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 $\gamma$-ray emission was detected with a significance of 5.5~$\sigma$. An integral flux above 200 GeV of $(2.3\pm0.7\times10^{-11}) {cm}^{-2}{s}^{-1}$ was measured, corresponding to 11% of the Crab Nebula flux. A rather soft spectrum with a photon index of $-3.3\pm0.7$ has been determined. No significant flux variation was found.