DETECTION OF THE gamma-RAY BINARY LS I+61 degrees 303 IN A LOW-FLUX STATE AT VERY HIGH ENERGY gamma-RAYS WITH THE MAGIC TELESCOPES IN 2009

Aleksic, J.; Alvarez, E. A.; Antonelli, L. A.; Antoranz, P.; Asensio, M.; Backes, M.; Barrio, J. A.; Bastieri, D.; Becerra Gonzalez, J.; Bednarek, W.; Berdyugin, A.; Berger, K.; Bernardini, E.; Biland, A.; Blanch, O.; Bock, R. K.; Boller, A.; Bonnoli, G.; Borla, Tridon D.; Bosch Ramon, V.; Braun, I.; Bretz, T.; Canellas, A.; Carmona, E.; Carosi, A.; Colin, P.; Colombo, E.; Contreras, J. L.; Cortina, J.; Cossio, L.; Covino, S.; Dazzi, F.; De Angelis, A.; De Caneva, G.; De Cea del Pozo, E.; De Lotto, B.; Delgado Mendez, C.; Diago Ortega, A.; Doert, M.; Dominguez, A.; Dominis, Prester D.; Dorner, D.; Doro, M.; Elsaesser, D.; Ferenc, D.; Fonseca, M. V.; Font, L.; Fruck, C.; Garcia Lopez, R. J.; Garczarczyk, M.; Garrido, D.; Giavitto, G.; Godinovic, N.; Hadasch, D.; Haefner, D.; Herrero, A.; Hildebrand, D.; Hoehne Moench, D.; Hose, J.; Hrupec, D.; Huber, B.; Jogler, T.; Kellermann, H.; Klepser, S.; Kraehenbuehl, T.; Krause, J.; La Barbera, A.; Lelas, D.; Leonardo, E.; Lindfors, E.; Lombardi, S.; Lopez, A.; Lopez, M.; Lorenz, E.; Makariev, M.; Maneva, G.; Mankuzhiyil, N.; Mannheim, K.; Maraschi, L.; Mariotti, M.; Martinez, M.; Mazin, D.; Meucci, M.; Miranda, J. M.; Mirzoyan, R.; Miyamoto, H.; Moldon, J.; Moralejo, A.; Munar Adrover, P.; Nieto, D.; Nilsson, K.; Orito, R.; Oya, I.; Paneque, D.; Paoletti, R.; Pardo, S.; Paredes, J. M.; Partini, S.; Pasanen, M.; Pauss, F.; Perez Torres, M. A.; Persic, M.; Peruzzo, L.; Pilia, M.; Pochon, J.; Prada, F.; PRADA MORONI, PIER GIORGIO; Prandini, E.; Puljak, I.; Reichardt, I.; Reinthal, R.; Rhode, W.; Ribo, M.; Rico, J.; Ruegamer, S.; Saggion, A.; Saito, K.; Saito, T. Y.; Salvati, M.; Satalecka, K.; Scalzotto, V.; Scapin, V.; Schultz, C.; Schweizer, T.; Shayduk, M.; Shore, STEVEN NEIL; Sillanpaa, A.; Sitarek, J.; Sobczynska, D.; Spanier, F.; Spiro, S.; Stamerra, A.; Steinke, B.; Storz, J.; Strah, N.; Suric, T.; Takalo, L.; Takami, H.; Tavecchio, F.; Temnikov, P.; Terzic, T.; Tescaro, D.; Teshima, M.; Tibolla, O.; Torres, D. F.; Treves, A.; Uellenbeck, M.; Vankov, H.; Vogler, P.; Wagner, R. M.; Weitzel, Q.; Zabalza, V.; Zandanel, F.; Zanin, R.

doi:10.1088/0004-637X/746/1/80

We present very high energy (E > 100 GeV) gamma-ray observations of the gamma-ray binary system LS I +61 degrees 303 obtained with the MAGIC stereo system between 2009 October and 2010 January. We detect a 6.3 sigma gamma-ray signal above 400 GeV in the combined data set. The integral flux above an energy of 300 GeV is F(E > 300 GeV) = (1.4 +/- 0.3(stat) +/- 0.4(syst)) x 10(-12) cm(-2) s(-1), which corresponds to about 1.3% of the Crab Nebula flux in the same energy range. The orbit-averaged flux of LS I +61 degrees 303 in the orbital phase interval 0.6-0.7, where a maximum of the TeV flux is expected, is lower by almost an order of magnitude compared to our previous measurements between 2005 September and 2008 January. This provides evidence for a new low-flux state in LS I +61 degrees 303. We find that the change to the low-flux state cannot be solely explained by an increase of photon-photon absorption around the compact star.