ar X iv : a st ro - p h / 06 06 63 0 v 1 2 6 Ju n 20 06 Discovery of very high energy γ - rays from Markarian 180 triggered by an optical outburst

The high-peaked BL Lacertae object Markarian 180 (Mkn 180) was observed to have an optical outburst in March 2006, triggering a Target of Opportunity observation with the MAGIC telescope. The source was observed for 12.4 hours and very high energy γ-ray emission was detected with a significance of 5.5 σ. An integral flux above 200 GeV of (2.3 ± 0.7 × 10) cms was measured, corresponding to 11% of the Crab Nebula flux. A rather soft spectrum with a Universität Würzburg, D-97074 Würzburg, Germany Institut de F́ısica d’Altes Energies, Edifici Cn., E-08193 Bellaterra (Barcelona), Spain ETH Zurich, CH-8093 Switzerland Universidad Complutense, E-28040 Madrid, Spain Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain Universität Dortmund, D-44227 Dortmund, Germany Max-Planck-Institut für Physik, D-80805 München, Germany Università di Padova and INFN, I-35131 Padova, Italy Università di Udine, and INFN Trieste, I-33100 Udine, Italy University of Lódź, PL-90236 Lodz, Poland Yerevan Physics Institute, AM-375036 Yerevan, Armenia Tuorla Observatory, FI-21500 Piikkiö, Finland Humboldt-Universität zu Berlin, D-12489 Berlin, Germany University of California, Davis, CA-95616-8677, USA Università di Siena, and INFN Pisa, I-53100 Siena, Italy Università di Trieste, and INFN Trieste, I-34100 Trieste, Italy Institute for Nuclear Research and Nuclear Energy, BG-1784 Sofia, Bulgaria INAF/Osservatorio Astronomico and INFN Trieste, I-34131 Trieste, Italy Università di Pisa, and INFN Pisa, I-56126 Pisa, Italy Institut de Ciències de l’Espai (CSIC-IEEC), E-08193 Bellaterra (Barcelona), Spain Universitat de Barcelona, E-08028 Barcelona, Spain deceased correspondence: [email protected], [email protected]