We consider the effect of radiation pressure from ionizing photons on black hole (BH) mass estimates based on the application of the virial theorem to broad emission lines in AGN spectra. BH masses based only on the virial product ∆V 2 R and neglecting the effect of radiation pressure can be severely underestimated especially in objects close to the Eddington limit. We provide an empirical calibration of the correction for radiation pressure and we show that it is consistent with a simple physical model in which BLR clouds are optically thick to ionizing radiation and have average column densities of N H ∼ 10 23 cm −2 . This value is remarkably similar to what is required in standard BLR photoionization models to explain observed spectra. With the inclusion of radiation pressure the discrepancy between virial BH masses based on single epoch spectra and on reverberation mapping data drops from 0.4 to 0.2 dex rms. The use of single epoch observations as surrogates of reverberation mapping campaigns can thus provide more accurate BH masses than previously thought. Finally, we show that Narrow Line Seyfert 1 (NLS1) galaxies have apparently low BH masses because they are radiating close to their Eddington limit. After the radiation pressure correction, NLS1 galaxies have BH masses similar to other broad line AGNs and follow the same M BH -σ e /L sph relations as other active and normal galaxies. Radiation forces arising from ionizing photon momentum deposition constitute an important physical effect which must be taken into account when computing virial BH masses.

We use a long (300 ks), continuous Suzaku X-ray observation of the active nucleus in NGC 1365 to investigate the structure of the circumnuclear broad line region (BLR) clouds through their occultation of the X-ray source. The variations of the absorbing column density and of the covering factor indicate that the clouds surrounding the black hole are far from having a spherical geometry (as sometimes assumed), instead they have a strongly elongated and cometary shape, with a dense head (n ∼ 10 11 cm −3 ) and an expanding, dissolving tail. We infer that the cometary tails must be longer than a few times 10 13 cm and their opening angle must be smaller than a few degrees. We suggest that the cometary shape may be a common feature of BLR clouds in general, but which has been difficult to recognize observationally so far. The cometary shape may originate from shocks and hydrodynamical instabilities generated by the supersonic motion of the BLR clouds into the intracloud medium. As a consequence of the mass loss into their tail, we infer that the BLR clouds probably have a lifetime of only a few months, implying that they must be continuously replenished. We also find a large, puzzling discrepancy (two orders of magnitude) between the mass of the BLR inferred from the properties of the absorbing clouds and the mass of the BLR inferred from photoionization models; we discuss the possible solutions to this discrepancy.

We present a new analysis of a 9-d long XMM-Newton monitoring of the narrow-line Seyfert 1 galaxy Mrk 766. We show that the strong changes in the spectral shape, which occurred during this observation, can be interpreted as due to broad-line region clouds crossing the line of sight to the X-ray source. Within the occultation scenario, the spectral and temporal analyses of the eclipses provide precise estimates of the geometrical structure, location and physical properties of the absorbing clouds. In particular, we show that these clouds have cores with column densities of at least a few 10 23 cm −2 and velocities in the plane of the sky of the order of thousands of km s −1 . The three different eclipses monitored by XMM-Newton suggest a broad range in cloud velocities (by a factor of ∼4-5). Moreover, two iron absorption lines clearly associated with each eclipse suggest the presence of highly ionized gas around the obscuring clouds and an outflow component of the velocity spanning from 3000 to 15 000 km s −1 .

The application of the virial theorem to the Broad Line Region of Active Galactic Nuclei allows Black Hole mass estimates for large samples of objects at all redshifts. In a recent paper we showed that ionizing radiation pressure onto BLR clouds affects virial BH mass estimates and we provided empirically calibrated corrections. More recently, a new test of the importance of radiation forces has been proposed: the M BH − σ relation has been used to estimate M BH for a sample of type-2 AGN and virial relations (with and without radiation pressure) for a sample of type-1 AGN extracted from the same parent population. The observed L/L Edd distribution based on virial BH masses is in good agreement with that based on M BH − σ only if radiation pressure effects are negligible, otherwise significant discrepancies are observed. In this paper we investigate the effects of intrinsic dispersions associated to the virial relations providing M BH , and we show that they explain the discrepancies between the observed L/L Edd distributions of type-1 and type-2 AGN. These discrepancies in the L/L Edd distributions are present regardless of the general importance of radiation forces, which must be negligible only for a small fraction of sources with large L/L Edd . Average radiation pressure corrections should then be applied in virial M BH estimators until their dependence on observed source physical properties has been fully calibrated. Finally, the comparison between M BH and L/L Edd distributions derived from σ-based and virial estimators can constrain the variance of BLR physical properties in AGN.

The X-ray spectra of accreting black hole systems generally contain components (sometimes dominating the total emission) which are well-fit by thermal Comptonization models with temperatures ∼ 100 keV. We demonstrate why, over many orders of magnitude in heating rate and seed photon supply, hot plasmas radiate primarily by inverse Compton scattering, and find equilibrium temperatures within a factor of a few of 100 keV. We also determine quantitatively the (wide) bounds on heating rate and seed photon supply for which this statement is true.Plasmas in thermal balance in this regime obey two simple scaling laws: ΘτT ≃ 0.1(l h /ls) 1/4 ; and α ≃ 1.6(ls/l h ) 1/4 . Here the hot plasma heating rate compactness is l h , the seed photon compactness is ls, the temperature in electron rest mass units is Θ, and the Thomson optical depth is τT . The coefficient in the first expression is weakly-dependent on plasma geometry; the second expression is independent of geometry. Only when ls/l h is a few tenths or greater is there a weak secondary dependence in both relations on τT .Because α is almost independent of everything but ls/l h , the observed power law index may be used to estimate ls/l h . In both AGN and stellar black holes, the mean value estimated this way is ls/l h ∼ 0.1, although there is much greater sample dispersion among stellar black holes than among AGN. This inference favors models in which the intrinsic (as opposed to reprocessed) luminosity in soft photons entering the hot plasma is small, or in which the hard X-ray production is comparatively distant from the source of soft photons. In addition, it predicts that ΘτT ≃ 0.1 -0.2, depending primarily on plasma geometry. It is possible to construct coronal models (i.e. models in which ls/l h ≃ 0.5) which fit the observed spectra, but they are tightly constrained: τT must be ≃ 0.08 and Θ ≃ 0.8.