Demystifying the powering mechanism of low-luminosity active galactic nuclei



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Universidad de Valparaíso



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Instituto de Fisica y Astronomia




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Despite the long history of studies of active galactic nuclei (AGN), details on the structure of the accretion mechanism are far from clear. The work presented in this thesis is directed at unveiling properties of the structures in AGN through broad- band X-ray spectroscopy, with particular emphasis on broad X-ray band using XMM- Newton+NuSTAR+Swift (0.5–110.0 keV) data. With its unprecedented spectral sensi- tivity in this energy band, the combination of these telescopes provides the key obser- vational diagnostics of the properties of the AGN X-ray source, the corona, and the surrounding gas in the accretion disk and the torus. In this thesis we focused on the study of of Low-luminosity AGN (LLAGN), aiming at disentangle their inner structure, with emphasis on the reflection and the accretion mechanism. The first part of the document is centered on the develop- ment of the methodology and its application to one particular LLAGN, NGC 3718 (L/LEdd ∼ 10−5). Developing a methodology to study properties of potential reflec- tors and explore the improvement in the spectral index estimation considering high energy data and torus or disk like reflection models. We found that the inclusion of these models and high energy data allows to place strong constraints on the geometry and physical features of the surroundings as well to obtain more restricted values of a torus and a disk reflector. Then we applied this methodology to a sample of 17 LLAGN from BASS/DR2 combining observations from XMM-Newton, NuSTAR and Swift and applying the same methodology that was previously developed in the first work in NGC 3718. From the analysis of the X-ray data of the AGN sample, we found a relation which is indicative of a change in the column density of the torus like reflector at lower ac- cretion rates. Also, we confirm an anti-correlation between Γ-Eddington ratio with smaller scatter than previously reported, thanks to the inclusion of high energy data and the reflection models. The change in the correlation Γ-λEdd at ∼10−3 that we found is in agreement with a different accretion mechanism compared with higher accretion AGN as was previously reported in the literature.


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