MAGIC detection of short-term variability of the high-peaked BL Lac object lES 0806+524

The high-frequency-peaked BL Lac (HBL) 1ES 0806+524 (z = 0.138) was discovered in very high energy (VHE) gamma-rays in 2008. Until now, the broad-band spectrum of lES 0806+524 has been only poorly characterized, in particular at high energies. We analysed multiwave-length 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 ES 0806+524 to date. The stereoscopic Major Atmospheric Gamma-Ray Imaging Cherenkov (MAGIC) observations yielded a gamma-ray signal above 250 GeV of (3.7 +/- 0.7) per cent of the Crab Nebula flux with a statistical significance of 9.9a. 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 lES 0806+524. The spectrum of this flare is well described by a power law with a photon index of 2.97 +/- 0.29 between similar to 150 GeV and 1 TeV and an integral flux of (9.3 +/- 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 lES 0806+524 is not substantially different from the HBLs previously detected.