Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Radiation Front Sweeping the Ambient Medium of Gamma-Ray Bursts   Gamma-ray bursts (GRBs) are emitted by relativistic ejecta from powerful
cosmic explosions. Their light curves suggest that the gamma-ray emission
occurs at early stages of the ejecta expansion, well before it decelerates in
the ambient medium. If so, the launched gamma-ray front must overtake the
ejecta and sweep the ambient medium outward. As a result a gap is opened
between the ejecta and the medium that surfs the radiation front ahead.
Effectively, the ejecta moves in a cavity until it reaches a radius
R_{gap}=10^{16}E_{54}^{1/2} cm where E is the isotropic energy of the GRB. At
R=R_{gap} the gap is closed, a blast wave forms and collects the medium swept
by radiation. Further development of the blast wave is strongly affected by the
leading radiation front: the front plays the role of a precursor where the
medium is loaded with e+- pairs and preaccelerated just ahead of the blast. It
impacts the emission from the blast at R < R_{load}=5R_{gap} (the early
afterglow). A spectacular observational effect results: GRB afterglows should
start in optical/UV and evolve fast (< min) to a normal X-ray afterglow. The
early optical emission observed in GRB 990123 may be explained in this way. The
impact of the front is especially strong if the ambient medium is a wind from a
massive progenitor of the GRB. In this case three phenomena are predicted: (1)
The ejecta decelerates at R<R_{load} producing a lot of soft radiation. (2) The
light curve of soft emission peaks at
t_{peak}=40(1+z)E_{54}^{1/2}(Gamma_{ej}/100)^{-2} s where Gamma_{ej} is the
Lorentz factor of the ejecta. Given measured redshift z and t_{peak}, one finds
Gamma_{ej}. (3) The GRB acquires a spectral break at 5 - 50 MeV because harder
photons are absorbed by radiation scattered in the wind.

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The categories of this paper are: alg-geom
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The categories of this paper are: gr-qc
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The categories of this paper are: q-bio
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The categories of this paper are: nucl-ex
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The categories of this paper are: stat
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The categories of this paper are: hep-ex
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The categories of this paper are: cs
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The categories of this paper are: math-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: mtrl-th
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The categories of this paper are: q-alg
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The categories of this paper are: quant-ph
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The categories of this paper are: eess
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-lat