Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Extended X-ray Halo of the Crab-like SNR G21.5-0.9   Recent XMM-Newton observations reveal an extended (150") low-surface
brightness X-ray halo in the supernova remnant G21.5-0.9. The near circular
symmetry, the lack of any limb brightening and the non-thermal spectral form,
all favour an interpretation of this outer halo as an extension of the central
synchrotron nebula rather than as a shell formed by the supernova blast wave
and ejecta. The X-ray spectrum of the nebula exhibits a marked spectral
softening with radius, with the power-law spectral index varying from Gamma =
1.63 +/- 0.04 in the core to Gamma = 2.45 +/- 0.06 at the edge of the halo.
Similar spectral trends are seen in other Crab-like remnants and reflect the
impact of the synchrotron radiation losses on very high energy electrons as
they diffuse out from the inner nebula. A preliminary timing analysis provides
no evidence for any pulsed X-ray emission from the core of G21.5-0.9.

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The categories of this paper are: q-bio
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The categories of this paper are: q-fin
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The categories of this paper are: patt-sol
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The categories of this paper are: alg-geom
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The categories of this paper are: atom-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: mtrl-th
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The categories of this paper are: solv-int
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The categories of this paper are: chem-ph
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The categories of this paper are: cs
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The categories of this paper are: eess
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The categories of this paper are: cond-mat
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The categories of this paper are: gr-qc
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The categories of this paper are: hep-ex
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The categories of this paper are: chao-dyn
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The categories of this paper are: nucl-ex