Formation of collimated beams

A mechanism is proposed for the formation of collinated beams in radio galaxies. Collimated flows are considered to be non-thermally driven by high energy particles and magneto-hydrodynamic (MHD) waves. The galactic nucleus is regarded as being surrounded by a cool gas. The cool gas accretes onto the nucleus, and then the high energy particles are completely locked to the MHD waves. When a quasi-radial magnetic field is embedded in the accretion flow, the resulting MHD wave packets are collimated into the direction of the symmetry axis of the galactic nuclear disc. The fluid around the nucleus is considered to be accelerated and heated by these MHD waves. The fluid beam is ejected along the symmetry axis.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Simulating Magnetized Jets

A suitable model for the macroscopic behavior of accretion disk-jet systems is provided by the equations of MagnetoHydroDynamics (MHD). These equations allow us to perform scale-encompassing numerical simulations of multidimensional nonlinear magnetized plasma flows. For that purpose, we continue the development and exploitation of the Versatile Advection Code (VAC) along with its recent extension which employs dynamically controlled grid adaptation. In the adaptive mesh refinement AMRVAC code, modules for simulating any-dimensional special relativistic hydro- and magnetohydrodynamic problems are currently operational. Here, we review recent 3D MHD simulations of fundamental plasma instabilities, relevant when dealing with cospatial shear flow and twisted magnetic fields. Such magnetized jet flows can be susceptible to a wide variety of hydro (e.g. Kelvin-Helmholtz) or magnetohydrodynamic (e.g. current driven kink) instabilities. Recent MHD computations of 3D jet flows have revealed how such mutually interacting instabilities can in fact aid in maintaining jet coherency. Another breakthrough from computational magnetofluid modeling is the demonstration of continuous, collimated, transmagnetosonic jet launching from magnetized accretion disks. Summarizing, MHD simulations are rapidly gaining realism and significantly advance our understanding of nonlinear astrophysical magnetofluid dynamics.     

3D MHD Simulations of Laboratory Plasma Jets

Jets and outflows are thought to be an integral part of accretion phenomena and are associated with a large variety of objects. In these systems, the interaction of magnetic fields with an accretion disk and/or a magnetized central object is thought to be responsible for the acceleration and collimation of plasma into jets and wider angle flows. In this paper we present three-dimensional MHD simulations of magnetically driven, radiatively cooled laboratory jets that are produced on the MAGPIE experimental facility. The general outflow structure comprises an expanding magnetic cavity which is collimated by the pressure of an extended plasma background medium, and a magnetically confined jet which develops within the magnetic cavity. Although this structure is intrinsically transient and instabilities in the jet and disruption of the magnetic cavity ultimately lead to its break-up, a well collimated, “knotty” jet still emerges from the system; such clumpy morphology is reminiscent of that observed in many astrophysical jets. The possible introduction in the experiments of angular momentum and axial magnetic field will also be discussed.     

Outflows from Dynamo-Active Protostellar Accretion Discs

An axisymmetric model of a cool, dynamo-active accretion disc is applied to protostellar discs. Thermally and magnetically driven outflows develop that are not collimated within 0.1 AU. In the presence of a central magnetic field from the protostar, accretion onto the protostar is highly episodic, which is in agreement with earlier work.     

The generation of MHD shock waves during the impulsive phase of the February 27, 1992 flare

In this paper a unique 2.3–4.2 GHz radio spectrum of the flare impulsive phase, showing fast positively drifting bursts superimposed on a slowly negatively drifting burst, is presented. Analyzing this radio spectrum it was found that the flare started somewhere near the transition region, where upward propagating MHD waves were generated during the whole impulsive phase. Moreover, it was found that behind a front of these ascending MHD waves the downward propagating electron beams, which bombarded dense layers of the solar atmosphere, were accelerated. It seems that, simultaneously with the increase of beam bombardment intensity, the intensity of MHD waves was increasing and thus the MHD shock wave generation and the electron beam acceleration and bombardment formed a self-consistently amplifying flare process. At higher coronal heights this process was followed by a type II radio burst, i.e. by the MHD flare shock. To verify this concept, the numerical modeling of the shock-wave generation and propagation in space from a flare site near the transition region up to 3 solar radii was made. Comparing the thermal and magnetic field disturbances, it was found that those of magnetic origin are more relevant in this case. Combining the results of interpretation and numerical simulation, a model of the February 27, 1992 flare is suggested and new aspects of this model are discussed.     

