Magnetic dissipation in the Crab Nebula

Magnetic dissipation is frequently invoked as a way of powering the observed emission of relativistic flows in Gamma Ray Bursts and Active Galactic Nuclei. Pulsar Wind Nebulae provide closer to home cosmic laboratories which can be used to test the hypothesis. To this end, we analyze the observational data on the spindown power of the Crab pulsar, energetics of the Crab nebula, and its magnetic field. We show that unless the magnetic inclination angle of the Crab pulsar is very close to 90 degrees the overall magnetization of the striped wind after total dissipation of its stripes is significantly higher than that deduced in the Kennel-Coroniti model and recent axisymmetric simulations of Pulsar Wind Nebulae. On the other hand, higher wind magnetization is in conflict with the observed low magnetic field of the Crab nebula, unless it is subject to efficient dissipation inside the nebula as well. For the likely inclination angle of 45 degrees the data require magnetic dissipation on the timescale about 80 years, which is short compared to the life-time of the nebula but long compared to the time scale of Crab's gamma-ray flares.Comment: Accepted for publication in MNRA

The remarkable AGN jets

The jets from active galactic nuclei exhibit stability which seems to be far superior compared to that of terrestrial and laboratory jets. They manage to propagate over distances up to a billion of initial jet radii. Yet this may not be an indication of some exotic physics but mainly a reflection of the specific environment these jets propagate through. The key property of this environment is a rapid decline of density and pressure along the jet, which promotes its rapid expansion. Such an expansion can suppress global instabilities, which require communication across the jet, and hence ensure its survival over huge distances. At kpc scales, some AGN jets do show signs of strong instabilities and even turn into plumes. This could be a result of the flattening of the external pressure distribution in their host galaxies or inside the radio lobes. In this regard, we discuss the possible connection between the stability issue and the Fanaroff-Riley classification of extragalactic radio sources. The observations of AGN jets on sub-kpc scale do not seem to support their supposed lack of causal connectivity. When interpreted using simple kinematic models, they reveal a rather perplexing picture with more questions than answers on the jets dynamics.Comment: Invited talk at the AU Symposium No. 324 "New Frontiers in Black Hole Astrophysics", Ljubljana, Slovenia, 201

3+1 Magnetodynamics

The Magnetodynamics, or Force-Free Degenerate Electrodynamics, is recognized as a very useful approximation in studies of magnetospheres of relativistic stars. In this paper we discuss various forms of Magnetodynamic equations which can be used to study magnetospheres of black holes. In particular, we focus on the 3+1 equations which allow for curved and dynamic spacetime.Comment: The revised version. Expanded by including the derivation of the force-free 4-curren

On the inadmissibility of non-evolutionary shocks

In recent years, numerical solutions of the equations of compressible magnetohydrodynamic (MHD) flows have been found to contain intermediate shocks for certain kinds of problems. Since these results would seem to be in conflict with the classical theory of MHD shocks, they have stimulated attempts to reexamine various aspects of this theory, in particular the role of dissipation. In this paper, we study the general relationship between the evolutionary conditions for discontinuous solutions of the dissipation-free system and the existence and uniqueness of steady dissipative shock structures for systems of quasilinear conservation laws with a concave entropy function. Our results confirm the classical theory. We also show that the appearance of intermediate shocks in numerical simulations can be understood in terms of the properties of the equations of planar MHD, for which some of these shocks turn out to be evolutionary. Finally, we discuss ways in which numerical schemes can be modified in order to avoid the appearance of intermediate shocks in simulations with such symmetry

Blandford-Znajek mechanism versus Penrose process

During the three decades since its theoretical discovery the Blandford-Znajek process of extracting the rotational energy of black holes has become one of the foundation stones in the building of modern relativistic astrophysics. However, it is also true that for a long time its physics was not well understood, as evidenced by the controversy that surrounded it since 1990s. Thanks to the efforts of many theorists during the last decade the state of affairs is gradually improving. In this lecture I attempt to explain the key ingredients of this process in more or less systematic, rigorous, and at the same time relatively simple fashion. A particular attention is paid to the similarities and differences between the Blandford-Znajek and Penrose processes. To this purpose I formulate the notion of energy counter flow. The concept of horizon membrane is replaced with the concept of vacuum as an electromagnetically active medium. The effect of negative phase velocity of electromagnetic waves in the black hole ergosphere is also discussed.Comment: To be published in the proceedings of the APCTP Winter School on Black Hole Astrophysics, 24-29 Jan 2008, Daejeon and Pohang, Kore