Wolfgang Kundt

The plane-fronted gravitational waves

This paper is a contribution to the theory of pure radiation in General Relativity. It gives a survey of the radiation fields which possess a twistfree and non-expanding null congruence, and characterizes their subclasses of different Petrov type by geometrical properties. The term “plane-fronted”, intuitive inMaxwells theory, is generalized toEinsteins theory.

ORTHOGONAL DECOMPOSITION OF AXI-SYMMETRIC STATIONARY SPACETIMES

We show that for a wide class of field equations the orbits of the isometry group defining axial symmetry and stationarity admit orthogonal 2-surfaces. The field equations covered by this result include those of a perfect fluid.

Astrophysical Jets and Their Engines

The Sources and Their Models.- The Central Engine.- The Jets.- The Quasar Family - an Introduction and Taxonomy.- Our Galactic Center.- Jets in X-Ray Binaries.- T Tauri Stars, Pre-T Tauri Stars, and Stellar Jets.- The Physics and the Structure of Agn.- Improved Accretion Disk Models of Continuum Emission from Active Galactic Nuclei.- Velocities in Radio Galaxies and Quasars.- Polarization and Magnetic Field Structure.- Numerical Simulation of Jets.- The Gas Dynamics of Jets.- Cosmological Evolution of Active Galaxies and Quasars.- Unified Beaming Models and Compact Radio Sources.- Magnetic Equilibria of Jets.- Stability of Magnetic Jet Equilibria.- Dynamical Effects of Large-Scale Magnetic Fields in Jets.- Particle Acceleration in Astrophysical Jets.- Optical Synchrotron Emission from Radio Hot Spots.- General Relativistic Effects on Collimation of a Jet.- Some Studies on Giant Radio Galaxies.

Hyperbolic Differential Equations in Two Dimensions

We analyze the general two‐dimensional hyperbolic differential equation of second order by means of a substitution method. Our main interest lies in the support of the solutions, i.e., in an answer to the question: under what circumstances can a signal be transmitted along null rays? It turns out that, in general, a signal spreads, i.e., fills the entire future of an event. However, reasonably large classes of differential equations do permit nonspreading (characteristic propagation) solutions. As examples it is shown that multipole solutions of the flat space‐time scalar wave equation and Maxwell equations fall into the non‐spreading class, whereas multipole solutions of the corresponding equations in a curved Schwarzschild background always show spreading (or continuous reflection).

“Golden Oldie”: The Bianchi Classification in the Schücking-Behr Approach

The historical development of the Bianchi classification of homogeneous cosmological models is described with special emphasis on the contributions by Schücking and Behr.

The galactic centre

An attempt is made to present all the relevant observations of our galactic centre and to explain them by means of a working scheme that involves a minimum number ofad hoc assumptions.In this scheme, the central engine is Sgr A*, a supermassive star of some 103M⊙ and surface temperature ≲3.6×104 K in Keplerian rotation, fuelled by the strongly magnetized disk. It drives both a non-thermal (pair-plasma) wind and a thermal wind. Interactions with the central star cluster and with the circumnuclear disk give rise to the thermal vortex Sgr A West and to the non-thermal spill-over bubble Sgr A East. The relativistic pair plasma escapes supersonically through the galactic chimney into the galactic twin jets, as in Seyfert galaxies.

Are supernova explosions driven by magnetic springs

SUPERNOVA explosions are generally believed1 to be powered by the gravitational energy of a collapsing core of (Chandrasekhar) mass ∼ 1.4M\cirċ. There is a difficulty, however, with the radial momentum balance: how is the liberated energy converted into the radial motion of an ejected shell? Neither thermal pressure gradients nor neutrino pressures seem to be sufficient to accelerate matter to the observed several per cent of the velocity of light, even after the 1974 rise in the calculated cross section for resonant neutrino scattering. On the other hand, there is increasing evidence that magnetic neutron stars1–3 form at the centre of (at least a large subclass of) supernovae. I should like to argue that their magnetic pressure is a serious candidate for driving the supernova motion.

Tr¤gheitsbahnen in einem von Gödel angegebenen kosmologischen Modell

ZusammenfassungIm Jahre 1949 wurde von K.Gödel ein kosmologisches Modell als Lösung derEinsteinschen Gravitationsfeldgleichungen mit kosmologischem Glied für verschwindenden Druck und konstante RuhmassendichteΜ angegeben. In der vorliegenden Arbeit sollen hierin die Differentialgleichungen der Geodätischen integriert werden in einem fortlaufenden Ruhinertialsystem einer Materieweltlinie; d. h. es sollen die Bahnen von Probekörpern ermittelt werden, wie sie ein in der felderzeugenden Materie mitschwimmender Beobachter während endlicher Zeitspannen wahrnimmt.

Note on the completeness of spacetimes

Three formally different definitions of completeness for normal hyperbolic manifolds are discussed and shown to be inequivalent.

A model for galactic centres

A rotating Supermassive Magnetized Disk is proposed as a model for all the violent phenomena occurring in the nuclei of galaxies, in the form of quasars, Lacertids, radio galaxies, Seyferts, exploding galaxies, etc. The cold disk feeds a fast-rotating supermassive core (some 103 Schwarzschild radii in extent), which emits (1) an unsteady thermal wind of filamentary geometry, (2) Low-Frequency magnetic Waves, and (3) relativistic electrons and positrons. The latter reach high γ-factors by phase-riding the LFWs, emit synchro-Compton radiation on crossing scattered waves (from γ-ray energies down to radio frequencies), and are eventually focused into two antipodal relativistic beams by their frozen-in toroidal magnetic field. Torsional oscillations between the core and disk give rise to a pulsed injection, and a breathing double-onion shape of the LFW windzone can explain the superluminal jetlike appearance. A big nuclear explosion ends each duty cycle, but many smaller explosions prevent the settling core from collapsing. In this model, the helium production of galactic centres is comparable to the observed cosmic helium.