Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Variability in Short Gamma-ray Bursts: Gravitationally Unstable Tidal
  Tails   Short gamma-ray bursts are thought to result from the mergers of two neutron
stars or a neutron star and stellar mass black hole. The final stages of the
merger are generally accompanied by the production of one or more tidal "tails"
of ejecta, which fall back onto the remnant-disc system at late times. Using
the results of a linear stability analysis, we show that if the material
comprising these tails is modeled as adiabatic and the effective adiabatic
index satisfies $\gamma \ge 5/3$, then the tails are gravitationally unstable
and collapse to form small-scale knots. We analytically estimate the properties
of these knots, including their spacing along the tidal tail and the total
number produced, and their effect on the mass return rate to the merger
remnant. We perform hydrodynamical simulations of the disruption of a
polytropic (with the polytropic and adiabatic indices $\gamma$ equal), $\gamma
=2$ neutron star by a black hole, and find agreement between the predictions of
the linear stability analysis and the distribution of knots that collapse out
of the instability. The return of these knots to the black hole induces
variability in the fallback rate, which can manifest as variability in the
lightcurve of the GRB and -- depending on how rapidly the instability operates
-- the prompt emission. The late-time variability induced by the return of
these knots is also consistent with the extended emission observed in some
GRBs.

17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex
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The categories of this paper are: alg-geom
20
The categories of this paper are: acc-phys
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The categories of this paper are: eess
31
The categories of this paper are: ao-sci
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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: bayes-an
2
The categories of this paper are: math
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The categories of this paper are: cmp-lg
1
The categories of this paper are: cs
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The categories of this paper are: cond-mat
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The categories of this paper are: math-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: stat