Discovery of very high energy γ-rays from Markarian 180 triggered by an optical outburst

The high-frequency-peaked BL Lacertae object Markarian 180 (Mrk 180) was observed to have an optical outburst in 2006 March, triggering a Target of Opportunity observation with the MAGIC telescope. The source was observed for 12.4 hr, and very high energy gamma-ray emission was detected with a significance of 5.5 sigma. An integral flux above 200 GeV of (2.3 +/- 0.7) x 10(-11) cm(-2) s(-1) was measured, corresponding to 11% of the Crab Nebula flux. A rather soft spectrum with a photon index of has been determined. No significant flux -3.3 x 0.7 variation was found.