No abstract

We associate the existence of short-lived compact radio sources with the intermittent activity of the central engine caused by a radiation pressure instability within an accretion disk. Such objects may constitute a numerous sub-class of Giga-Hertz Peaked Spectrum sources, in accordance with the population studies of radio-loud active galaxies, as well as detailed investigations of their radio morphologies. We perform the model computations assuming the viscosity parametrization as proportional to a geometrical mean of the total and gas pressure. The implied timescales are consistent with the observed ages of the sources. The duration of an active phase for a moderate accretion rate is short enough (< 10 3 − 10 4 years) that the ejecta are confined within the host galaxy and thus these sources cannot evolve into large size radio galaxies unless they are close to the Eddington limit.

Aims. Although broad emission lines are the most reliable signature of the nuclear activity of a galaxy and the location of the emitting material is well measured by the reverberation method, the physical cause of the formation of the broad line region remains unclear. We attempt to place some constraints on its origin. Methods. We study the properties of the accretion disk underlying the broad line region. Results. We find that the effective temperature at the disk radius corresponding to the location of the broad line region, as inferred from the Hβ line, is universal in all monitored sources and equal to 1000 K. This value is close to the limiting value that permits the existence of the dust. Conclusions. The likely origin of the low ionization part of the broad line region is the strong local dusty wind from the disk. This wind becomes exposed to the irradiation by the central regions when moving higher above the disk surface and subsequently behaves like a failed wind, thus leading to a local mixture of inflow and outflow. This may provide the physical explanation of the turbulence needed both to smooth the line profiles as well as provide additional mechanical heating.

The time-dependent evolution of the accretion disk around the black hole is computed. The classical description of the -viscosity is adopted so the evolution is driven by the instability operating in the innermost radiation pressure-dominated part of the accretion disk. We assume that the optically thick disk always extends down to the marginally stable orbit, so it is never evacuated completely. We include the effect of the advection, coronal dissipation, and vertical outflow. We show that the presence of the corona and/or the outflow reduces the amplitude of the outburst. If only about half of the energy is dissipated in the disk (with the other half dissipated in the corona and carried away by the outflow), the outburst amplitude and duration are consistent with observations of the microquasar GRS 1915+105. Viscous evolution explains in a natural way the lack of direct transitions from the state C to the state B in the color-color diagram of this source. Further reduction of the fraction of energy dissipated in the optically thick disk switches off the outbursts, which may explain why they are not seen in all high accretion rate sources being in the very high state.

Abstract. The spectra of quasars and NLS1 galaxies show surprising similarity in their spectral shape. They seem to scale only with the accretion rate. This is in contradiction with the simple expectations from the standard disk model which predicts lower disk temperature for higher black hole mass. Here we consider two mechanisms modifying the disk spectrum: the irradiation of the outer disk due to the scattering of the flux by the extended ionized medium (warm absorber) and the development of the warm Comptonizing disk skin under the effect of the radiation pressure instability. Those two mechanisms seem to lead to a spectrum which indeed roughly scales, as observed, only with the accretion rate. The scenario applies only to objects with relatively high Eddington ratio for which disk evaporation is inefficient.

We observed 17 optically-selected, radio-quiet high-redshift quasars with the Chandra Observatory ACIS, and detected 16 of them. The quasars have redshift between 3.70 and 6.28 and include the highest redshift quasars known. When compared to low-redshift quasars observed with ROSAT, these high redshift quasars are significantly more X-ray quiet. We also find that the X-ray spectral index of the high redshift objects is flatter than the average at lower redshift. These trends confirm the predictions of models where the accretion flow is described by a cold, optically-thick accretion disk surrounded by a hot, optically thin corona, provided the viscosity parameter α ≥ 0.02. The high redshift quasars have supermassive black holes with masses ∼ 10 10 M ⊙ , and are accreting material at ∼0.1 the Eddington limit. We detect 10 X-ray photons from the z = 6.28 quasar SDS 1030+0524, which may have a Gunn-Peterson trough and be near the redshift of reionization of the intergalactic medium. The X-ray data place an upper limit on the optical depth of the intergalactic medium τ (IGM) < 10 6 , compared to the lower limit from the spectrum of Lyα and Lyβ, which implies τ (IGM) > 20.

We construct a quasar extinction curve based on the blue and red composite quasar spectra of Richards et al. (2003) prepared from the SDSS survey. This extinction curve does not show any traces of the 2200Å feature characteristic of the Interstellar Medium, and this indicates that graphite grains are likely absent close to quasar nuclei. The extinction is best modeled by AC amorphous carbon grains, assuming a standard distribution of grain sizes (p = 3.5) but slightly larger minimum grain size (a min = 0.016µm) and lower maximum grain size (a min = 0.12µm) than the respective canonical values for the interstellar medium. The dust composition is thus similar to that of the dust in AGB stars. Since graphite grains form from amorphous carbon exposed to strong UV irradiation the results indicate that either the dust forms surprisingly far from the active nucleus or in a wind that leaves the nucleus quickly enough to avoid crystallization into graphite.

The concept of the quasar main sequence is very attractive since it stresses correlations between various parameters and implies the underlying simplicity. In the optical plane defined by the width of the Hβ line and the ratio of the equivalent width of the Fe II to Hβ observed objects form a characteristic pattern. In this paper, we use a physically motivated model to explain the distribution of quasars in the optical plane. Continuum is modelled as an accretion disk with a hard X-ray power law uniquely tight to the disk at the basis of observational scaling, and the Broad Line Region distance is determined also from observational scaling. We perform the computations of the FeII and Hβ line production with the code CLOUDY. We have only six free parameters for an individual source: maximum temperature of the accretion disk, Eddington ratio, cloud density, cloud column density, microturbulence, and iron abundance, and only the last four remain as global parameters in our modelling of the whole sequence. Our theoretically computed points cover well the optical plane part populated with the observed quasars, particularly if we allow for super-Solar abundance of heavy elements. Explanation of the exceptionally strong Fe II emitter requires a stronger contribution from the dark sides of the clouds. Analyzing the way how our model covers the optical plane we conclude that there is no single simple driver behind the sequence, as neither the Eddington ratio nor broad band spectrum shape plays the dominant role. Also, the role of the viewing angle in providing the dispersion of the quasar main sequence is apparently not as strong as expected.