676 research outputs found
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
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