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Observations of fast TeV $\gamma$-ray flares from blazars reveal the extremecompactness of emitting regions in blazar jets. Combined withvery-long-baseline radio interferometry measurements, they probe the structureand emission mechanism of the jet. We report on a fast TeV $\gamma$-ray flarefrom BL Lacertae observed by VERITAS, with a rise time of about 2.3 hours and adecay time of about 36 minutes. The peak flux at $>$200 GeV measured with the4-minute binned light curve is $(4.2 \pm 0.6) \times 10^{-6} \;\text{photons}\;\text{m}^{-2}\, \text{s}^{-1}$, or $\sim$180% the Crab Nebula flux.Variability in GeV $\gamma$-ray, X-ray, and optical flux, as well as in opticaland radio polarization was observed around the time of the TeV $\gamma$-rayflare. A possible superluminal knot was identified in the VLBA observations at43 GHz. The flare constrains the size of the emitting region, and is consistentwith several theoretical models with stationary shocks.

Multiwavelength observations of the blazar BL Lacertae: a new fast TeV $\gamma$-ray flare