Detection of the $\gamma$-Ray Binary LS I +61\deg303 in a Low-flux State at Very High Energy {$\gamma$}-Rays with the MAGIC Telescopes in 2009}

We present very high energy (E {\gt} 100 GeV) {$\gamma$}-ray observations of the {$\gamma$}-ray binary system LS I +61{\deg}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 {\gt} 300 GeV) = (1.4 {\plusmn} 0.3$_{stat}$ {\plusmn} 0.4$_{syst}$) {\times} 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{\deg}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{\deg}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.