Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A Two-Step Model for Gamma-Ray Bursts Associated with Supernovae   We here propose a two-step model for gamma-ray bursts (GRBs) associated with
supernovae. In the first step, the core collapse of a star with mass $\ge
19M_\odot$ leads to a massive neutron star and a normal supernova, and
subsequently hypercritical accretion of the neutron star from the supernova
ejecta may give rise to a jet through neutrino annihilation and/or Poynting
flux along the stellar rotation axis. However, because of too much surrounding
matter, this jet rapidly enters a nonrelativistic phase and evolves to a large
bubble. In the second step, the neutron star promptly implodes to a rapidly
rotating black hole surrounded by a torus when the mass of the star increases
to the maximum mass and meanwhile its rotation frequency increases to the upper
limit due to the accreted angular momentum. The gravitational binding energy of
the torus may be dissipated by a magnetized relativistic wind, which may then
be absorbed by the supernova ejecta, thus producing an energetic hypernova. The
rotational energy of the black hole may be extracted by the Blandford-Znajek's
mechanism, leading to another jet. This jet is relatively free of baryons and
thus may be accelerated to an ultrarelativistic phase because the first jet has
pushed out of its front matter and left a baryon-free exit. We expect that the
second jet generates a GRB and its afterglow. Our two-step model may alleviate
the baryon-contamination problem suffered possibly from in the hypernova
models. Furthermore, this model not only accounts for association of several
GRBs with supernovae but also explains well the features of the afterglows of
these bursts.

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The categories of this paper are: q-alg
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The categories of this paper are: supr-con
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The categories of this paper are: alg-geom
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The categories of this paper are: funct-an
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The categories of this paper are: nucl-th
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The categories of this paper are: econ
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The categories of this paper are: solv-int
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The categories of this paper are: q-fin
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The categories of this paper are: chem-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: nucl-ex
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The categories of this paper are: math-ph
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The categories of this paper are: hep-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: physics