Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: SGR 0418+5729 - How does a Young Neutron Star Spin Down to a 9 s Period
  with a Dipole Field less than 10^13 G?   The period derivative bound for SGR 0418+5729 (Rea et al. 2010) establishes
the magnetic dipole moment to be distinctly lower than the magnetar range,
placing the source beyond the regime of isolated pulsar activity in the P -
dP/dt diagram and giving a characteristic age > 2 \times 10^{7} years, much
older than the 10^5 year age range of SGRs and AXPs. So the spindown must be
produced by a mechanism other than dipole radiation in vacuum. A fallback disk
will spin down a neutron star with surface dipole magnetic field in the 10^{12}
G range and initial rotation period P_0 ~ 100 ms to the 9.1 s period of SGR
0418+5729 in a few 10^4 to ~10^5 years. The current upper limit to the period
derivative gives a lower limit of \sim 10^5 years to the age that is not
sensitive to the neutron star's initial conditions. The total magnetic field on
the surface of SGR 0418+5729 could be significantly larger than its 10^{12} G
dipole component.

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The categories of this paper are: physics
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The categories of this paper are: acc-phys
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The categories of this paper are: gr-qc
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The categories of this paper are: econ
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The categories of this paper are: chem-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: math
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The categories of this paper are: stat
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The categories of this paper are: bayes-an
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The categories of this paper are: comp-gas
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The categories of this paper are: cmp-lg
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The categories of this paper are: plasm-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: math-ph
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The categories of this paper are: q-fin
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The categories of this paper are: hep-th