Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: GRB 091208B: First Detection of the Optical Polarization in Early
  Forward Shock Emission of a Gamma-Ray Burst Afterglow   We report that the optical polarization in the afterglow of GRB 091208B is
measured at t = 149 - 706 s after the burst trigger, and the polarization
degree is P = 10.4% +/- 2.5%. The optical light curve at this time shows a
power-law decay with index -0.75 +/- 0.02, which is interpreted as the forward
shock synchrotron emission, and thus this is the first detection of the
early-time optical polarization in the forward shock (rather than that in the
reverse shock reported by Steele et al. (2009). This detection disfavors the
afterglow model in which the magnetic fields in the emission region are random
on the plasma skin depth scales, such as amplified by the plasma instabilities,
e.g., Weibel instability. We suggest that the fields are amplified by the
magnetohydrodynamic instabilities, which would be tested by future observations
of the temporal changes of the polarization degrees and angles for other
bursts.

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The categories of this paper are: acc-phys
7
The categories of this paper are: hep-th
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The categories of this paper are: alg-geom
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The categories of this paper are: funct-an
3
The categories of this paper are: physics
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The categories of this paper are: econ
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The categories of this paper are: eess
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The categories of this paper are: math-ph
31
The categories of this paper are: ao-sci
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The categories of this paper are: chem-ph
9
The categories of this paper are: nlin
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The categories of this paper are: cs
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The categories of this paper are: hep-ex
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The categories of this paper are: hep-lat
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The categories of this paper are: quant-ph
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The categories of this paper are: bayes-an