The high-frequency-peaked BL Lac ( HBL ) 1ES 0806+524 ( z = 0.138 ) was discovered in VHE \gamma rays in 2008 .
Until now , the broad-band spectrum of 1ES 0806+524 has been only poorly characterized , in particular at high energies .
We analysed multiwavelength observations from \gamma rays to radio performed from 2011 January to March , which were triggered by the high activity detected at optical frequencies .
These observations constitute the most precise determination of the broad-band emission of 1ES 0806+524 to date .
The stereoscopic MAGIC observations yielded a \gamma -ray signal above 250 GeV of ( 3.7 \pm 0.7 ) per cent of the Crab Nebula flux with a statistical significance of 9.9 \sigma .
The multiwavelength observations showed significant variability in essentially all energy bands , including a VHE \gamma -ray flare that lasted less than one night , which provided unprecedented evidence for short-term variability in 1ES 0806+524 .
The spectrum of this flare is well described by a power law with a photon index of 2.97 \pm 0.29 between \sim 150 GeV and 1 TeV and an integral flux of ( 9.3 \pm 1.9 ) per cent of the Crab Nebula flux above 250 GeV .
The spectrum during the non-flaring VHE activity is compatible with the only available VHE observation performed in 2008 with VERITAS when the source was in a low optical state .
The broad-band spectral energy distribution can be described with a one-zone Synchrotron Self Compton model with parameters typical for HBLs , indicating that 1ES 0806+524 is not substantially different from the HBLs previously detected .