X-ray spectroscopic survey of highly-accreting AGN

Abstract

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, ${\lambda }_{\mathrm{Edd}}\gtrsim 1$ AGN at $0.4\le z\le 0.75$, 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 ($M_{\mathrm{BH}}\sim 10^{8-8.5}{M}_{\odot }$) 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 $k_{\mathrm{bol},\mathrm{X}}$ 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 $\Gamma \ge 2$ would be an ubiquitous hallmark of AGN with ${\lambda }_{\mathrm{Edd}}\sim 1$. We found that $\sim 30\%$ of the sources are X-ray weak, with an X-ray emission about a factor of $\sim 10-80$ 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.

17 pages, 8 figures (with 2 more figures in Appendix A and B). Accepted for publication in A&A

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