Abstract
Validating our understanding of the innermost structure of Active Galactic Nuclei (AGN) would require resolving sub-parsec scales. Lacking adequate direct imaging, X-ray astronomy can still contribute to this undertaking through the study of spectral variability on time-scales from days to years. This bears information on the location of gaseous and dusty systems in the innermost regions around the accreting supermassive black hole. In this paper I discuss the application of this concept in two specific contexts: a) fast column density variations in heavily obscured AGN; b) reverberation of optically-thick reprocessing. These results lend support to a scenario where obscuration and optically thick reprocessing are due to a variety of different systems, ranging from the Broad Line Regions to a clumpy structure extended on larger scales up to hundreds of parsecs.
Highlights
Validating our understanding of the innermost structure of Active Galactic Nuclei (AGN) requires resolving subparsec scales
In the three obscured AGN for which such measurements have been possible so far (NGC 1068, the Circinus Galaxy, and NGC 4151), the data are consistent with a two-temperature distribution of dust, with most the of the obscuration occurring on scales smaller than 1 pc [6, 14, 44]
In Type 1 AGN, near-IR studies [15, 16] indicate that the size of the innermost torus is primarily driven by the ∝ L1/2 dust sublimation radius
Summary
Validating our understanding of the innermost structure of Active Galactic Nuclei (AGN) requires resolving subparsec scales. In Type 1 AGN, near-IR studies [15, 16] indicate that the size of the innermost torus is primarily driven by the ∝ L1/2 dust sublimation radius Once this dependence is removed, the dusty structure is more compact in more luminous objects. Reprocessing of the AGN high-energy primary emission These methods rely on measuring either the smallest possible variability timescales of the column density in heavily obscured AGN (§2), or the response to changes of the primary continuum in spectral regions dominated by reprocessing (§3). In a few cases, extended emission on the arcsecond ( 100 pc) scale allows direct imaging of the X-ray torus, as well as of the X-ray Extended Narrow Line Regions The latter system will not be discussed in this paper. Readers are referred to a vast recent literature on this field [2, 40, 45]
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