PhD Thesis Project

“Comptonization mechanisms in hot coronae in AGN. The NuSTAR view.”

NGC 1068 as seen by Hubble Space Telescope (HST) with an artistic impression zoom-in on the central active galactic nucleus. Image credit: NASAESA/JPL-Caltech.  

My thesis project was based mainly on the study of the X-ray emission spectrum of Active Galactic Nuclei (AGN), to constrain the coronal parameters of AGN through a detailed spectral analysis of high-quality X-rays data. In this project I looked for correlations between these parameters and other physical parameters, such as the geometry and the position of the emitting corona, with the aim of better understanding the complex environment that can be found in AGN. The approach of this work was based both on new observations of NuSTARXMM-Newton and Swift X-ray satellites and on archival data. The detailed analysis of single sources allows to built and constraints physical models while the analysis of a large sample give us insights into the average properties of AGN. I present the data analysis of three sources, namely GRS 1734-292, MCG +8-11-11 and NGC 6814. These three sources are nearby X-ray-bright Seyfert 1 galaxies. GRS 1734-292 is a Seyfert 1 galaxy, located near the Galactic plane. It shows one of the lowest high energy cutoff measurements made up so far by NuSTAR. The results of the analysis are presented in Tortosa et al., 2017. MCG +8-11-11 and NGC 6814 are two bright Seyfert 1 galaxies that show very similar coronal properties even if they had different properties overall (black hole masses, luminosity and Eddington ratios). The analysis of the data of these sources is presented in Tortosa et al., 2018a. After the analysis of single sources I discussed a project based on the analysis of a small catalogue of AGN we build up choosing the unobscured nearby, non-jetted, Seyfert galaxies that have been observed by NuSTAR (often in coordination with XMM-Newton, Suzaku or Swift) present on literature. The aim of the project, presented in Tortosa et al., 2018b,  is to look for correlations between coronal spectral parameters, i.e. the photon index and cut-off energy, and physical parameters, i.e. the optical depth and coronal temperature, (when a Comptonization model is used), and other parameters of the systems, such as the black hole mass or the Eddington ratio. To this end we analysed the coronal parameters of the selected sample, founding an anti-correlation with a significance level >98% between the coronal optical depth and the coronal temperature of the sources from our sample. On the other hand, no correlation between the above parameters and the black hole mass, the accretion rate, and the intrinsic spectral slope of the sources is found.

Fit (red line) and error on the fitting relation (red shaded region) of the optical depth versus the electron coronal temperature in the case of a disc shape corona (left) and of a a spherical corona.(right), from Tortosa et al., 2018b.

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