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Detection of the gamma-ray binary LS I +61 303 in a low flux state at Very High Energy gamma- rays with the MAGIC Telesopes in 2009

We present very high energy (VHE, E > 100 GeV) gamma-ray observations of the high mass X-ray binary system LS I +61 303 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) * 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 there is a maximum of the TeV flux, 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 emission state in LS I +61 303. We find that the change in the emission state can not be explained by an increase of photon-photon absorption around the compact star.

binaries: general; gamma rays: observations; stars: emission-line; stars: individual (LS I +61 303); X-rays: binaries; X-rays: individual (LS I +61 303)

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