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

1 AIM (UMR7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation
2 IRAP - Institut de recherche en astrophysique et planétologie
3 LLR - Laboratoire Leprince-Ringuet
4 INFN, sezione di Bari - Istituto Nazionale di Fisica Nucleare, sezione di Bari
5 IEEC-CSIC
6 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble
7 FERMI
8 LUPM - Laboratoire Univers et Particules de Montpellier

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.

Domaines: Phénomènes cosmiques de haute energie [astro-ph.HE]; Phénomènes cosmiques de haute energie [astro-ph.HE]

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, 2013, 773, pp.L35. ⟨10.1088/2041-8205/773/2/L35⟩. ⟨in2p3-00915658⟩