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AGN in the X-ray band
AGN consist of the following components:
AGN consist of the following components:
Supermassive Black Hole
Corona
Accretion Disk
Broad Line Region (BLR)
Narrow Line Region (NLR) Torus like stucture
Structure of an AGN. Credits: Ramos Almeida C. and Ricci C. 2017, Nature.
The central engine emits UV photons which interact with energetic electrons in the so-called corona (Nandra & Pounds 1994) producing the X-ray emission (e.g., Haardt & Maraschi 1993). This emission interacts with the circumnuclear dust and gas, producing the obscuration observed in the spectra. The dust emission can be observed in the infrared energy range, as a result of the thermalization of the UV photons by the dust. On the other hand, the gas will absorb and scatter the X-ray continuum producing the X-ray absorption, most noticeable at energies below 10 keV in the Xray spectrum (Brightman & Nandra 2011).
Obscuration gives evidence of material in the line of sight, which could be associated with the torus. Gas that is not in the line of sight of the observer can also imprint some features on the X-ray spectrum. Between 10 keV and up to hundreds of keV there is a reflection hump created by X-rays being reflected at the accretion disk or more distant material, like the torus.
Reprocessed X-ray radiation in AGN (Credits: Ricci 2011, PhD thesis).
The primary X-ray emission originates in a corona close to the accretion disc and is well represented by a power-law model (red dotted line in the spectra below). When this X-ray continuum is scattered by the surrounding gas, new features are imprinted in the spectrum, producing fluorescent emission lines, most notably Fe Kα 6.4 keV, and a broad hump-like continuum peaking around 10–30 keV (blue dashed line below). The relative strength of these two features is related to the column density of the scattering gas, while their overall flux is proportional to the gas covering fraction as seen from the central engine. Therefore, a careful analysis of the scattered X-ray spectrum can reveal the presence and properties of the obscuring torus even in unobscured AGN. Therefore, the X-ray spectrum is an useful tool to study the properties of obscuration in AGN because it gives information on the circumnuclear material even if it does not lie in the line of sight to the corona
Reprocessed X-ray radiation in AGN (Credits: Ricci 2011, PhD thesis).
It is not clear how the reflecting structure is formed, but clues can be gathered from the relation between reflection strength and the nuclear accretion rate. From the observational point of view, the torus in the infrared (IR) becomes weaker in the low luminosity regime (i.e., for low accretion rates - below 10^−3 , González-Martín et al. 2017).
My research focuses on modeling the dusty torus, with particular emphasis on the reflection components arising from both the torus itself and the polar distributions of dust and gas. My objective is to establish a robust connection between infrared observations, which trace dust, and X-ray data (gas).
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