  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.