Sound waves in the intracluster medium

We present an analysis of the behaviour of a perturbed radio cocoon. Comparisons with observations of sound waves detected in the Perseus and Virgo clusters suggest the separations of observed ripples correspond to the natural oscillation frequency of the cocoon. An energy injection rate consistent with active galactic nucleus power is required to offset the strong acoustic damping of cocoon oscillations, suggesting the sources are in equilibrium with the intracluster medium (ICM), and the oscillations are effectively undamped. Viscous dissipation of sound waves provides ICM heating that can quench cooling flows on time-scales greatly exceeding the oscillation time-scale. Thermal conductivity is likely to be heavily suppressed.     

Magnetized accretion flows: effects of gas pressure

We study how axisymmetric magnetohydrodynamic (MHD) accretion flows depend on γ adiabatic index in the polytropic equation of state. This work is an extension of Mościbrodzka & Proga, where we investigated the γ dependence of two-dimensional Bondi-like accretion flows in the hydrodynamical (HD) limit. Our main goal is to study if simulations for various γ can give us insights into the problem of various modes of accretion observed in several types of accretion systems, such as black hole binaries (BHBs), active galactic nuclei (AGN) and gamma-ray bursts. We find that for  γ≳ 4/3  , the fast-rotating flow forms a thick torus that is supported by rotation and gas pressure. As shown before for  γ= 5/3  , such a torus produces a strong, persistent bipolar outflow that can significantly reduce the polar funnel accretion of a slowly rotating flow. For low γ, close to 1, the torus is thin and is supported by rotation. The thin torus produces an unsteady outflow which is too weak to propagate throughout the polar funnel inflow. Compared to their HD counterparts, the MHD simulations show that the magnetized torus can produce an outflow and does not exhibit regular oscillations. Generally, our simulations demonstrate how the torus thickness affects the outflow production. They also support the notion that the geometrical thickness of the torus correlates with the power of the torus outflow. Our results, applied to observations, suggest that the torus ability to radiatively cool and become thin can correspond to a suppression of a jet as observed in the BHBs during a transition from a hard/low to soft/high spectral state and a transition from a quiescent to hard/low state in AGN.     

The Dynamics of Gas around Active Galactic Nuclei

This paper examines the relationship between the accretion flows into the accretion disk around the black holes of active galactic nuclei (AGN), and the jets which they produce. A large-scale accretion flow around the nucleus is proposed as the thick dusty toroid structure of the unified model. Physically, this is similar to the Kahn ‘cocoon star’ model. High (super-Eddington) accretion rates into the accretion disk are needed. It is assumed that all black holes produce relativistic jets. However, entrainment of thermal (wind) material into the jet determines the AGN class, radio-loud or radio quiet. The jet interacts with the ambient medium as it expands, generating a cocoon of fast radiative or partially-radiative shocks around it. Such a model can explain both the radio properties and the emission line properties over a wide variety of AGN classes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Acceleration and radiation processes around active galactic nuclei

It is proposed that the region containing fast particles, electrostatic and electromagnetic fields, around active galactic nuclei is responsible for generating electromagnetic emissions from -rays to radio waves. The electrons are accelerated by Langmuir turbulence originating through the process of Raman forward scattering (RFS). The radiation mechanism is stimulated Raman backward scattering (RBS) where the fast electron beam loses energy by scattering over spatially periodic magnetic field. The spatially periodic magnetic field results from the magnetic modulational instability of the Langmuir waves. This model accounts well for the large luminosities observed in active galactic nuclei over -rays to radio waves and in addition it relates physically the emission regions at different wavelengths.     

