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

Abstract

We present very high energy (E > 100 GeV) γ-ray observations of the γ-ray binary system LS I +61°303 obtained with the MAGIC stereo system between 2009 October and 2010 January. We detect a 6.3σ γ-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 ± 0.3stat ± 0.4syst) × 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°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°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.