Narrow Results

We model the propagation of pulsars through the inhomogeneous ISM using nonrelativistic axisymmetric magneto-hydrodynamic (MHD) simulations. We take into account the wind from the star, which carries predominantly azimuthal magnetic field, and investigate the PWN at different levels of magnetization (the ratio of magnetic to matter energy-densities) in the wind. We consider the interaction of PWN with large-scale and small-scale imhomogeneities in the ISM at different values of magnetization. We conclude that the inhomogeneities in the ISM can change the shapes of the bow shocks and magnetotails at different values of the magnetization.

We present the results of MHD simulation of the laboratory experiment for creating plasma jets. In the experiment on a NEODIM laser installation the plasma jet is formed as a result of the action of a powerful laser on the target. We simulated plasma flow, we chose a numerical method, boundary and initial conditions. We investigated the picture of the flow and compared it with the experiment.

Many neutron stars propagate through the interstellar medium with supersonic velocities, and their magnetospheres interact with the interstellar medium (ISM), forming bow shocks and magnetotails. Using numerical MHD simulations, we investigated the propagation of a magnetized neutron stars through a non-uniform ISM, the interaction of the magnetospheres with the ISM and the influence of ISM density on the shape of the magnetosphere tail. We consider the interaction of magnetized neutron stars with small-scale and large-scale inhomogeneities in the ISM. We conclude that the inhomogeneities in the ISM can change the shapes of the bow shocks and magnetotails at different values of the magnetization.

We consider first order perturbations on locally rotationally symmetric (LRS) class II spacetimes. In particular, we investigate the interactions between electromagnetic, gravitational, and plasma related perturbations when allowing for a non-zero magnetic field on the background spacetime. In doing so we focus on mechanisms for generating and magnifying the magnetic fields, as these kinds of mechanisms may have some relevance when trying to explain the origin of the observed large scale cosmic magnetic fields. The equations governing the behavior of the perturbations are gathered from the Ricci identities for certain preferred vector fields, the Bianchi identities, Maxwell’s equations, and from relations describing energy-momentum conservation and particle conservation for the cosmological plasma. After employing a simplified cold magnetohydrodynamic (MHD) description, and harmonically decomposing the spatial dependencies, we arrive at a closed set of ordinary differential equations in time. On analyzing this system, which decouples into an even and an odd subsector, we indeed observe possible mechanisms for generating magnetic field perturbations to first order.

Liquid metal first wall is attractive in fusion reactor designs because of its high heat removal and self-refreshment capabilities. In liquid wall study, method of forming stable liquid flows on the front surface of blanket has to be found. In order to do this, free surface magneto-hydrodynamic (MHD) effects and flow velocity distributions of liquid metal under gravity have been studied. In our study, liquid metal flows down along ducts half-opened to face the plasma. Net electromagnetic force forms from induced eddy current interacting with the confinement magnetic field (12T) in the liquid metal flow. For liquid metal lithium (about 4cm thick), distributions of velocity along the flow direction have been obtained by combined calculations of free surface flow and electromagnetic analysis. The results show that MHD baffle might be used to get stable in front of the blanket.

A variety of topics related to our understanding of turbulence and angular momentum transport in astrophysical accretion disks are discussed, including (1) new numerical algorithms for magnetohydrodynamics required to study these processes, (2) turbulence and the decay of vortices in hydrodynamic disks, (3) MHD turbulence driven by the magnetorotational instability (MRI), and (4) studying the MRI through laboratory experiments. A brief outline of some of the outstanding challenges in understanding MHD turbulence in astrophysical disks is given.

Recent observations and theoretical considerations have linked gamma-ray bursts with ultra-bright type Ibc supernovae (‘hypernovae’). We here work out a specific scenario for this connection. Based on earlier work, we argue that especially the longest bursts must be powered by the Blandford–Znajek mechanism of electromagnetic extraction of spin energy from a black hole. Such a mechanism requires a high angular momentum in the progenitor object. The observed association of gamma-ray bursts with type Ibc supernovae leads us to consider massive helium stars that form black holes at the end of their lives as progenitors. In our analysis we combine the numerical work of MacFadyen and Woosley with analytic calculations in Kerr geometry, to show that about 10⁵³ erg each are available to drive the fast GRB ejecta and the supernova. The GRB ejecta are driven by the power output through the open field lines threading the black hole, whereas the supernova can be powered both by the shocks driven into the envelope by the jet, and by the power delivered into the disk via field lines connecting the disk with the black hole. We also present a much simplified approximate derivation of these energetics.

