X-ray spectroscopic survey of highly accreting AGN

Abstract

Improving our understanding of the nuclear properties of high-Eddington ratio (λEdd) 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-λEdd AGN, which have been largely studied so far. We present here the X-ray spectral analysis of 14 radio-quiet, λEdd & 1 AGN at 0.4 ≤ z ≤ 0.75, observed with XMM-Newton. Optical/UV data from simultaneous Optical Monitor observations have been also considered. These quasars have been selected to have relatively high values of black hole mass (MBH ∼ 108−8.5 M ) and bolometric luminosity (Lbol ∼ 1046 erg s−1), in order to complement previous studies of high-λEdd AGN at lower MBH and Lbol. We studied the relation between λEdd and other key X-ray spectral parameters, such as the photon index (Γ) of the power-law continuum, the X-ray bolometric correction kbol,X and the optical/UV-to-X-ray spectral index αox. Our analysis reveals that, despite the homogeneous optical and SMBH accretion properties, the X-ray properties of these high-λEdd AGN are quite heterogeneous. We indeed measured values of Γ comprised between 1.3 and 2.5, at odds with the expectations based on previously reported Γ − λEdd relations, by which Γ ≥ 2 would be an ubiquitous hallmark of AGN with λEdd ∼ 1. Interestingly, we found that ∼ 30% of the sources are X-ray weak, with an X-ray emission about a factor of ∼ 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-λEdd 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 sources suggest that these periods can last for several years.