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 unveiled a sudden decrease in flux above 100 MeV over a timescale shorter than a week. The "jump" was near MJD 55850 (2011 October 16), with the flux decreasing from (8.33 0.08) 10<SUP>-10</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP> to (6.86 0.13) 10<SUP>-10</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>. Simultaneously, the frequency spindown rate increased from (7.8 0.1) 10<SUP>-13</SUP> Hz s<SUP>-1</SUP> to (8.1 0.1) 10<SUP>-13</SUP> Hz s<SUP>-1</SUP>. Significant (>5sigma) changes in the pulse profile and marginal (<3sigma) changes in the emission spectrum occurred at the same time. There is also evidence for a small, steady flux increase over the 3 yr 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.