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.

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.

We investigate the properties and evolution of accretion tori formed after the coalescence of two compact objects. At these extreme densities and temperatures, the accreting torus is cooled mainly by neutrino emission produced primarily by electron and positron capture on nucleons (β reactions). We solve for the disc structure and its time evolution by introducing a detailed treatment of the equation of state which includes photodisintegration of helium, the condition of β-equilibrium, and neutrino opacities. We self-consistently calculate the chemical equilibrium in the gas consisting of helium, free protons, neutrons and electron-positron pairs and compute the chemical potentials of the species, as well as the electron fraction throughout the disc. We find that, for sufficiently large accretion rates (Ṁ 10M ⊙ /s), the inner regions of the disk become opaque and develop a viscous and thermal instability. The identification of this instability might be relevant for GRB observations.

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.