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The ratio of the disk thickness to the radius as a function of radius in AGN (P gas heating). The black hole mass is 10 8 M ⊙ and the accretion rate is ˙ m = 0.35.
Source publication
We develop a two-flow model of accretion onto a black hole which incorporates the effect of the local magneto-rotational instability. The flow consists of an accretion disk and an accreting corona, and the local dynamo affects the disk/corona mass exchange. The model is aimed to explain the power spectrum density of the sources in their soft, disk-...
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Citations
... The model parameters are stored in the configuration file dysk-zwm.ini. Their values refer to the physical setup as in [12], [13], and [10]. The code stores the output in various formats: ASCII and HDF5. ...
... The model parameters are stored in the configuration file dysk-zwm.ini. Their values refer to the physical setup as in [5], [6], and [9]. The code stores the output in various formats: ASCII and HDF5. ...
I present the publicly available code GLADIS (GLobal Accretion Disk Instability Simulation) developed in my reserach group over the years 2002-2017. It can be freely downloaded and modified by the users via the link from the Astrophysics Source Code Library. The software computes time-dependent evolution of a black hole accretion disk, in one-dimensional, axisymmetric, vertically integrated scheme. The main applications are to explain the variability of accretion disks that can be subject to radiation-pressure instability. The phenomenon is relevant for fast variable microquasars, as well as for a class of changing-look AGN.
... The models of the time-dependent evolution have been calculated by a number of authors [45][46][47][48][49][50][51], generally with the aim of comparing with some observational data for objects showing outbursts. Thus, in order to fit the outburst amplitude, some modifications of the model are usually introduced since generally the outbursts calculated without any modification had amplitudes that were too high. ...
The concept of slim accretion disks emerged over 30 years ago as an answer to several unsolved problems. Since that time there has been a tremendous increase in the amount of observational data where this model applies. However, many critical issues on the theoretical side remain unsolved, as they are inherently difficult. This is the issue of the disk stability under radiation pressure, the role of the magnetic field in the energy transfer inside the disk, the formation (or not) of a warm corona, and outflows. Thus the progress has to be done both through further developments of the model and through careful comparison with the observational data.
... The models of the time-dependent evolution has been calculated by a number of autors [37][38][39][40][41][42][43], generally with the aim to compare to some observational data for objects showing outbursts. Thus, to fit the outburst amplitude, some modification of the model were usually introduced since in general the outbursts calculated without any modification had too high amplitudes. ...
Slim accretion disks idea emerged over 30 years ago as an answer to several unsolved problems. Since that time there was a tremendous increase in the amount of observational data where this model applies. However, many critical issues on the theoretical side remain unsolved, as they are inherently difficult. This is the issue of the disk stability under the radiation pressure, the role of the magnetic field in the energy transfer inside the disk and the formation (or not) of a warm corona, and outflows. Thus the progress has to be done both through further developments of the model and through the careful comparison to the observational data.
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We analyze the general relativistic oscillations of thin accretion disks
around compact astrophysical objects interacting with the surrounding medium
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respectively. The general equations describing the stochastically perturbed
disks are derived by considering the perturbations of trajectories of the test
particles in equatorial orbits, assumed to move along the geodesic lines. By
taking into account the presence of a viscous dissipation and of a stochastic
force we show that the dynamics of the stochastically perturbed disks can be
formulated in terms of a general relativistic Langevin equation. The stochastic
energy transport equation is also obtained. The vertical oscillations of the
disks in the Schwarzschild and Kerr geometries are considered in detail, and
they are analyzed by numerically integrating the corresponding Langevin
equations. The vertical displacements, velocities and luminosities of the
stochastically perturbed disks are explicitly obtained for both the
Schwarzschild and the Kerr cases.
Context. Some accreting black holes exhibit much stronger variability patterns than the usual stochastic variations. Radiation pressure instability is one of the proposed mechanisms that might account for this effect.
Aims. We model luminosity changes for objects with a black hole mass of 10, 10 ⁵ , and 10 ⁷ solar masses, using the time-dependent evolution of an accretion disk that is unstable as a result of the dominant radiation pressure. We concentrate on the outburst timescales. We explore the influence of the hot coronal flow above the cold disk, the inner purely hot flow, and the effect of the magnetic field on the time evolution of the disk-corona system. For intermediate-mass black holes and active galactic nuclei, we also explore the role of the disk outer radius because a disk that is fed by tidal disruption events (TDE) can be quite small.
Methods. We used a 1D vertically integrated time-dependent numerical scheme that models the simultaneous evolution of the disk and corona, which is coupled by the vertical mass exchange. We parameterized the strength of the large-scale toroidal magnetic fields according to a local accretion rate. We also discuss a possible inner optically thin flow, the advection-dominated accretion flow (ADAF). This flow would require modification of the inner boundary condition of the cold disk flow. For the set of the global parameters, we calculated the variability timescales and outburst amplitudes of the disk and the corona.
