Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Diffusion in Pulsar Wind Nebulae: An Investigation using
  Magnetohydrodynamic and Particle Transport Models   We study the transport of high-energy particles in pulsar wind nebulae (PWN)
using three-dimensional MHD (see Porth et al. (2014) for details) and
test-particle simulations, as well as a Fokker-Planck particle transport model.
The latter includes radiative and adiabatic losses, diffusion, and advection on
the background flow of the simulated MHD nebula. By combining the models, the
spatial evolution of flux and photon index of the X-ray synchrotron emission is
modelled for the three nebulae G21.5-0.9, the inner regions of Vela, and 3C 58,
thereby allowing us to derive governing parameters: the magnetic field
strength, average flow velocity and spatial diffusion coefficient. For
comparison, the nebulae are also modelled with the semi-analytic Kennel &
Coroniti (1984) model but the Porth et al. (2014) model generally yields better
fits to the observational data. We find that high velocity fluctuations in the
turbulent nebula (downstream of the termination shock) give rise to efficient
diffusive transport of particles, with average P\'eclet number close to unity,
indicating that both advection and diffusion play an important role in particle
transport. We find that the diffusive transport coefficient of the order of
$\sim2\times 10^{27} (L_{\rm s}/0.42\rm Ly) cm^{2}s^{-1}$ ($L_{\rm s}$ is the
size of the termination shock) is independent of energy up to extreme particle
Lorentz factors of $\gamma_{p}\sim10^{10}$.

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The categories of this paper are: ao-sci
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The categories of this paper are: q-alg
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The categories of this paper are: alg-geom
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The categories of this paper are: adap-org
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The categories of this paper are: chao-dyn
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The categories of this paper are: quant-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-lat
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The categories of this paper are: eess
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The categories of this paper are: funct-an
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The categories of this paper are: nucl-ex
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The categories of this paper are: nlin
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-ex
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The categories of this paper are: atom-ph
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The categories of this paper are: bayes-an