Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Particle Acceleration in Supernova Remnants and Pulsar Wind Nebulae   While supernova remnants (SNRs) have long been considered prime candidates
for the source of cosmic rays, at least to energies up to ~10^14 eV, it is only
over the past several years that direct evidence of such energetic particles in
SNRs has been uncovered. X-ray observations of several shell-type SNRs have now
revealed sites dominated by nonthermal emission, indicating an electron
population whose energy extends far beyond the thermal distribution typical of
such SNRs. In other remnants, discrepancies between the shock velocity and the
electron temperature points to a strong cosmic ray component that has
essentially thrived at the expense of the thermal component of the gas.
Modeling of the radio, X-ray, and gamma-ray emission provides strong
constraints on the acceleration mechanism as well as the properties of the
ambient medium in which the mechanism prospers. In the innermost regions of
some SNRs, particle acceleration is taking place over much different scales.
The formation of Crab-like pulsar wind nebulae (PWNe) is understood to require
the presence of a termination shock at which the relativistic pulsar wind is
forced to join the slow expansion of the outer nebula. While the acceleration
mechanism is necessarily different, these shocks also act as sites in which
particles are boosted to high energies. In the Crab Nebula, optical wisps mark
the location of this termination shock. Recent X-ray observations have begun to
reveal the termination shock zones in other PWNe, and are now allowing us to
constrain the nature of the pulsar wind as well as the flow conditions in the
outer nebula. Here I present a summary of the properties of shock acceleration
in these two distinct regions of SNRs, and review recent observational results
in which the properties of the shocks are finally being revealed.

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The categories of this paper are: q-alg
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The categories of this paper are: solv-int
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The categories of this paper are: eess
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The categories of this paper are: nucl-ex
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The categories of this paper are: physics
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The categories of this paper are: adap-org
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The categories of this paper are: acc-phys
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The categories of this paper are: plasm-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: econ
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The categories of this paper are: math-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: supr-con
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The categories of this paper are: nucl-th
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The categories of this paper are: cmp-lg
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The categories of this paper are: hep-ph