Helium stars that leave massive black-hole remnants can only be made in fairly specific binary evolution scenarios, namely the kind that also leads to the formation of soft X-ray transients with black-hole primaries, or in very massive WNL stars. Since the binary progenitors will inevitably possess the high angular momentum we need, we propose a natural link between balck-hole transients and gamma-ray bursts. Recent observations of one such transient, GRO J1655-40/Nova Scorpii 1994, explicitly support this connection: its high space velocity indicates that substantial mass was ejected in the formation of the black hole, and the overabundance of α-nuclei, especially sulphur, indicates that the explosion energy was extreme, as in SN 1998bw/GRB 980425. Furthermore, X-ray studies of this object indicate that the black hole may still be spinning quite raidly, as expected in our model. We also show that the presence of a disk during the powering of the GRB and the explosion is required to deposit enough of the α nuclei on the companion. © 2000 Elsevier Science B.V. All rights reserved.

This paper presents a computational scheme for compressible magnetohydrodynamics (MHD). The scheme is based on the same elements that make up many modern compressible gas dynamics codes: a high-resolution upwinding based on an approximate Riemann solver for MHD and limited reconstruction; an optimally smoothing multi-stage time-stepping scheme; and solution-adaptive refinement and coarsening. The pieces of the scheme are described, and the scheme is validated and its accuracy assessed by comparison with exact solutions. A domain-decomposition-based parallelization of the code has been carried out; parallel performance on a number of architectures is presented.

Very Long Baseline Interferometry (VLBI) at sub-millimeter/millimeter wavelengths shows promise at resolving the silhouette of the supermassive black hole at the Galactic Center, Sagittarius A* (Sgr A*), in the near future. In order to accurately test theoretical models of Sgr A* using these observations, a direct comparison of VLBI data to numerical models must be made. We present calculated images and spectra of Sgr A* using accretion disk simulation data from general relativistic magnetohydrodynamics (GRMHD) evolutions. Synchrotron and bremsstrahlung emission models are considered in the optically thin limit, which allows us to solve the radiative transfer equations using only simulation and geodesic data in a post-processing step. We show predictions of millimeter observations at the expected angular resolution limit and the spectrum's variability.

Many astrophysical sources of high energy emission, such as black hole magnetospheres, superstrongly magnetized neutron stars (magnetars), and probably relativistic jets in Active Galactic Nuclei and Gamma Ray Bursts involve relativistically magnetically dominated plasma. In such plasma the energy density of magnetic field greatly exceeds the thermal and the rest mass energy density of particles. Therefore the magnetic field is the main reservoir of energy and its dissipation may power the bursting emission from these sources, in close analogy to Solar flares. One of the principal dissipative instabilities that may lead to release of magnetic energy is the tearing instability. In this paper we study (see also¹⁰), both analytically and numerically, the development of tearing instability in relativistically magnetically-dominated plasma using the framework of resistive magnetodynamics. We confirm and elucidate the previously obtained result on the growth rate of the tearing mode: the shortest growth time is the same as in the case of classical non-relativistic MHD, namely where τ_a is the Alfvén crossing time and τ_d is the resistive time of a current layer. The reason for this coincidence is the close similarity between the governing equations, especially in the quasi-equilibrium approximation. In particular, the role of the mass density of nonrelativistic MHD is played by the mass-energy density of the magnetic field,ρ = B²/8πc².

This contribution is a review of some talks presented at the session “MHD processes near compact objects” of the Fourteenth Marcel Grossmann Meeting MG14. We discuss the contemporary developments of MHD processes that occur near black holes and neutron stars. The influence of magneto-plasma processes on the structure of the compact objects and accretion processes is considered.

Using numerical MHD simulations, we investigated the motion of a magnetized neutron star through a non-uniform interstellar medium (ISM), the interaction of the magnetosphere with the ISM and the influence of ISM density on the shape of the magnetosphere tail.

We show that the removal of angular momentum is possible in the presence of large scale magnetic stresses, arisen by fields much stronger than that required for magnetorotational instability, in geometrically thick, advective, sub-Keplerian accretion flows around black holes in steady-state, in the complete absence of alpha-viscosity. The efficiency of such angular momentum transfer via Maxwell stress, with the field well below its equipartition value, could be equivalent to that of alpha-viscosity, arisen via Reynolds stress, with α = 0.01 − 0.08. We find in our simpler vertically averaged advective disk model that stronger the magnetic field and/or larger the vertical-gradient of azimuthal component of magnetic field, stronger the rate of angular momentum transfer is, which in turn may lead to a faster rate of outflowing matter, which has important implications to describe the hard spectral states of black hole sources. When the generic origin of alpha-viscosity is still being explored, mechanism of efficient angular momentum transfer via magnetic stresses alone is very interesting.

