939 research outputs found
Interaction of a magnetized shell with an ambient medium: limits on impulsive magnetic acceleration
The interaction of relativistic magnetized ejecta with an ambient medium is
studied for a range of structures and magnetization of the unshocked ejecta. We
particularly focus on the effect of the ambient medium on the dynamics of an
impulsive, high-sigma shell. It is found that for sufficiently high values of
the initial magnetization the evolution of the system is
significantly altered by the ambient medium well before the shell reaches its
coasting phase. The maximum Lorentz factor of the shell is limited to values
well below ; for a shell of initial energy ergs and
size cm expelled into a medium having a uniform density
we obtain in the
high sigma limit. The reverse shock and any internal shocks that might form if
the source is fluctuating are shown to be very weak. The restriction on the
Lorentz factor is more severe for shells propagating in a stellar wind.
Intermittent ejection of small sub-shells doesn't seem to help, as the shells
merge while still highly magnetized. Lower sigma shells start decelerating
after reaching the coasting phase and spreading away. The properties of the
reverse shock then depend on the density profiles of the coasting shell and the
ambient medium. For a self-similar cold shell the reverse shock becomes strong
as it propagates inwards, and the system eventually approaches the self-similar
solution derived recently by Nakamura \& Shigeyama.Comment: 22 pages, 8 figs, post referee versio
Role of Reconnection in AGN Jets
We discuss the possible role of reconnection in electro-magnetically
dominated cores of relativistic AGN jets. We suggest that reconnection may
proceed in a two-fold fashion: initial explosive collapse on the Alfven
time-scale of a current-carrying jet (which is of the order of the light
crossing time) and subsequent slow quasi-steady reconnection. Sites of
explosive collapse are associated with bright knots, while steady-state
reconnection re-energizes particles in the ``bridges'' between the knots.
Ohmic dissipation in reconnection layers leads to particle acceleration
either by inductive electric fields or by stochastic particle acceleration in
the ensuing electro-magnetic turbulence.Comment: 4 pages, Proceedings of the conference "The Physics of Relativistic
Jets in the CHANDRA and XMM Era", 23-27 September 2002, Bologn
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