Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: GRB 081007 and GRB 090424: the surrounding medium, outflows and
  supernovae   We discuss the results of the analysis of multi-wavelength data for the
afterglows of GRB 081007 and GRB 090424, two bursts detected by Swift. One of
them, GRB 081007, also shows a spectroscopically confirmed supernova, SN
2008hw, which resembles SN 1998bw in its absorption features, while the maximum
luminosity is only about half as large as that of SN 1998bw. Bright optical
flashes have been detected in both events, which allows us to derive solid
constraints on the circumburst-matter density profile. This is particularly
interesting in the case of GRB 081007, whose afterglow is found to be
propagating into a constant-density medium, yielding yet another example of a
GRB clearly associated with a massive star progenitor which did not sculpt the
surroundings with its stellar wind. There is no supernova component detected in
the afterglow of GRB 090424, likely due to the brightness of the host galaxy,
comparable to the Milky Way. We show that the afterglow data are consistent
with the presence of both forward- and reverse-shock emission powered by
relativistic outflows expanding into the interstellar medium. The absence of
optical peaks due to the forward shock strongly suggests that the reverse shock
regions should be mildly magnetized. The initial Lorentz factor of outflow of
GRB 081007 is estimated to be \Gamma ~ 200, while for GRB 090424 a lower limit
of \Gamma > 170 is derived. We also discuss the prompt emission of GRB 081007,
which consists of just a single pulse. We argue that neither the external
forward-shock model nor the shock-breakout model can account for the prompt
emission data and suggest that the single-pulse-like prompt emission may be due
to magnetic energy dissipation of a Poynting-flux dominated outflow or to a
dissipative photosphere.

29
The categories of this paper are: funct-an
16
The categories of this paper are: stat
31
The categories of this paper are: ao-sci
1
The categories of this paper are: cs
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The categories of this paper are: patt-sol
18
The categories of this paper are: econ
23
The categories of this paper are: solv-int
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The categories of this paper are: mtrl-th
22
The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
24
The categories of this paper are: dg-ga
3
The categories of this paper are: physics
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The categories of this paper are: comp-gas
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The categories of this paper are: nucl-ex
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The categories of this paper are: atom-ph
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The categories of this paper are: gr-qc