Results. We found that the role of the inner ADAF and the accreting corona are relatively unimportant, but the outburst character strongly depends on the magnetic field and on the outer radius of the disk if this radius is smaller (due to the TDE phenomenon) than the size of the instability zone in a stationary disk with infinite radius. For microquasars, the dependence on the magnetic field is monotonic, and the period decreases with the field strength. For higher black hole masses, the dependence is nonmonotonic, and an initial rise of the period is later replaced with a relatively rapid decrease as the magnetic field continues to rise. A still stronger magnetic field stabilizes the disk. When we assumed a smaller disk outer radiusfor 10 ⁵ and 10 ⁷ M ⊙ , the outbursts were shorter and led to complex multiscale outbursts for some parameters, thus approaching the behavior of deterministic chaos.
Conclusions. Our computations confirm that the radiation pressure instability model can account for heartbeat states in microquasars. The rapid variability detected in intermediate-mass black holes in the form of quasi-periodic eruptions can be consistent with the model, but only when it is combined with the TDE phenomenon. The yearly repeating variability in changing-look active galactic nuclei in our model also requires a small outer radius either due to the recent TDE or due to the gap in the disk that is related to a secondary black hole.
Apart from regular, low-level stochastic variability, some active galactic nuclei (AGN) occasionally show exceptionally large changes in the luminosity, spectral shape, and/or X-ray absorption. The most notable are the changes of the spectral type when the source classified as a Seyfert 1 becomes a Seyfert 2 galaxy or vice versa. Thus, a name was coined of “changing-look AGN” (CL AGN). The origin of this phenomenon is still unknown, but for most of the sources, there are strong arguments in favor of the intrinsic changes. Understanding the nature of such rapid changes is a challenge to the models of black hole accretion flows since the timescales of the changes are much shorter than the standard disk viscous timescales. We aim to model the CL AGN phenomenon assuming that the underlying mechanism is the time-dependent evolution of a black hole accretion disk unstable due to the dominant radiation pressure. We use a one-dimensional, vertically integrated disk model, but we allow for the presence of the hot coronal layer above the disk and the presence of the inner purely hot flow. We focus on the variability timescales and amplitudes, which can be regulated by the action of large-scale magnetic fields, the description of the disk-corona coupling and the presence of an inner optically thin flow, like advection-dominated accretion flow. We compare model predictions for the accretion disk around black hole mass .
Context. The changing-look phenomenon observed in a growing number of active galaxies challenges our understanding of the accretion process close to a black hole.
Aims. We propose a simple explanation for the sources where multiple semi-periodic outbursts are observed, and the sources are operating close to the Eddington limit.
Methods. The outburst are caused by the radiation pressure instability operating in the narrow ring between the standard gas-dominated outer disk and the hot optically thin inner advection-dominated accretion flow. The corresponding limit cycle is responsible for periodic outbursts, and the timescales are much shorter than the standard viscous timescale due to the narrowness of the unstable radial zone.
Results. Our toy model gives quantitative predictions and works well for multiple outbursts like those observed in NGC 1566, NGC 4151, NGC 5548, and GSN 069, although the shapes of the outbursts are not yet well modeled, and further development of the model is necessary.
The emission from black hole binaries (BHBs) and active galactic nuclei (AGNs) displays significant aperiodic variabilities. The most promising explanation for these variabilities is the propagating fluctuations in the accretion flow. It is natural to expect that the mechanism driving variabilities in BHBs and AGNs may operate in a black hole hyper-accretion disk, which is believed to power gamma-ray bursts (GRBs). We study the variabilities of jet power in GRBs based on the model of propagating fluctuations. It is found that the variabilities of jet power and the temporal profile of erratic spikes in this scenario are similar to those in observed light curves of prompt gamma-ray emission of GRBs. Our results show that the mechanism driving X-ray variabilities in BHBs and AGNs may operate in the central engine to drive the variabilities of GRBs.
AGN exhibit rapid, high amplitude stochastic flux variability across the entire electromagnetic spectrum on timescales ranging from hours to years. The cause of this variability is poorly understood. We present a new method for using variability to (1) measure the time-scales on which flux perturbations evolve and (2) characterize the driving flux perturbations. We model the observed light curve of an AGN as a linear differential equation driven by stochastic impulses. Physically, the impulses could be local `hot-spots' in the accretion disk---the linear differential equation then governs how the hot spots evolve and dissipate. The impulse-response function of the accretion disk material is given by the Green's function of the linear differential equation. The timescales on which the hot-spots radiate energy is characterized by the powerspectrum of the driving stochastic impulses. We analyze the light curve of the \Kepler AGN Zw 229-15 and find that the observed variability behavior can be modeled as a damped harmonic oscillator perturbed by a colored noise process. The model powerspectrum turns over on time-scale $385$~d. On shorter time-scales, the log-powerspectrum slope varies between $2$ and $4$, explaining the behavior noted by previous studies. We recover and identify both the $5.6$~d and $67$~d timescales reported by previous work. These timescales represent the time-scale on which flux perturbations grow, and the time-scale on which flux perturbations decay back to the steady-state flux level respectively. We make the software package used to study light curves using our method, \textsc{k\={a}l\={i}}, available to the community.
