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

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.