Magnetic acceleration of relativistic AGN jets

astro-ph/0703146 [abs, ps, pdf, other] :

Title: Magnetic acceleration of relativistic AGN jets


Authors:
S.S.Komissarov,
M.V.Barkov,
N.Vlahakis,
A.Konigl


Comments: Submitted to MNRAS

We present numerical simulations of axisymmetric, magnetically driven
relativistic jets. To eliminate the dissipative effects induced by a free
boundary with an ambient medium we assume that the flow is confined by a rigid
wall of a prescribed shape, which we take to be $z\propto r^a$ (in cylindrical
coordinates, with $a$ ranging from 1 to 3). The outflows are initially cold,
sub-Alfv\'enic and Poynting flux-dominated, with a total--to--rest-mass energy
flux ratio $\mu \sim 15$. We find that in all cases they converge to a steady
state characterized by a spatially extended acceleration region. The
acceleration process is very efficient: on the outermost scale of the
simulation as much as $\sim 77%$ of the Poynting flux has been converted into
kinetic energy flux, and the terminal Lorentz factor approaches its maximum
possible value ($\Gamma_\infty \simeq \mu$). We also find a high collimation
efficiency: all our simulated jets develop a cylindrical core. We argue that
this could be the rule for current-carrying outflows that start with a low
initial Lorentz factor ($\Gamma_0 \sim 1$). Our conclusions on the high
acceleration and collimation efficiencies are not sensitive to the particular
shape of the confining boundary or to the details of the injected current
distribution, and they are qualitatively consistent with the semi-analytic
self-similar solutions derived by Vlahakis K\"onigl. We apply our results to
the interpretation of relativistic jets in AGNs: we argue that they naturally
account for the spatially extended accelerations inferred in these sources
($\Gamma_\infty \ga 10$ attained on radial scales $R\ga 10^{17} {\rm cm}$) and
are consistent with the transition to the matter-dominated regime occurring
already at $R\ga 10^{16} {\rm cm}$.

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