Investigating the nuclear properties of highly accreting active galactic nuclei with XMM-Newton

TL;DR

This study analyzes the X-ray spectral and variability properties of a large sample of highly accreting AGNs, revealing complex spectral behavior and correlations with accretion rates, and exploring their evolution and soft excess features.

Contribution

It presents the first systematic X-ray spectral analysis of a large, diverse sample of highly accreting AGNs, including variability and epoch-dependent accretion properties.

Findings

Large scatter in the Gamma - Eddington ratio distribution.

Significant Gamma - Eddington ratio correlation only in lower-Eddington AGNs.

Possible anti-correlation between Gamma and soft excess strength.

Abstract

Our understanding of highly accreting AGNs is hampered by the lack of a complete systematic investigation in terms of their main spectral and variability properties, and by the relative paucity of them in the local Universe, especially those powered by supermassive black holes with $M_\mathrm{BH} > 10^8\,M_\odot$. To overcome this limitation, we present here the X-ray spectral analysis of a new, large sample of 61 highly accreting AGNs named as the \emph{XMM-Newton} High-Eddington Serendipitous AGN Sample, or X-HESS, obtained by cross-correlating the 11th release of the \emph{XMM-Newton} serendipitous catalogue and the catalogue of spectral properties of quasars from the SDSS DR14. The X-HESS AGNs are spread across wide intervals with a redshift of $0.06<z<3.3$, a black hole mass of $6.8<\log(M_\mathrm{BH}/M_\odot)<9.8$, a bolometric luminosity of $44.7<\log(L_\mathrm{bol}/\mathrm{erg\,s}^{-1})<48.3$, and an Eddington ratio of $-0.2<\log\lambda_\mathrm{Edd}<0.5$, and more than one third of these AGNs can rely on multiple observations at different epochs, allowing us to investigate also their variability. We find a large scatter in the $\Gamma - \lambda_\mathrm{Edd}$ distribution of the highly accreting X-HESS AGNs. A significant correlation is only found by considering a sample of lower-\ledd\ AGNs with $\lambda_\mathrm{Edd}\lesssim0.3$. The $\Gamma - \lambda_\mathrm{Edd}$ relation appears to be more statistically sound for AGNs with lower $M_\mathrm{BH}$ and/or $L_\mathrm{bol}$. We investigate the possibility of transforming the $\Gamma - \lambda_\mathrm{Edd}$ plane into a fully epoch-dependent frame by calculating the Eddington ratio from the simultaneous optical/UV data from the optical monitor, $\lambda_\mathrm{Edd,O/UV}$. Finally, we also get a mild indication of a possible anti-correlation between $\Gamma$ and the strength of the soft excess.