Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Binary Neutron Star (BNS) merger: What we learned from relativistic
  ejecta of GW/GRB~170817A   Gravitational waves from coalescence of a Binary Neutron Star (BNS) and its
accompagning short Gamma-Ray Burst GW/GRB~170817A confirmed the presumed origin
of these puzzeling transients and opened up the way for relating properties of
short GRBs to those of their progenitor stars and their surroundings. Here we
review an extensive analysis of the prompt gamma-ray and late afterglows of
this event. We show that a fraction of polar ejecta from the merger had been
accelerated to ultra-relativistic speeds. This structured jet had an initial
Lorentz factor of about $260$ in our direction - $\mathcal{O}(10^\circ)$ from
the jet's axis - and was a few orders of magnitude less dense than in typical
short GRBs. At the time of arrival to circum-burst material the
ultra-relativistic jet had a close to Gaussian profile and a Lorentz factor
$\gtrsim 130$ in its core. It had retained in some extent its internal
collimation and coherence, but had extended laterally to create mildly
relativistic lobes - a {\it cocoon}. External shocks on the far from center
inhomogeneous circum-burst material and low density of colliding shells
generated slow rising afterglows. The circum-burst material was somehow
correlated with the merger and it is possible that it contained recently
ejected material from glitching, which had resumed due to the deformation of
neutron stars crust by tidal forces in the latest stages of inspiral but well
before their merger. By comparing these findings with the results of
relativistic MHD simulations and observed gravitational waves we conclude that
progenitor neutron stars were old, had close masses and highly reduced magnetic
fields. In addition, they probably had oppositely directed spins due to the
encounter and gravitational interaction with other stars.

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The categories of this paper are: hep-th
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The categories of this paper are: q-bio
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The categories of this paper are: math-ph
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The categories of this paper are: hep-ex
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The categories of this paper are: q-fin
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The categories of this paper are: acc-phys
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The categories of this paper are: atom-ph
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The categories of this paper are: econ
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The categories of this paper are: nlin
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The categories of this paper are: cs
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The categories of this paper are: gr-qc
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The categories of this paper are: physics
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The categories of this paper are: chao-dyn
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The categories of this paper are: supr-con
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The categories of this paper are: math
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The categories of this paper are: eess