Bolometric luminosities and Eddington ratios of both X‐ray selected broad‐line (Type‐1) and narrow‐line (Type‐2) active galactic nuclei (AGN) from the XMM–Newton survey in the Cosmic Evolution Survey field are presented. The sample is composed of 929 AGN (382 Type‐1 AGN and 547 Type‐2 AGN) and it covers a wide range of redshifts, X‐ray luminosities and absorbing column densities. About 65 per cent of the sources are spectroscopically identified as either Type‐1 or Type‐2 AGN (83 and 52 per cent, respectively), while accurate photometric redshifts are available for the rest of the sample. The study of such a large sample of X‐ray selected AGN with a high‐quality multiwavelength coverage from the far‐infrared (now with the inclusion of Herschel data at 100 and 160 μm) to the optical–ultraviolet allows us to obtain accurate estimates of bolometric luminosities, bolometric corrections and Eddington ratios. The kbol ‐ Lbol relations derived in this work are calibrated for the first time against a sizable AGN sample, and rely on observed redshifts, X‐ray luminosities and column density distributions. We find that kbol is significantly lower at high Lbol with respect to previous estimates by Marconi et al. and Hopkins et al. Black hole (BH) masses and Eddington ratios are available for 170 Type‐1 AGN, while BH masses for Type‐2 AGN are computed for 481 objects using the BH mass–stellar mass relation and the morphological information. We confirm a trend between kbol and λEdd, with lower hard X‐ray bolometric corrections at lower Eddington ratios for both Type‐1 and Type‐2 AGN. We find that, on average, the Eddington ratio increases with redshift for all types of AGN at any given MBH, while no clear evolution with redshift is seen at any given Lbol.

We present a critical analysis of the usual interpretation of the multicolour disc model parameters for black hole candidates in terms of the inner radius and temperature of the accretion disc. Using a self‐consistent model for the radiative transfer and the vertical temperature structure in a Shakura–Sunyaev disc, we simulate the observed disc spectra, taking into account Doppler blurring and gravitational redshift, and fit them with multicolour models. We show not only that such a model systematically underestimates the value of the inner‐disc radius, but that when the accretion rate and/or the energy dissipated in the corona are allowed to change, the inner edge of the disc, as inferred from the multicolour model, appears to move even when it is in fact fixed at the innermost stable orbit.

A B S T R A C TThe spectral-energy distributions of galactic black holes in the low/hard state and of lowluminosity active galactic nuclei (AGNs) possess many common features, the most prominent being: compact, flat-(or inverted-)spectrum radio cores with high brightness temperatures; excess red and infrared emission, often correlated with the radio flux; an extremely weak (or absent) quasi-thermal hump and a hard-X-ray power-law with high-energy cut-off. These sources are thought to be accreting at low rates, and advection-(or convection-)dominated accretion flows are usually considered the best candidates to explain them. Here we present an alternative possibility, involving strong, unbound, magnetic coronae generated by geometrically thin, optically thick accretion discs at low accretion rates. First we show that, if angular momentum transport in the disc is due to magnetic turbulent stresses, the magnetic energy density and effective viscous stresses inside the disc are proportional to the geometric mean of the total (gas plus radiation) and gas pressure. Therefore the corona is less powerful in a radiation-pressure dominated disc, and the relative fraction of the power liberated in the corona increases as the accretion rate decreases. Furthermore, we discuss why energetically dominant coronae are ideal sites for launching powerful jets/outflows, both MHD-and thermally driven. In analysing the spectral properties of such coronal-outflow dominated accretion discs, we reach the important conclusion that if the jet/outflow is, as is likely, radiatively inefficient, then so is the source overall, even without advection of energy into the black hole being relevant for the dynamics of the accretion flow.

A model for the inner regions of accretion flows is presented where, owing to disc instabilities, cold and dense material is clumped into deep sheets or rings. Surrounding these density enhancements is hot, tenuous gas where coronal dissipation processes occur. We expect this situation to be most relevant when the accretion rate is close to Eddington and the disc is radiation‐pressure‐dominated, and so may apply to narrow‐line Seyfert 1 (NLS1) galaxies. In this scenario, the hard X‐ray source is obscured for most observers, and so the detected X‐ray emission would be dominated by reflection off the walls of the sheets. A simple Comptonization calculation shows that the large photon‐indices characteristic of NLS1s would be a natural outcome of two reprocessors closely surrounding the hard X‐ray source. We test this model by fitting the XMM‐Newton spectrum of the NLS1 1H 0707–495 between 0.5 and 11 keV with reflection‐dominated ionized disc models. A very good fit is found with three different reflectors each subject to the same Γ=2.35 power law. An iron overabundance is still required to fit the sharp drop in the spectrum at around 7 keV. We note that even a small corrugation of the accretion disc may result in Γ>2 and a strong reflection component in the observed spectrum. Therefore, this model may also explain the strength and the variability characteristics of the MCG–6‐30‐15 Fe Kα line. The idea needs to be tested with further broad‐band XMM‐Newton observations of NLS1s.

Most astrophysical sources powered by accretion onto a black hole, either of stellar mass or supermassive, when observed with hard X-rays show signs of a hot Comptonizing component in the flow, the so-called corona, with observed temperatures and optical depths lying in a narrow range (0.1 < \tau < 1 and 1x10^9 K < T < 3x10^9 K). Here we argue that these facts constitute strong supporting evidence for a magnetically-dominated corona. We show that the inferred thermal energy content of the corona, in all black hole systems, is far too low to explain their observed hard X-ray luminosities, unless either the size of the corona is at least of the order of 10^3 Schwarzschild radii, or the corona itself is in fact a reservoir, where the energy is mainly stored in the form of a magnetic field generated by a sheared rotator (probably the accretion disc). We briefly outline the main reasons why the former possibility is to be discarded, and the latter preferred.Comment: 5 pages, accepted for publication in MNRA