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

Markus Ackermann Marco Ajello Jean Ballet 1 Guido Barbiellini D. Bastieri R. Bellazzini E. Bonamente T. J. Brandt 2 J. Bregeon M. Brigida Pascal Bruel 3 R. Buehler S. Buson G. A. Caliandro R. A. Cameron P. A. Caraveo J. M. Casandjian 1 E. Cavazzuti C. Cecchi A. Chekhtman J. Chiang G. Chiaro S. Ciprini R. Claus J. Cohen-Tanugi L. R. Cominsky J. Conrad S. Cutini M. Dalton F. d'Ammando A. de Angelis P. R. den Hartog F. de Palma C. D. Dermer S. W. Digel L. Di Venere P. S. Drell R. Dubois C. Favuzzi S. J. Fegan 3 E. C. Ferrara W. B. Focke A. Franckowiak S. Funk P. Fusco 4 F. Gargano 4 D. Gasparrini S. Germani N. Giglietto F. Giordano M. Giroletti T. Glanzman G. Godfrey I. A. Grenier 1 S. Guiriec D. Hadasch 5 Y. Hanabata A. K. Harding M. Hayashida E. Hays A. B. Hill 6 D. Horan 3 R. E. Hughes T. Jogler G. J ́ohannesson A.S. Johnson T. J. Johnson T. Kawano M. Kerr J. Kn ̈odlseder M. Kuss J. Lande S. Larsson L. Latronico M. Lemoine-Goumard 7 J. Li F. Longo M. N. Lovellette P. Lubrano M. Mayer M. N. Mazziotta J. E. Mcenery P. F. Michelson T. Mizuno M. E. Monzani A. Morselli I. V. Moskalenko S. Murgia R. Nemmen E. Nuss 8 T. Ohsugi A. Okumura M. Orienti E. Orlando J. F. Ormes D. Paneque A. Papitto J. S. Perkins M. Pesce-Rollins F. Piron 8 G. Pivato S. Rain'O R. Rando M. Razzano N. Rea A. Reimer O. Reimer J. D. Scargle A. Schulz C. Sgr'O E. J. Siskind G. Spandre P. Spinelli H. Takahashi J. G. Thayer J. B. Thayer M. Tinivella D. F. Torres G. Tosti E. Troja Y. Uchiyama T. L. Usher J. Vandenbroucke V. Vasileiou 8 G. Vianello V. Vitale M. Werner B. L. Winer K. S. Wood
Abstract : 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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Submitted on : Monday, December 9, 2013 - 10:57:55 AM
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Markus Ackermann, Marco Ajello, Jean Ballet, Guido Barbiellini, D. Bastieri, et al.. Associating long-term γ-ray variability with the superorbital period of LS I + 61◦ 303. The Astrophysical journal letters, Bristol : IOP Publishing, 2013, 773, pp.L19. ⟨10.1088/2041-8205/773/2/L19⟩. ⟨in2p3-00915658⟩

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