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We study the multifrequency emission and spectral properties of the quasar 3C279. We observed 3C 279 in very high energy (VHE, E>100GeV) gamma rays, withthe MAGIC telescopes during 2011, for the first time in stereoscopic mode. Wecombine these measurements with observations at other energy bands: in highenergy (HE, E>100MeV) gamma rays from Fermi-LAT, in X-rays from RXTE, in theoptical from the KVA telescope and in the radio at 43GHz, 37GHz and 15GHz fromthe VLBA, Mets\"ahovi and OVRO radio telescopes and optical polarisationmeasurements from the KVA and Liverpool telescopes. During the MAGICobservations (February to April 2011) 3C 279 was in a low state in optical,X-ray and gamma rays. The MAGIC observations did not yield a significantdetection. These upper limits are in agreement with the extrapolation of the HEgamma-ray spectrum, corrected for extragalactic background light absorption,from Fermi-LAT. The second part of the MAGIC observations in 2011 was triggeredby a high activity state in the optical and gamma-ray bands. During the opticaloutburst the optical electric vector position angle rotatated of about 180degrees. There was no simultaneous rotation of the 43GHz radio polarisationangle. No VHE gamma rays were detected by MAGIC, and the derived upper limitssuggest the presence of a spectral break or curvature between the Fermi-LAT andMAGIC bands. The combined upper limits are the strongest derived to date forthe source at VHE and below the level of the previously detected flux by afactor 2. Radiation models that include synchrotron and inverse Comptonemissions match the optical to gamma-ray data, assuming an emission componentinside the broad line region (BLR) responsible for the high-energy emission andone outside the BLR and the infrared torus causing optical and low-energyemission. We interpreted the optical polarisation with a bent trajectory model.

MAGIC observations and multifrequency properties of the Flat Spectrum Radio Quasar 3C 279 in 2011