We search for low-dimensional chaotic signatures in the optical lightcurve of the Kepler field blazar W2R 1926+42. The frequently used correlation integral method is employed in our analysis. We find no apparent evidence for the presence of low-dimensional chaos in the lightcurve. If further confirmed, these results could be of importance for modeling the blazar emission mechanisms.

Our consistent effort to unravel the mystery of super-Chandrasekhar white dwarfs (WDs), by exploiting the potential of magnetic fields, has brought this topic considerable attention. This is also evident from the recent surge in the corresponding literature. In the present work, by means of full-scale general relativistic magnetohydrodynamic (GRMHD) numerical analysis, we confirm the existence of stable, highly magnetized, significantly super-Chandrasekhar WDs having mass exceeding 3 solar mass. We have explored various possible field configurations, namely, poloidal, toroidal and mixed, by self-consistently incorporating the departure from spherical symmetry induced by a strong magnetic field. Such super-Chandrasekhar WDs can be ideal progenitors of peculiar, over-luminous type Ia supernovae.

Over the last few decades, developing efficient iterative methods for solving discretized partial differential equations (PDEs) has been a topic of intensive research. Though these efforts have yielded many mathematically optimal solvers, such as the multigrid method, the unfortunate reality is that multigrid methods have not been used much in practical applications. This marked gap between theory and practice is mainly due to the fragility of traditional multigrid methodology and the complexity of its implementation. This paper aims to develop theories and techniques that will narrow this gap. Specifically, its aim is to develop mathematically optimal solvers that are robust and easy to use for a variety of problems in practice. One central mathematical technique for reaching this goal is a general framework called the Fast Auxiliary Space Preconditioning (FASP) method. FASP methodology represents a class of methods that (1) transform a complicated system into a sequence of simpler systems by using auxiliary spaces and (2) produces an efficient and robust preconditioner (to be used with Krylov space methods such as CG and GMRes) in terms of efficient solvers for these simpler systems. By carefully making use of the special features of each problem, the FASP method can be efficiently applied to a large class of commonly used partial differential equations including equations of Poisson, diffusion-convection-reaction, linear elasticity, Stokes, Brinkman, Navier-Stokes, complex fluids models, and magnetohydrodynamics. This paper will give a summary of results that have been obtained mostly by the author and his collaborators on this topic in recent years.

Unsteady mixed convection flow under the influence of gravity modulation and magnetic field has been investigated. The conducting fluid flows past a vertical porous plate of infinite length in a porous medium subjected to oscillating suction and temperature. The solution has been obtained by using regular perturbation method. Velocity profiles, temperature profiles, skin friction and heat transfer coefficients have been derived and shown graphically. It is noted that the fluid flow and heat transfer are significantly affected by gravity modulation.

Using a fully general relativistic implementation of ideal magnetohydrodynamics with no assumed symmetries in three spatial dimensions, the dynamics of magnetized, rigidly rotating neutron stars are studied. Beginning with fully consistent initial data constructed with Magstar, part of the Lorene project, we study the dynamics and stability of rotating, magnetized polytropic stars as models of neutron stars. Evolutions suggest that some of these rotating, magnetized stars may be minimally unstable occurring at the threshold of black hole formation.

We study polar Alfvén oscillations of relativistic stars endowed with a strong global poloidal dipole magnetic field. Here we focus only on the axisymmetric oscillations which are studied by evolving numerically the two-dimensional perturbation equations. Our study shows that the spectrum of the polar Alfvén oscillations is discrete in contrast to the spectrum of axial Alfvén oscillations which is continuous. We also show that the typical fluid modes, such as the f and p modes, are not significantly affected by the presence of the strong magnetic field.

We investigate numerically the spinning-down of magnetars rotating at the propeller stage and propagating supersonically through the interstellar medium. Fast moving magnetars lose angular momentum due to direct interaction of the rotating magnetospheres with the non-rotating interstellar medium. Simulations have shown that magnetars spin-down very fast due to such interaction, much faster compared to non-moving ones.