AGN in the X-ray band

Primary X-ray radiation
X-ray emission is ubiquitous in AGN, and is believed to be produced by Comptonization of optical/UV disk photons by a corona of hot electrons (Fig. 1) located above the SMBH.
This Comptonization produces a power-law emission, with a photon index typically of Γ~1.8-2. The power-law continuum often shows a high-energy roll-over, which is usually located around few hundreds keV. This feature is directly related to the temperature and optical depth of the plasma of hot electrons responsible for the power-law emission.

Fig. 1 Schematic representation of X-ray production in AGN.


Reflected X-ray radiation
Part of the primary X-ray emission is reprocessed by the molecular torus, the BLR and the disk, giving rise to reflection features.
This reprocessed radiation consists mainly of two features: a Compton hump peaking around 30-40 keV, and an iron Kα emission line at 6.4 keV. While the Compton hump is produced only if the reprocessing material is Compton thick (CT, Nh > 1e24 cm^−2), the iron Kα line can be produced also by Compton-thin material. The iron Kα line is often observed as the superposition of two components: a broad and a narrow one. While the broad component might arise from reflection from the inner part of the accretion flow, the narrow component is likely produced by material located further away (i.e. in the BLR or in the torus).

Fig. 2 Reprocessing of X-ray radiation in AGN

A large fraction of AGN show an excess over the power-law continuum below ~2 keV. This Soft excess has a different
origin for absorbed and unabsorbed AGN. In the case of unabsorbed objects, several interpretations have been given:
blurred reflection from an ionized disk, smeared absorption by an ionized wind, or a second cooler corona

Fig. 3 The typical 0.1-1000 keV spectrum of an unabsorbed AGN
High resolution image (1.2 MB)


Absorption affects the X-ray emission through two processes: photoelectric absorption and Compton scattering. Photoelectric absorption starts being effective already at column densities of NH~1e21cm^-2, is energy-dependent and does not play a significant role above 10 keV for NH<1e24cm^-2. Compton scattering becomes significant above Nh~1e+24 and does not have a significant dependence on energy below ~150 keV.

Check out this video to see the effects of absorption on the X-ray spectra of AGN for different values of the column density NH.