Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Magnetic Collapse of a Neutron Gas: No Magnetar Formation   A degenerate neutron gas in equilibrium with a background of electrons and
protons in a magnetic field exerts its pressure anisotropically, having a
smaller value perpendicular than along the magnetic field. For critical fields
the magnetic pressure may produce the vanishing of the equatorial pressure of
the neutron gas, and the outcome could be a transverse collapse of the star.
This fixes a limit to the fields to be observable in stable pulsars as a
function of their density. The final structure left over after the implosion
might be a mixed phase of nucleons and meson ($\pi^{\pm,0},\kappa^{\pm,0}$)
condensate (a strange star also likely) or a black string, but no magnetar at
all.

2
The categories of this paper are: math
29
The categories of this paper are: funct-an
9
The categories of this paper are: nlin
8
The categories of this paper are: hep-ex
13
The categories of this paper are: quant-ph
25
The categories of this paper are: patt-sol
36
The categories of this paper are: cmp-lg
14
The categories of this paper are: math-ph
22
The categories of this paper are: chao-dyn
4
The categories of this paper are: cond-mat
12
The categories of this paper are: nucl-th
26
The categories of this paper are: bayes-an
20
The categories of this paper are: acc-phys
28
The categories of this paper are: alg-geom
18
The categories of this paper are: econ
1
The categories of this paper are: cs