Discovery of VHE $\gamma$-rays from the blazar 1ES 1215+303 with the MAGIC telescopes and simultaneous multi-wavelength observations}

Context. We present the discovery of very high energy (VHE, E {\gt} 100 GeV) {$\gamma$}-ray emission from the BL Lac object 1ES 1215+303 by the MAGIC telescopes and simultaneous multi-wavelength data in a broad energy range from radio to {$\gamma$}-rays. Aims: We study the VHE {$\gamma$}-ray emission from 1ES 1215+303 and its relation to the emissions in other wavelengths. Methods: Triggered by an optical outburst, MAGIC observed the source in 2011 January - February for 20.3 h. The target was monitored in the optical R-band by the KVA telescope that also performed optical polarization measurements. We triggered target of opportunity observations with the Swift satellite and obtained simultaneous and quasi-simultaneous data from the Fermi Large Area Telescope and from the Metsähovi radio telescope. We also present the analysis of older MAGIC data taken in 2010. Results: The MAGIC observations of 1ES 1215+303 carried out in 2011 January - February resulted in the first detection of the source at VHE with a statistical significance of 9.4{$\sigma$}. Simultaneously, the source was observed in a high optical and X-ray state. In 2010 the source was observed in a lower state in optical, X-ray, and VHE, while the GeV {$\gamma$}-ray flux and the radio flux were comparable in 2010 and 2011. The spectral energy distribution obtained with the 2011 data can be modeled with a simple one zone SSC model, but it requires extreme values for the Doppler factor or the electron energy distribution.