Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Radio-quiet neutron star 1E 1207.4-5209: a possible strong
  Gravitational-wave source   There are four puzzles on 1E 1207.4-5209: (1) the characteristic age of the
pulsar is much higher than the estimated age of the supernova remnant; (2) the
magnetic field inferred from spin-down is significantly different from the
value obtained from the cyclotron absorption lines; (3) the spinning down of
the pulsar is non-monotonic; (4) the magnitude of the frequency's first
derivative varies significantly and its sign is also variable. The third puzzle
can be explained by a wide binary system, with orbital period from 0.2 to 6 yr.
This letter proposes that all four puzzles can be explained naturally by an
ultra-compact binary with orbital period of between 0.5 and 3.3 min. With the
shortest orbital period and a close distance of 2 kpc, the characteristic
amplitude of gravitational waves is $h\sim3\times10^{-21}$. It would be an
excellent source of gravitational-wave detectors such as the Laser
Interferometer Space Antenna.

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The categories of this paper are: nlin
32
The categories of this paper are: atom-ph
22
The categories of this paper are: chao-dyn
27
The categories of this paper are: comp-gas
4
The categories of this paper are: cond-mat
25
The categories of this paper are: patt-sol
29
The categories of this paper are: funct-an
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The categories of this paper are: hep-ex
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The categories of this paper are: supr-con
3
The categories of this paper are: physics
15
The categories of this paper are: nucl-ex
31
The categories of this paper are: ao-sci
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The categories of this paper are: econ
10
The categories of this paper are: q-bio
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The categories of this paper are: quant-ph
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The categories of this paper are: solv-int