Abdo AA, Ackermann M, Ajello M, Atwood WB, Axelsson M, Baldini L, Ballet J, Barbiellini G, Bastieri D, Baughman BM, Bechtol K, Bellazzini R, Berenji B, Blandford R, Bloom ED, Bonamente E, Borgland AW, Bregeon J, Brez A, Brigida M, Bruel P, Burnett TH, Caliandro GA, Cameron RA, Caraveo PA, Casandjian JM, Cavazzuti E, Cecchi C, Celik O, Charles E, Chaty S, Chekhtman A, Cheung CC, Chiang J, Ciprini S, Claus R, Cohen-Tanugi J, Cominsky LR, Conrad J, Corbel S, Corbet R, Cutini S, Dermer CD, De Angelis A, De Luca A, De Palma F, Digel SW, Dormody M, Do Couto E Silva E, Drell PS, Dubois R, Dubus G, Dumora D, Farnier C, Favuzzi C, Fegan SJ, Focke WB, Frailis M, Fukazawa Y, Funk S, Fusco P, Gargano F, Gasparrini D, Gehrels N, Germani S, Giebels B, Giglietto N, Giordano F, Glanzman T, Godfrey G, Grenier IA, Grondin MH, Grove JE, Guillemot L, Guiriec S, Hanabata Y, Harding AK, Hayashida M, Hays E, Hill AB, Hughes RE, Johannesson G, Johnson AS, Johnson RP, Johnson TJ, Johnson WN, Kamae T, Katagiri H, Kataoka J, Kawai N, Kerr M, Knoedlseder J, Kocian ML, Kuehn F, Kuss MW, Lande J, Larsson S, Latronico L, Longo F, Loparco F, Lott B, Lovellette MN, Lubrano P, Madejski GM, Makeev A, Marelli M, Mazziotta MN, Mcenery JE, Meurer C, Michelson PF, Mitthumsiri W, Mizuno T, Monte C, Monzani ME, Morselli A, Moskalenko IV, Murgia S, Nolan PL, Nuss E, Ohsugi T, Okumura A, Omodei N, Orlando E, Ormes JF, Paneque D, Panetta JH, Parent D, Pelassa V, Pepe M, Pesce-Rollins M, Piron F, Porter TA, Raino S, Rando R, Ray PS, Razzano M, Rea N, Reimer A, Reimer O, Reposeur T, Ritz S, Rochester LS, Rodriguez AY, Romani RW, Ryde F, Sadrozinski HFW, Sanchez D, Sander A, Parkinson PMS, Scargle JD, Sgro C, Shaw MS, Sierpowska-Bartosik A, Siskind EJ, Smith DA, Smith PD, Spandre G, Spinelli P, Striani E, Strickman MS, Suson DJ, Tajima H, Takahashi HH, Takahashi T, Tanaka T, Thayer JB, Thayer JG, Thompson DJ, Tibaldo L, Torres DF, Tosti G, Tramacere A, Uchiyama Y, Usher TL, Vasileiou V, Vilchez N, Vitale V, Waite AP, Wang P, Winer BL, Wood KS, Ylinen T, Ziegler M (2009)

