Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Torsional Oscillations of a Magnetar with a Tangled Magnetic Field   Motivated by stability considerations and observational evidence, we argue
that magnetars possess highly-tangled internal magnetic fields. We propose that
the quasi-periodic oscillations (QPOs) seen to accompany giant flares can be
explained as torsional modes supported by a tangled magnetic field, and we
present a simple model that supports this hypothesis for SGR 1900+14. Taking
the strength of the tangle as a free parameter, we find that the magnetic
energy in the tangle must dominate that in the dipolar component by a factor of
$\sim 14$ to accommodate the observed 28 Hz QPO. Our simple model provides
useful scaling relations for how the QPO spectrum depends on the bulk
properties of the neutron star and the tangle strength. The energy density in
the tangled field inferred for SGR 1900+14 renders the crust nearly dynamically
irrelevant, a significant simplification for study of the QPO problem. The
predicted spectrum is about three times denser than observed, which could be
explained by preferential mode excitation or beamed emission. We emphasize that
field tangling is needed to stabilize the magnetic field, so should not be
ignored in treatment of the QPO problem.

2
The categories of this paper are: math
35
The categories of this paper are: mtrl-th
31
The categories of this paper are: ao-sci
29
The categories of this paper are: funct-an
33
The categories of this paper are: plasm-ph
18
The categories of this paper are: econ
17
The categories of this paper are: q-fin
8
The categories of this paper are: hep-ex
19
The categories of this paper are: eess
34
The categories of this paper are: chem-ph
30
The categories of this paper are: q-alg
16
The categories of this paper are: stat
4
The categories of this paper are: cond-mat
0
The categories of this paper are: hep-ph
26
The categories of this paper are: bayes-an
11
The categories of this paper are: hep-lat