Abstract

Improving our understanding of the nuclear properties of high-Eddington-ratio (lambda(Edd)) active galactic nuclei (AGN) is necessary since at this regime the radiation pressure is expected to affect the structure and efficiency of the accretion disc-corona system. This may cause departures from the typical nuclear properties of low-lambda(Edd) AGN, which have been largely studied so far. We present here the X-ray spectral analysis of 14 radio-quiet, lambda(Edd)1 AGN at 0.4x0.75, observed with XMM-Newton. Optical/UV data from simultaneous Optical Monitor observations have also been considered. These quasars were selected to have relatively high values of black hole mass (M(BH)10(8 - 8.5)M(circle dot)) and bolometric luminosity (L(bol)10(46) erg s(-1)) in order to complement previous studies of high-lambda(Edd) AGN at lower M-BH and L-bol. We studied the relation between lambda(Edd) and other key X-ray spectral parameters, such as the photon index (Gamma) of the power-law continuum, the X-ray bolometric correction (k(bol,X)), and the optical/UV-to-X-ray spectral index (alpha(ox)). Our analysis reveals that, despite the homogeneous optical and supermassive black hole accretion properties, the X-ray properties of these high-lambda(Edd) AGN are quite heterogeneous. We indeed measured values of Gamma between 1.3 and 2.5, at odds with the expectations based on previously reported Gamma - lambda(Edd) relations, for which Gamma 2 would be a ubiquitous hallmark of AGN with lambda(Edd)1. Interestingly, we found that similar to 30% of the sources are X-ray weak, with an X-ray emission about a factor of similar to 10 - 80 fainter than that of typical AGN at similar UV luminosities. The X-ray weakness seems to be intrinsic and not due to the presence of absorption along the line of sight to the nucleus. This result may indicate that high-lambda(Edd) AGN commonly undergo periods of intrinsic X-ray weakness. Furthermore, results from follow-up monitoring with Swift of one of these X-ray weak sources suggest that these periods can last for several years.