Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optical Spectropolarimetry of the GRB 020813 Afterglow   The optical afterglow of gamma-ray burst 020813 was observed for 3 hours with
the LRIS spectropolarimeter at the Keck-I telescope, beginning 4.7 hours after
the burst was detected by HETE-2. The spectrum reveals numerous metal
absorption lines that we identify with two systems at z=1.223 and z=1.255. We
also detect an O II 3727 emission line at z=1.255 and we identify this galaxy
as the likely host of the GRB. After a correction for Galactic interstellar
polarization, the optical afterglow has a linear polarization of 1.8-2.4%
during 4.7-7.9 hours after the burst. A measurement of p = 0.80% +/- 0.16% on
the following night by Covino et al. demonstrates significant polarization
variability over the next 14 hours. The lack of strong variability in the
position angle of linear polarization indicates that the magnetic field in the
jet is likely to be globally ordered rather than composed of a number of
randomly oriented cells. Within the framework of afterglow models with
collimated flows, the relatively low observed polarization suggests that the
magnetic field components perpendicular and parallel to the shock front are
only different by about 20%.

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The categories of this paper are: bayes-an
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The categories of this paper are: adap-org
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The categories of this paper are: econ
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The categories of this paper are: mtrl-th
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The categories of this paper are: ao-sci
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The categories of this paper are: math-ph
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The categories of this paper are: q-fin
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The categories of this paper are: chao-dyn
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The categories of this paper are: q-alg
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The categories of this paper are: gr-qc
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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: cond-mat
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The categories of this paper are: cmp-lg
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The categories of this paper are: chem-ph
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The categories of this paper are: stat