Agnieszka Janiuk

Accretion Disk Model of Short-Timescale Intermittent Activity in Young Radio Sources

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.

Chandra Survey of Radio-quiet, High-Redshift Quasars

We observed 17 optically selected, radio-quiet, high-redshift quasars with the Chandra ACIS and detected 16 of them. The quasars have redshifts between 3.70 and 6.28 and include the highest-redshift quasars known. When compared with 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 in which 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 of ~1010 M☉, and are accreting material at ~0.1 times the Eddington limit. We detect 10 X-ray photons from the z = 6.28 quasar SDSS 1030+0524, which might 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) 20.

Radiation Pressure Instability Driven Variability in the Accreting Black Holes

The time-dependent evolution of the accretion disk around the black hole is computed. The classical description of the α-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.

Instabilities in the Time-Dependent Neutrino Disk in Gamma-Ray Bursts

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 disk 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 disk. We find that, for sufficiently large accretion rates ( 10 M☉ s-1), 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.

On the Turbulent α-Disks and the Intermittent Activity in Active Galactic Nuclei

We consider effects of MHD turbulence on viscosity during the evolution of the thermal-viscous ionization instability in standard α-accretion disks. We consider the possibility that accretion onto a supermassive black hole proceeds through an outer standard accretion disk and an inner radiatively inefficient and advection-dominated flow. In this scenario we follow the time evolution of the accretion disk, in which the viscosity parameter α is constant throughout the whole instability cycle, as implied by the strength of MHD turbulence. We conclude that the hydrogen ionization instability is a promising mechanism to explain the intermittent activity in AGNs.We consider effects of the MHD turbulence on the viscosity during the evolution of the thermal-viscous ionization instability in the standard

Evolution of a neutrino-cooled disc in gamma-ray bursts

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Radiation Pressure Instability as a Variability Mechanism in the Microquasar GRS 1915+105

-accretion disks. We consider the possibility that the accretion onto a supermassive black hole proceeds through an outer standard accretion disk and inner, radiatively inefficient and advection dominated flow. In this scenario we follow the time evolution of the accretion disk in which the viscosity parameter

On different types of instabilities in black hole accretion discs: implications for X-ray binaries and active galactic nuclei

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On different types of instabilities in black hole accretion discs. Implications for X-ray binaries and AGN

is constant throughout the whole instability cycle, as implied by the strength of MHD turbulence. We conclude that the hydrogen ionization instability is a promising mechanism to explain the intermittent activity in AGN.

Time‐delays between the soft and hard X‐ray bands in GRS 1915 + 105

Rapid, hyper-Eddington accretion is likely to power the central engines of gamma-ray bursts (GRBs). In the extreme conditions of densities and temperatures the accreting torus is cooled by neutrino emission rather than by radiation. Another important cooling mechanism is the advection of energy into the central black hole. We compute the time evolution of a neutrino-dominated disc that proceeds during the burst and investigate the changes in its density and temperature. The discrimination between short and long bursts is made on the basis of the different rates of material inflow to the outer parts of the disc, thus favouring the binary merger scenario for the short GRBs and the collapsar scenario for the long GRBs. Within the context of the collapsar model, we also study the evolution of the photon luminosity of the remnant disc up to times of ∼ I d, and we discuss its implications for the production of emission lines in GRB spectra.