Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Resolving the Crab pulsar wind nebula at teraelectronvolt energies   The Crab nebula is one of the most studied cosmic particle accelerators,
shining brightly across the entire electromagnetic spectrum up to very
high-energy gamma rays. It is known from radio to gamma-ray observations that
the nebula is powered by a pulsar, which converts most of its rotational energy
losses into a highly relativistic outflow. This outflow powers a pulsar wind
nebula (PWN), a region of up to 10~light-years across, filled with relativistic
electrons and positrons. These particles emit synchrotron photons in the
ambient magnetic field and produce very high-energy gamma rays by Compton
up-scattering of ambient low-energy photons. While the synchrotron morphology
of the nebula is well established, it was up to now not known in which region
the very high-energy gamma rays are emitted. Here we report that the Crab
nebula has an angular extension at gamma-ray energies of 52 arcseconds
(assuming a Gaussian source width), significantly larger than at X-ray
energies. This result closes a gap in the multi-wavelength coverage of the
nebula, revealing the emission region of the highest energy gamma rays. These
gamma rays are a new probe of a previously inaccessible electron and positron
energy range. We find that simulations of the electromagnetic emission
reproduce our new measurement, providing a non-trivial test of our
understanding of particle acceleration in the Crab nebula.

9
The categories of this paper are: nlin
10
The categories of this paper are: q-bio
13
The categories of this paper are: quant-ph
19
The categories of this paper are: eess
7
The categories of this paper are: hep-th
12
The categories of this paper are: nucl-th
14
The categories of this paper are: math-ph
36
The categories of this paper are: cmp-lg
1
The categories of this paper are: cs
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The categories of this paper are: nucl-ex
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The categories of this paper are: ao-sci
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The categories of this paper are: mtrl-th
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The categories of this paper are: chao-dyn
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The categories of this paper are: gr-qc
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The categories of this paper are: alg-geom
2
The categories of this paper are: math