Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Superradiance and stimulated scattering in SNR 1987A   The rings observed around supernova remnant 1987A are emitted by a plasma
mainly made of ionized and neutral hydrogen atoms. With a density of 10 power
10 atoms per cubic metre, and at least a dimension of plasma of 0.01
light-year, the column density is 10 power 24 atoms per square metre, much more
than needed for an optically thick gas at Lyman frequencies (Case B). While, at
10000 K, the bulky gas would absorb Lyman lines fully, at 50000K it emits
superradiant lines. As superradiance de-excites the atoms strongly, nearly all
available energy is emitted in a few competing modes: Superradiance appears
only for beams which cross the largest column densities; for an observation
from Earth, these beams generate three elliptical hollow cylinders whose bases
are the observed rings; admitting that the Earth is not in a privileged
direction, these cylinders envelope ellipsoidal shells observed, for the
external rings, by photon echoes. For the equatorial ring, the brightness of
the superradiant beams is multiplied by a quasi-resonant induced scattering of
the rays emitted by the star. The de-excitation of atoms by emitted beams cools
the gas strongly, so that ionization decreases fast, the process self
accelerates. The energy of the high radiance rays from the star is strongly
scattered to the ring while the low radiance of the glow which surrounds the
star is rather amplified. The fast absorption of the radial beams produces the
radial decrease of radiance of the ring while a competition of modes produces
the pearls.

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The categories of this paper are: cond-mat
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The categories of this paper are: econ
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The categories of this paper are: cmp-lg
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The categories of this paper are: astro-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: hep-ex
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The categories of this paper are: acc-phys
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The categories of this paper are: chem-ph
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The categories of this paper are: solv-int
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The categories of this paper are: nlin
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The categories of this paper are: mtrl-th
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The categories of this paper are: eess
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The categories of this paper are: adap-org
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The categories of this paper are: hep-ph
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The categories of this paper are: math-ph
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The categories of this paper are: math