We investigate how the total radio luminosity of AGN-powered radio sources depends on their accretion luminosity and the central black hole mass. Our studies cover about seven orders of magnitude in accretion luminosity (expressed in Eddington units, i.e. as Eddington ratios) and the full range of AGN black hole masses. We find that AGNs form two distinct and well separated sequences on the radio-loudness -Eddington-ratio plane. The 'upper' sequence is formed by radio selected AGNs, the 'lower' sequence contains mainly optically selected objects. Whereas an apparent 'gap' between the two sequences may be an artifact of selection effects, the sequences themselves mark the real upper bounds of radio-loudness of two distinct populations of AGNs: those hosted respectively by elliptical and disk galaxies. Both sequences show the same dependence of the radioloudness on the Eddington ratio (an increase with decreasing Eddington ratio), which suggests that the normalization of this dependence is determined by the black hole spin. This implies that central black holes in giant elliptical galaxies have (on average) much larger spins than black holes in spiral/disc galaxies. This galaxy-morphology related radio-dichotomy breaks down at high accretion rates where the dominant fraction of luminous quasars hosted by elliptical galaxies is radio quiet. This led to speculations in the literature that formation of powerful jets at high accretion rates is intermittent and related to switches between two disk accretion modes, as directly observed in some BH X-ray binaries. We argue that such intermittency can be reconciled with the spin paradigm, provided that successful formation of relativistic jets by rotating black holes requires collimation by MHD outflows from accretion disks.
Many luminous blazars which are associated with quasar-type active galactic nuclei display broadband spectra characterized by a large luminosity ratio of their high-energy (γ -ray) and low-energy (synchrotron) spectral components. This large ratio, reaching values up to 100, challenges the standard synchrotron self-Compton models by means of substantial departures from the minimum power condition. Luminous blazars also typically have very hard X-ray spectra, and those in turn seem to challenge hadronic scenarios for the high-energy blazar emission. As shown in this paper, no such problems are faced by the models which involve Comptonization of radiation provided by a broad-line region, or dusty molecular torus. The lack or weakness of bulk-Compton and KleinNishina features indicated by the presently available data favors the production of γ -rays via upscattering of infrared photons from hot dust. This implies that the blazar emission zone is located at parsec-scale distances from the nucleus, and as such is possibly associated with the extended, quasi-stationary reconfinement shocks formed in relativistic outflows. This scenario predicts characteristic timescales for flux changes in luminous blazars to be days/weeks, consistent with the variability patterns observed in such systems at infrared, optical, and γ -ray frequencies. We also propose that the parsec-scale blazar activity can be occasionally accompanied by dissipative events taking place at sub-parsec distances and powered by internal shocks and/or reconnection of magnetic fields. These could account for the multiwavelength intraday flares occasionally observed in powerful blazar sources.
For the first time, detailed radiative transfer calculations of Comptonized X-ray and γ-ray radiation in a hot pair plasma above a cold accretion disk are performed using two independent codes and methods. The simulations include both energy and pair balance as well as reprocessing of the X-and γ-rays by the cold disk. We study both plane-parallel coronae as well as active dissipation regions having shapes of hemispheres and pill boxes located on the disk surface. It is shown, contrary to earlier claims, that plane-parallel coronae in pair balance have difficulties in selfconsistently reproducing the ranges of 2-20 keV spectral slopes, high energy cutoffs, and compactnesses inferred from observations of type 1 Seyfert galaxies. Instead, the observations are consistent with the X-rays coming from a number of individual active regions located on the surface of the disk.A number of effects such as anisotropic Compton scattering, the reflection hump, feedback to the soft photon source by reprocessing, and an active region in pair equilibrium all conspire to produce the observed ranges of X-ray slopes, high energy cutoffs, and compactnesses. The spread in spectral X-ray slopes can be due to a spread in the properties of the active regions such as their compactnesses and their elevations above the disk surface. Simplified models invoking isotropic Comptonization in spherical clouds are no longer sufficient when interpreting the data.
We investigate the conjecture by Sikora, Stawarz & Lasota (2007) that the observed AGN-radioloudness bimodality can be explained by the morphology -related bimodality of black-hole spin distribution in the centers of galaxies: central black holes in giant elliptical galaxies may have (on average) much larger spins than black holes in spiral/disc galaxies. We study how accretion from a warped disc influences the evolution of black hole spins and conclude that within the cosmological framework, where the most massive BHs have grown in mass via merger driven accretion, one indeed expects most supermassive black holes in elliptical galaxies to have on average higher spin than black holes in spiral galaxies, where random, small accretion episodes (e.g. tidally disrupted stars, accretion of molecular clouds) might have played a more important role.
X-ray observations of blazars associated with the OVV (Optically Violently Variable) quasars put strong constraints on the e + e − pair content of radio-loud quasar jets. From those observations, we infer that jets in quasars contain many more e + e − pairs than protons, but dynamically are still dominated by protons. In particular, we show that pure e + e − jet models can be excluded, as they overpredict soft X-ray radiation; likewise, pure proton-electron jets can be excluded, as they predict too weak nonthermal X-ray radiation. An intermediate case is viable. We demonstrate that jets which are initially proton-electron ("proto-jets") can be pair-loaded via interaction with 100 -300 keV photons produced in hot accretion disc coronae, likely to exist in active galactic nuclei in general. If the coronal radiation is powered by magnetic flares, the pair loading is expected to be non-uniform and non-axisymmetric. Together with radiation drag, this leads to velocity and density perturbations in a jet and formation of shocks, where the pairs are accelerated. Such a scenario can explain rapid (time scale of ∼ a day) variability observed in OVV quasars.
The formation of relativistic astrophysical jets is presumably mediated by magnetic fields threading accretion disks and central, rapidly rotating objects. As it is accelerated by magnetic stresses, the jet's kinetic energy flux grows at the expense of its Poynting flux. However, it is unclear how efficient the conversion from magnetic to kinetic energy is and whether there are any observational signatures of this process. We address this issue in the context of jets in quasars. Using data from all spatial scales, we demonstrate that in these objects the conversion from Poynting fluxdominated to matter-dominated jets is very likely to take place closer to the black hole than in the region where most of the Doppler-boosted radiation observed in blazars is produced. We briefly discuss the possibility that blazar activity could be induced by global MHD instabilities, e.g., via the production of localized velocity gradients that lead to dissipative events such as shocks or magnetic reconnection, in which acceleration of relativistic particles and production of nonthermal flares is taking place.
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