X-ray spectroscopic survey of highly-accreting AGN

Improving our understanding of the nuclear properties of high-Eddington ratio ($lambda_mathrm{Edd}$) active galactic nuclei (AGN) is necessary since the bulk of X-ray spectroscopic studies have been focused on low-Eddington AGN. We present here the X-ray spectral analysis of 14 radio-quiet, $lambda_mathrm{Edd}gtrsim1$ AGN at $0.4leq z leq 0.75$, observed with XMM-Newton. Optical/UV data from simultaneous Optical Monitor observations have been also considered. These AGN have been selected to have relatively high values of black hole mass ($M_mathrm{BH}sim10^{8-8.5}M_odot$) and bolometric luminosity ($L_mathrm{bol} sim 10^{46}$ erg s$^{-1}$), in order to complement previous studies of high-Eddington AGN at lower $M_mathrm{BH}$ and $L_mathrm{bol}$. We studied the relation between $lambda_mathrm{Edd}$ and other key X-ray spectral parameters, such as the photon index of the power-law continuum $Gamma$, the X-ray bolometric correction $k_mathrm{bol,X}$ and $alpha_mathrm{ox}$. Despite the homogeneous optical and SMBH accretion properties, the X-ray properties of these high-Eddington AGN are quite heterogeneous. We measured values of $Gamma$ comprised between 1.3 and 2.5, at odds with the expectations based on previously reported $Gamma-lambda_mathrm{Edd}$ relations, by which $Gammageq2$ would be an ubiquitous hallmark of AGN with $lambda_mathrm{Edd}sim1$. We found that $sim30%$ of the sources are X-ray weak, with an X-ray emission about a factor of $sim10-80$ fainter than that of typical AGN at similar UV luminosities. The X-ray weakness seems to be intrinsic and not due to intervening obscuration. This may indicate that high-Eddington AGN commonly undergo periods of intrinsic X-ray weakness. Furthermore, results from a follow-up monitoring with Swift of one of these X-ray weak AGN suggest that these periods can last for several years.