Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: PSR J2021+4026 in the Gamma Cygni region: the first variable gamma-ray
  pulsar seen by the Fermi LAT   Long-term monitoring of PSR J2021+4026 in the heart of the Cygnus region with
the Fermi Large Area Telescope (LAT) unveiled a sudden decrease in flux above
100 MeV over a time scale shorter than a week. The "jump" was near MJD 55850
(2011 October 16), with the flux decreasing from $(8.33\pm0.08) \times
10^{-10}$ erg cm$^{-2}$ s$^{-1}$ to $(6.86\pm0.13) \times 10^{-10}$ erg
cm$^{-2}$ s$^{-1}$. Simultaneously, the frequency spindown rate increased from
$(7.8\pm 0.1) \times 10^{-13}$ Hz s$^{-1}$ to $(8.1\pm 0.1) \times 10^{-13}$ Hz
s$^{-1}$. Significant (> 5 sigma's) changes in the pulse profile and marginal
(< 3 sigma's) changes in the emission spectrum occurred at the same time. There
is also evidence for a small, steady flux increase over the three years
preceding MJD 55850. This is the first observation at gamma-ray energies of
mode changes and intermittent behavior, observed at radio wavelengths for other
pulsars. We argue that the change in pulsed gamma-ray emission is due to a
change in emission beaming and we speculate that it is precipitated by a shift
in the magnetic field structure, leading to a change of either effective
magnetic inclination or effective current.

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The categories of this paper are: quant-ph
36
The categories of this paper are: cmp-lg
10
The categories of this paper are: q-bio
37
The categories of this paper are: supr-con
33
The categories of this paper are: plasm-ph
2
The categories of this paper are: math
35
The categories of this paper are: mtrl-th
7
The categories of this paper are: hep-th
11
The categories of this paper are: hep-lat
4
The categories of this paper are: cond-mat
22
The categories of this paper are: chao-dyn
12
The categories of this paper are: nucl-th
34
The categories of this paper are: chem-ph
18
The categories of this paper are: econ
21
The categories of this paper are: adap-org
1
The categories of this paper are: cs