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Associating long-term γ-ray variability with the superorbital period of LS I + 61◦ 303

Ackermann, M. / Ajello, M. / Ballet, J. / Barbiellini, G. / Bastieri, D. / Bellazzini, R. / Bonamente, E. / J. Brandt, T. / Bregeon, J. / Brigida, M. / Bruel, P. / Buehler, R. / Buson, S. / A. Caliandro, G. / A. Cameron, R. / A. Caraveo, P. / M. Casandjian, J. / Cavazzuti, E. / Cecchi, C. / Chekhtman, A. / Chiang, J. / Chiaro, G. / Ciprini, S. / Claus, R. / Cohen-Tanugi, J. / R. Cominsky, L. / Conrad, J. / Cutini, S. / Dalton, M. / D'Ammando, F. / De Angelis, A. / R. Den Hartog, P. / De Palma, F. / D. Dermer, C. / W. Digel, S. / Di Venere, L. / S. Drell, P. / Dubois, R. / Favuzzi, C. / J. Fegan, S. / C. Ferrara, E. / B. Focke, W. / Franckowiak, A. / Funk, S. / Fusco, P. / Gargano, F. / Gasparrini, D. / Germani, S. / Giglietto, N. / Giordano, F. / Giroletti, M. / Glanzman, T. / Godfrey, G. / A. Grenier, I. / Guiriec, S. / Hadasch, D. / Hanabata, Y. / K. Harding, A. / Hayashida, M. / Hays, E. / B. Hill, A. / Horan, D. / E. Hughes, R. / Jogler, T. / J ́ohannesson, G. / Johnson, A.S. / J. Johnson, T. / Kawano, T. / Kerr, M. / Kn ̈odlseder, J. / Kuss, M. / Lande, J. / Larsson, S. / Latronico, L. / Lemoine-Goumard, M. / Li, J. / Longo, F. / N. Lovellette, M. / Lubrano, P. / Mayer, M. / N. Mazziotta, M. / E. Mcenery, J. / F. Michelson, P. / Mizuno, T. / E. Monzani, M. / Morselli, A. / V. Moskalenko, I. / Murgia, S. / Nemmen, R. / Nuss, E. / Ohsugi, T. / Okumura, A. / Orienti, M. / Orlando, E. / F. Ormes, J. / Paneque, D. / Papitto, A. / S. Perkins, J. / Pesce-Rollins, M. / Piron, F. / Pivato, G. / Rain'O, S. / Rando, R. / Razzano, M. / Rea, N. / Reimer, A. / Reimer, O. / D. Scargle, J. / Schulz, A. / Sgr'O, C. / J. Siskind, E. / Spandre, G. / Spinelli, P. / Takahashi, H. / G. Thayer, J. / B. Thayer, J. / Tinivella, M. / F. Torres, D. / Tosti, G. / Troja, E. / Uchiyama, Y. / L. Usher, T. / Vandenbroucke, J. / Vasileiou, V. / Vianello, G. / Vitale, V. / Werner, M. / L. Winer, B. / S. Wood, K. - 2015
Gamma-ray binaries are stellar systems for which the spectral ene rgy distribu- tion (discounting the thermal stellar emission) peaks at high energie s. Detected from radio to TeV gamma rays, the γ -ray binary LS I +61 ◦ 303 is highly variable across all frequencies. One aspect of this system's variability is the modula- tion of its emission with the timescale set by the ∼ 26 . 4960-day orbital period. Here we show that, during the time of our observations, the γ -ray emission of LS I +61 ◦ 303 also presents a sinusoidal variability consistent with the previou sly- known superorbital period of 1667 days. This modulation is more pro minently seen at orbital phases around apastron, whereas it does not intr oduce a visible change close to periastron. It is also found in the appearance and d isappearance of variability at the orbital period in the power spectrum of the data . This be- havior could be explained by a quasi-cyclical evolution of the equator ial outflow of the Be companion star, whose features influence the conditions for generating gamma rays. These findings open the possibility to use γ -ray observations to study the outflows of massive stars in eccentric binary systems.

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