Publication Type: Journal article

Publication year: 2009


Book Volume: 701

Pages Range: L123-L128

Journal Issue: 2

DOI: 10.1088/0004-637X/701/2/L123


This Letter presents the first results from the observations of LS I + 61 degrees 303 using Large Area Telescope data from the Fermi Gamma-Ray Space Telescope between 2008 August and 2009 March. Our results indicate variability that is consistent with the binary period, with the emission being modulated at 26.6 +/- 0.5 days. This constitutes the first detection of orbital periodicity in high-energy gamma rays (20 MeV-100 GeV, HE). The light curve is characterized by a broad peak after periastron, as well as a smaller peak just before apastron. The spectrum is best represented by a power law with an exponential cutoff, yielding an overall flux above 100 MeV of 0.82 +/- 0.03(stat) +/- 0.07(syst) 10(-6) ph cm(-2) s(-1), with a cutoff at 6.3 +/- 1.1(stat) +/- 0.4(syst) GeV and photon index Gamma = 2.21 +/- 0.04(stat) +/- 0.06(syst). There is no significant spectral change with orbital phase. The phase of maximum emission, close to periastron, hints at inverse Compton scattering as the main radiation mechanism. However, previous very high-energy gamma ray (>100 GeV, VHE) observations by MAGIC and VERITAS show peak emission close to apastron. This and the energy cutoff seen with Fermi suggest that the link between HE and VHE gamma rays is nontrivial.

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Involved external institutions

National Academy of Sciences US United States (USA) (US) University of California Santa Cruz US United States (USA) (US) Stanford University US United States (USA) (US) University of Montpellier / Université Montpellier FR France (FR) École Polytechnique - Université Paris-Saclay FR France (FR) U.S. Naval Research Laboratory US United States (USA) (US) Italian Space Agency / Agenzia Spaziale Italiana (ASI) IT Italy (IT) Riken Institute of Physical and Chemical Research / 理研 JP Japan (JP) Université Joseph Fourier (UJF Grenoble 1) FR France (FR) Hiroshima University / 広島大学 JP Japan (JP) Politecnico di Bari IT Italy (IT) National Institute for Nuclear Physics / Istituto Nazionale di Fisica Nucleare (INFN) IT Italy (IT) University of Maryland, Baltimore County (UMBC) US United States (USA) (US) University of Maryland US United States (USA) (US) The Oskar Klein Centre SE Sweden (SE) University of Udine / Università degli Studi di Udine IT Italy (IT) National Center for Scientific Research / Centre national de la recherche scientifique (CNRS) FR France (FR) Istituto Universitario Di Studi Superiori Di Pavia IT Italy (IT) Ohio State University US United States (USA) (US) University of Paris 7 - Denis Diderot / Université Paris VII Denis Diderot FR France (FR) University of Alabama in Huntsville (UAH) US United States (USA) (US) Sonoma State University US United States (USA) (US) Institut d'Estudis Espacials de Catalunya (IEEC) ES Spain (ES) University of Washington US United States (USA) (US) Purdue University Northwest / Purdue University Calumet US United States (USA) (US) National Institute for Astrophysics / Istituto Nazionale Astrofisica (INAF) IT Italy (IT) NYCB Real-Time Computing, Inc. US United States (USA) (US) Università degli Studi di Roma 'Tor Vergata' IT Italy (IT) University of Denver US United States (USA) (US) Université Toulouse III - Paul Sabatier FR France (FR) Anton Pannekoek Institute for Astronomy NL Netherlands (NL) University of Tokyo JP Japan (JP) National Aeronautics and Space Administration (NASA) US United States (USA) (US) Waseda University / 早稲田大学 JP Japan (JP) Japan Aerospace eXploration Agency (JAXA) JP Japan (JP) Max-Planck-Institut für extraterrestrische Physik (MPE) / Max Planck Institute for Extraterrestrial Physics DE Germany (DE)

How to cite


Abdo, A.A., Ackermann, M., Ajello, M., Atwood, W.B., Axelsson, M., Baldini, L.,... Ziegler, M. (2009). FERMI LAT OBSERVATIONS OF LS I+61 degrees 303: FIRST DETECTION OF AN ORBITAL MODULATION IN GeV GAMMA RAYS. Astrophysical Journal Letters, 701(2), L123-L128. https://doi.org/10.1088/0004-637X/701/2/L123


Abdo, A. A., et al. "FERMI LAT OBSERVATIONS OF LS I+61 degrees 303: FIRST DETECTION OF AN ORBITAL MODULATION IN GeV GAMMA RAYS." Astrophysical Journal Letters 701.2 (2009): L123-L128.

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