Baring, M.G.Allafort, A.Baldini, L.Ballet, J.Barbiellini, G.Bastieri, D.Bellazzini, R.Bonamente, E.Bottacini, E.Brandt, T.J.Bregeon, J.Bruel, P.Buehler, R.Buson, S.Caliandro, G.A.Cameron, R.A.Caraveo, P.A.Cecchi, C.Chaves, R.C.G.Chekhtman, A.Chiang, J.Chiaro, G.Ciprini, S.Claus, R.D'Ammando, F.de Palma, F.Digel, S.W.Di Venere, L.Drell, P.S.Favuzzi, C.Ferrara, E.C.Franckowiak, A.Fusco, P.Gargano, F.Gasparrini, D.Giglietto, N.Giroletti, M.Glanzman, T.Godfrey, G.Grenier, I.A.Guiriec, S.Hadasch, D.Harding, A.K.Hayashida, M.Hayashi, K.Hays, E.Hewitt, J.Hill, A.B.Horan, D.Hou, X.Jogler, T.Johnson, A.S.Johnson, T.J.Kerr, M.Knödlseder, J.Kuss, M.Lande, J.Larsson, S.Latronico, L.Lemoine-Goumard, M.Lande, J.Loparco, F.Lubrano, P.Malyshev, D.Marelli, M.Mayer, M.Mazziotta, M.N.Mehault, J.Mizuno, T.Monzani, M.E.Morselli, A.Murgia, S.Nemmen, R.Nuss, E.Ohsugi, T.Omodei, N.Orienti, M.Orlando, E.Paneque, D.Pesce-Rollins, M.Pierbattista, M.Piron, F.Pivato, G.Porter, T.A.Rainò, S.Rando, R.Ray, P.S.Razzano, M.Reimer, O.Reposeur, T.Romani, R.W.Sartori, A.Saz Parkinson, P.M.Sgrò, C.Siskind, E.J.Smith, D.A.Spinelli, P.Strong, A.W.Takahashi, H.Thayer, J.B.Thompson, D.J.Tibaldo, L.Tinivella, M.Torres, D.F.Tosti, G.Uchiyama, Y.Usher, T.L.Vandenbroucke, J.Vasileiou, V.Venter, C.Vianello, G.Vitale, V.Winer, B.L.Wood, K.S.2013-10-182014-04-192013Baring, M.G., Allafort, A., Baldini, L., et al.. "PSR J2021+4026 in the Gamma Cygni Region: The First Variable y-Ray Pulsar Seen by the Fermi Lat." <i>The Astrophysical Journal Letters,</i> 777, no. L2 (2013) The American Astronomical Society: http://dx.doi.org/10.1088/2041-8205/777/1/L2.https://hdl.handle.net/1911/72217Long-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) × 10–10 erg cm–2 s–1 to (6.86 ± 0.13) × 10–10 erg cm–2 s–1. Simultaneously, the frequency spindown rate increased from (7.8 ± 0.1) × 10–13 Hz s–1 to (8.1 ± 0.1) × 10–13 Hz s–1. Significant (>5σ) changes in the pulse profile and marginal (<3σ) 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 γ-ray energies of mode changes and intermittent behavior, observed at radio wavelengths for other pulsars. We argue that the change in pulsed γ-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.engArticle is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.Journal articlegamma raysstars – pulsarsindividual (PSR J2021+4026) – starsneutronhttp://dx.doi.org/10.1088/2041-8205/777/1/L2