The mechanisms producing fast variability of the \gamma-ray emission in active galactic nuclei are under debate. The MAGIC telescopes detected a fast very high energy (VHE, E>100 GeV) \gamma-ray flare from BL Lacertae on 2015 June 15. The flare had a maximum flux of (1.5\pm 0.3)\times 10^{-10} photons cm^{-2} s^{-1}... Show more