Accretion disc models around compact objects

We present a survey of accretion disc models around compact objects — in particular the accretion onto white dwarfs, neutron stars, and black holes. We discuss both the thin disc as well as thick disc models and also the feaibility where either of these can be applied in the astrophysical systems. The crucial role of magnetic field in facilitating the formation of accretion discs in neutron stars is indicated. The prime significance of accretion discs in the generation of soft and hard X-rays is also discussed. Thick disc models are found to explain the observations of active galactic nuclei and also collimated and persistent jets in some of the radio sources.     

A simplified model of ADAF with the jet driven by the large-scale magnetic field

We propose a simplified model of outflow/jet driven by the Blandford–Payne (BP) process from advection-dominated accretion flows (ADAF) and derive the expressions of the BP power and disk luminosity based on the conservation laws of mass, angular momentum and energy. We fit the 2–10 keV luminosity and kinetic power of 15 active galactic nucleus (AGNs) of sub-Eddington luminosity. It is found that there exists an anti-correlation between the accretion rate and the advection parameter, which could be used to explain the correlation between Eddington-scaled kinetic power and bolometric luminosity of the 15 samples. In addition, the Ledlow–Owen relation for FR I/II dichotomy is re-expressed in a parameter space consisting of logarithm of dimensionless accretion rate versus that of the BH mass. It turns out that the FR I/II dichotomy is determined mainly by the dimensionless accretion rate, being insensitive to the BH mass. And the dividing accretion rate is less than the critical accretion rate for ADAFs, suggesting that FR I sources are all in the ADAF state.     

Electron surfing acceleration in oblique magnetic fields

Initially, inhomogeneous plasma jets, ejected by active galactic nuclei and associated with gamma-ray bursts, are thermalized by the formation of internal shocks. Jet subpopulations can hereby collide at Lorentz factors of a few. As the resulting relativistic shock expands into the upstream plasma, a significant fraction of the upstream ions is reflected. These ions, together with downstream ions that leak through the shock, form relativistic beams of ions that outrun the shock. The thermalization of these beams via the two-stream instability is thought to contribute significantly to plasma heating and particle acceleration by the shock. Here, the capability of a two-stream instability to generate relativistic field-aligned and cross-field electron flow, is examined for a magnetized plasma by means of a particle-in-cell (PIC) simulation. The electrons interact with the developing quasi-electrostatic waves and oblique magnetic fields. The simulation results bring forward evidence that such waves, by their non-linear interactions with the plasma, produce a highly relativistic field-aligned electron flow and electron energies, which could contribute to the radio synchrotron emissions from astrophysical jets, to ultrarelativistic leptonic subpopulations propagating with the jet and to the halo particles surrounding the accretion disc of the black hole.     

Hot accretion flow with ordered magnetic field, outflow, and saturated conduction

The importance of thermal conduction on hot accretion flow is confirmed by observations of hot gas that surrounds Sgr A^? and a few other nearby galactic nuclei. On the other hand, the existence of outflow in accretion flows is confirmed by observations and magnetohydrodynamic (MHD) simulations. In this research, we study the influence of both thermal conduction and outflow on hot accretion flows with ordered magnetic field. Since the inner regions of hot accretion flows are, in many cases, collisionless with an electron mean free path due to Coulomb collision larger than the radius, we use a saturated form of thermal conduction, as is appropriate for weakly collisional systems. We also consider the influence of outflow on accretion flow as a sink for mass, and the radial and the angular momentum, and energy taken away from or deposited into the inflow by outflow. The magnetic field is assumed to have a toroidal component and a vertical component as well as a stochastic component. We use a radially self-similar method to solve the integrated equations that govern the behavior of such accretion flows. The solutions show that with an ordered magnetic field, both the surface density and the sound speed decrease, while the radial and angular velocities increase. We found that a hot accretion flow with thermal conduction rotates more quickly and accretes more slowly than that without thermal conduction. Moreover, thermal conduction reduces the influences of the ordered magnetic field on the angular velocities and the sound speed. The study of this model with the magnitude of outflow parameters implies that the gas temperature decreases due to mass, angular momentum, and energy loss. This property of outflow decreases for high thermal conduction.     

