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AGN in the X-ray band

A typical AGN can be imagined as having several components:


An accretion disk where matter is funnelled onto the SMBH.


A broad line region (BLR) where the broad and optical/UV lines are produced.

A dusty torus, which is located within few

parsecs from the SMBH.

A narrow line region (NLR), which is located at ~100s pc from the SMBH, where the narrow optical lines are created.


Typical structure (not on scale) of an AGN (Credits: Ricci 2011, PhD thesis).

Primary X-ray radiation
X-ray emission is ubiquitous in AGN, and is thought to be produced by Comptonization of optical/UV disk photons by a corona of hot electrons (see figure) 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.


Schematic representation of X-ray production in AGN (Credits: Ricci 2011, PhD thesis).

Reflected X-ray radiation
Part of the primary X-ray emission is reprocessed by the 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).


Reprocessed X-ray radiation in AGN (Credits: Ricci 2011, PhD thesis).


Typical X-ray spectrum of an unobscured AGN (Credits: Ricci 2011, PhD thesis).

Absorption in the X-rays

Absorption affects the X-ray emission through two processes: photoelectric absorption and Compton scattering. Photoelectric absorption starts being effective at column densities of NH~1e21 cm^-2, and is strongly energy-dependent. Compton scattering becomes significant above Nh~1e24 cm^-2 and does not have a significant dependence on energy below ~100 keV.

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

The effects of X-ray obscuration on the X-ray spectra of AGN (Credits: Ricci 2011, PhD thesis).

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