfer equation containing Compton scattering processes is an integro-differential equation, which is solved numerically for the corona with a parallel plane geometry. We find that the specific intensity of X-ray radiation from the corona changes little with the viewing angle \theta when \theta is small (nearly face-on), and it is sensitive to \theta if the viewing angle is large (\theta> 40 degrees). The radiation from the cold disc, mostly in infrared/optical/UV bands, is almost proportional to cos\theta when \theta< 40 degrees, while it decreases more rapidly than cos\theta when \theta> 40 degrees because of strong absorption in the corona in this case. For seyfert galaxies, the Fe K\alpha line may probably be emitted from the disc irradiated by the X-ray continuum emission. The observed equivalent width (EW) difference between Seyfert 1s and Seyfert 2s can be reproduced by our model calculations, provided Seyfert 1s are observed in nearly face-on direction and the average inclination angle of Seyfert 2s ~65 dgrees.