On the excitation of the Fe Kα line in AGNs by the impact of beams of highly relativistic electrons on the disk

The propagation of highly relativistic electron beams in dense matter induces a cascade of secondary particles that spreads in the environment redistributing efficiently the beam energy in the medium. Such a highly relativistic beams are expected to be produced in the magnetic reconnection events associated with the flaring activity of the magnetized accretion disks of the AGNs. This contribution presents a quantitative analysis of the possible role of these beams in the excitation of the Fe Kα line. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Continuing galactic formation: A new concept of galactic evolution

A new theory for galactic arm formation shows the arms to be continually eminating from the galactic nucleus due to a continual influx of cosmic dust. In the neighborhood of the nucleus the problem is treated as a fluid flow and a simple solution is given using conservation of momentum. When rotational dynamics are included the spinning arm system is the result. This solution resolves the problem of the missing mass, accounts for warped disk galaxies and gives a probable source for the gravity waves measured by Weber which eminate from our galactic center. Reversal of arm direction is demonstrated and examples of such reversals are cited. An approximate theoretical estimate of the age of our Sun is found to be in good agreement with radio isotope dating. A general result shows why twin star systems are in such great abundance in a galaxy. It gives a model of galactic evolution which begins with only a single massive nucleus with the collapsing gas clouds forming the arms.     

wham: a WENO-based general relativistic numerical scheme – I. Hydrodynamics

Active galactic nuclei, X-ray binaries, pulsars and gamma-ray bursts are all believed to be powered by compact objects surrounded by relativistic plasma flows driving phenomena such as accretion, winds and jets. These flows are often accurately modelled by the relativistic magnetohydrodynamic (MHD) approximation. Time-dependent numerical MHD simulations have proven to be especially insightful, but one regime that remains difficult to simulate is when the energy scales (kinetic, thermal, magnetic) within the plasma become disparate. We develop a numerical scheme that significantly improves the accuracy and robustness of the solution in this regime. We use a modified form of the weighted essentially non-oscillatory (WENO) method to construct a finite-volume general relativistic hydrodynamics code called wham that converges at fifth order. We avoid (1) field-by-field decomposition by adaptively reducing down to two-point stencils near discontinuities for a more accurate treatment of shocks and (2) excessive reduction to low-order stencils, as in the standard WENO formalism, by maintaining high-order accuracy in smooth monotonic flows. Our scheme performs the proper surface integral of the fluxes, converts cell-averaged conserved quantities to point-conserved quantities before performing the reconstruction step, and correctly averages all source terms. We demonstrate that the scheme is robust in strong shocks, very accurate in smooth flows and maintains accuracy even when the energy scales in the flow are highly disparate.     

Massive black holes in nuclei of galaxies and quasars

In the first part of the paper the known results on the gravitational interaction of a massive black hole with the surrounding stars in a galactic nucleus are discussed. The tidal disruption of stars in close encounters with a black hole is reviewed. Expressions for the flux of stars on a black hole are given, taking into account energy and angular momentum diffusion of stellar orbits. The scenario of star disruption and accretion of the released stellar matter is depicted. The growth of a black hole in a typical galactic nucleus on account of gas accretion from disrupted stars is discussed. A comparison with the upper limit to the luminosity of the nucleus of our Galaxy puts rather severe constraints on the mass of a hypothetical black hole at the galactic centre. Possible mechanisms preventing the formation and growth of black holes in normal galactic nuclei are discussed.The second part of the paper (Section 8) deals with the hypothesis that massive black holes are the primary energy sources in active galaxies and quasars. The luminosity requirements of bright quasars and weak Seyferts can probably be accounted for in such a model, but there are difficulties in explaining the intermediate range. Mass ejection from Seyferts and quasars is not a severe problem. The same applies to the spectrum. A much more serious objection is the observed periodic and quasi-periodic variability. Another unsatisfactory feature of this hypothesis is that one needs two different evolutionary tracks for quasars and active galaxies, and for normal galaxies.     

