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Originally published in Science Express on 18 May 2006
Science 23 June 2006:
Vol. 312. no. 5781, pp. 1771 - 1773
DOI: 10.1126/science.1128177
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Reports

Variable Very-High-Energy Gamma-Ray Emission from the Microquasar LS I +61 303

J. Albert,1 E. Aliu,2 H. Anderhub,3 P. Antoranz,4 A. Armada,2 M. Asensio,4 C. Baixeras,5 J. A. Barrio,4 M. Bartelt,6 H. Bartko,7 D. Bastieri,8 S. R. Bavikadi,9 W. Bednarek,10 K. Berger,1 C. Bigongiari,8 A. Biland,3 E. Bisesi,9 R. K. Bock,7 P. Bordas,11 V. Bosch-Ramon,11 T. Bretz,1 I. Britvitch,3 M. Camara,4 E. Carmona,7 A. Chilingarian,12 S. Ciprini,13 J. A. Coarasa,7 S. Commichau,3 J. L. Contreras,4 J. Cortina,2 V. Curtef,6 V. Danielyan,12 F. Dazzi,8 A. De Angelis,9 R. de los Reyes,4 B. De Lotto,9 E. Domingo-Santamaría,2 D. Dorner,1 M. Doro,8 M. Errando,2 M. Fagiolini,14 D. Ferenc,15 E. Fernández,2 R. Firpo,2 J. Flix,2 M. V. Fonseca,4 L. Font,5 M. Fuchs,7 N. Galante,14 M. Garczarczyk,7 M. Gaug,8 M. Giller,10 F. Goebel,7 D. Hakobyan,12 M. Hayashida,7 T. Hengstebeck,16 D. Höhne,1 J. Hose,7 C. C. Hsu,7 P. G. Isar,7 P. Jacon,10 O. Kalekin,16 R. Kosyra,7 D. Kranich,1,15 M. Laatiaoui,7 A. Laille,15 T. Lenisa,9 P. Liebing,7 E. Lindfors,13 S. Lombardi,8 F. Longo,17 J. López,2 M. López,4 E. Lorenz,3,7 F. Lucarelli,4 P. Majumdar,7 G. Maneva,18 K. Mannheim,1 O. Mansutti,9 M. Mariotti,8 M. Martínez,2 K. Mase,7 D. Mazin,7 C. Merck,7 M. Meucci,14 M. Meyer,1 J. M. Miranda,4 R. Mirzoyan,7 S. Mizobuchi,7 A. Moralejo,2 K. Nilsson,13 E. Oña-Wilhelmi,2 R. Orduña,5 N. Otte,7 I. Oya,4 D. Paneque,7 R. Paoletti,14 J. M. Paredes,11 M. Pasanen,13 D. Pascoli,8 F. Pauss,3 N. Pavel,16* R. Pegna,14 M. Persic,19 L. Peruzzo,8 A. Piccioli,14 M. Poller,1 G. Pooley,20 E. Prandini,8 A. Raymers,12 W. Rhode,6 M. Ribó,11 J. Rico,2{dagger} B. Riegel,1 M. Rissi,3 A. Robert,5 G. E. Romero,21,22 S. Rügamer,1 A. Saggion,8 A. Sánchez,5 P. Sartori,8 V. Scalzotto,8 V. Scapin,8 R. Schmitt,1 T. Schweizer,16 M. Shayduk,16 K. Shinozaki,7 S. N. Shore,23 N. Sidro,2{dagger} A. Sillanpää,13 D. Sobczynska,10 A. Stamerra,14 L. S. Stark,3 L. Takalo,13 P. Temnikov,18 D. Tescaro,2 M. Teshima,7 N. Tonello,7 A. Torres,5 D. F. Torres,2,24 N. Turini,14 H. Vankov,18 V. Vitale,9 R. M. Wagner,7 T. Wibig,10 W. Wittek,7 R. Zanin,8 J. Zapatero5

Microquasars are binary star systems with relativistic radio-emitting jets. They are potential sources of cosmic rays and can be used to elucidate the physics of relativistic jets. We report the detection of variable gamma-ray emission above 100 gigaelectron volts from the microquasar LS I 61 + 303. Six orbital cycles were recorded. Several detections occur at a similar orbital phase, which suggests that the emission is periodic. The strongest gamma-ray emission is not observed when the two stars are closest to one another, implying a strong orbital modulation of the emission or absorption processes.

1 Universität Würzburg, D-97074 Würzburg, Germany.
2 Institut de Física d'Altes Energies, E-08193 Bellaterra (Barcelona), Spain.
3 Eidgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland.
4 Universidad Complutense, E-28040 Madrid, Spain.
5 Universitat Autónoma de Barcelona, E-08193 Bellaterra, Spain.
6 Universität Dortmund, D-44227 Dortmund, Germany.
7 Max-Planck-Institut für Physik, D-80805 München, Germany.
8 Università di Padova and Istituto Nazionale di Fisica Nucleare (INFN), I-35131 Padova, Italy.
9 Università di Udine and INFN Trieste, I-33100 Udine, Italy.
10 University of Lodz, PL-90236 Lodz, Poland.
11 Universitat de Barcelona, E-08028 Barcelona, Spain.
12 Yerevan Physics Institute, 375036 Yerevan, Armenia.
13 Tuorla Observatory, FI-21500 Piikkiö, Finland.
14 Università di Siena and INFN Pisa, I-53100 Siena, Italy.
15 University of California, Davis, CA 95616, USA.
16 Humboldt-Universität zu Berlin, D-12489 Berlin, Germany.
17 Università di Trieste and INFN Trieste, I-34100 Trieste, Italy.
18 Institute for Nuclear Research and Nuclear Energy, BG-1784 Sofia, Bulgaria.
19 Osservatorio Astronomico and INFN Trieste, I-34100 Trieste, Italy.
20 Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK.
21 Facultad de Ciencias Astronómicas y Geofísicas, UNLP, 1900 La Plata, Argentina.
22 Instituto Argentino de Radioastronomía CC5, 1894 Villa Elisa, Argentina.
23 Università di Pisa and INFN Pisa, I-56100 Pisa, Italy.
24 Institut de Ciències de l'Espai/Consejo Superior de Investigaciones Cientificas, E-08193 Bellaterra (Barcelona), Spain.

* Deceased.

{dagger} To whom correspondence should be addressed. E-mail: nsidro{at}ifae.es (N.S.); jrico{at}ifae.es (J.R.)

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