Driven waves in a two-fluid plasma

We study the physics of wave propagation in a weakly ionized plasma, as it applies to the formation of multifluid, magnetohydrodynamics (MHD) shock waves. We model the plasma as separate charged and neutral fluids which are coupled by ion–neutral friction. At times much less than the ion–neutral drag time, the fluids are decoupled and so evolve independently. At later times, the evolution is determined by the large inertial mismatch between the charged and neutral particles. The neutral flow continues to evolve independently; the charged flow is driven by and slaved to the neutral flow by friction. We calculate this driven flow analytically by considering the special but realistic case where the charged fluid obeys linearized equations of motion. We carry out an extensive analysis of linear, driven, MHD waves. The physics of driven MHD waves is embodied in certain Green functions which describe wave propagation on short time-scales, ambipolar diffusion on long time-scales and transitional behaviour at intermediate times. By way of illustration, we give an approximate solution for the formation of a multifluid shock during the collision of two identical interstellar clouds. The collision produces forward and reverse J shocks in the neutral fluid and a transient in the charged fluid. The latter rapidly evolves into a pair of magnetic precursors on the J shocks, wherein the ions undergo force-free motion and the magnetic field grows monotonically with time. The flow appears to be self-similar at the time when linear analysis ceases to be valid.     

Radio-loudness of active galaxies and the black hole evolution

Active galactic nuclei (AGNs) form two distinct sequences on the radio-loudness–Eddington ratio plane. The ‘upper’ sequence contains radio selected AGNs, the ‘lower’ sequence is composed mainly of optically selected AGNs. The sequences mark the upper bounds for the radio-loudness of two distinct populations of AGNs, hosted, respectively, by elliptical and disk galaxies. Both sequences show the same dependence of the radio-loudness on the Eddington ratio (an increase with decreasing Eddington ratio), which suggests that another parameter in addition to the accretion rate must play a role in determining the efficiency of jet production in AGNs. We speculate that this additional parameter is the spin of the black hole, assuming that black holes in giant elliptical galaxies have (on average) much larger spins than black holes in disc galaxies. Possible evolutionary scenarios leading to such a spin dichotomy are discussed. The galaxy-morphology related radio dichotomy breaks down at high accretion rates where the dominant fraction of luminous quasars being hosted by giant ellipticals is radio-quiet. This indicates that the production of powerful jets at high accretion rates is in most cases suppressed and, in analogy to X-ray binary systems (XRB) during high and very high states, may be intermittent. Such intermittency can be caused by switches between two different accretion modes, assuming that only during one of them an outflow from the central engine is sufficiently collimated to form a relativistic jet.     

3D global simulations of galactic magnetic fields and gas flows

The evolution of three-dimensional (3D), dynamo excited galactic magnetic fields under the influence of a time-dependent gas flow in spiral arms is already well investigated. Our principal goal is to check how the dynamo-driven turbulent magnetic fields affect the gas flows. Numerical solutions of the full set of 3D MHD equations for dynamos in spiral galaxies are presented. Further we try to investigate the nonlinear evolution of magnetic instabilities in a global galactic model. The model includes differential rotation, eddy diffusivity and tensorial alpha-effect. In a first step the flow is driven by a prescribed gravitational potential. The vertical density stratification and the radial-azimutal spiral pattern are taken closely to observational data. We use a modified variant of the highly parallelized time-stepping ZeusMP code for the simulations of global galactic magnetic fields and gas flows. This revised version was published online in August 2006 with corrections to the Cover Date.