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

Combined with very-long-baseline interferometrymeasurements, the observations of fast TeV gamma-ray flaresprobe the structure and emission mechanism of blazarjets. However, only a handful of such flareshave been detected to date, and only within the last few years have these flaresbeen observed from lower-frequency-peaked BL~Lac objectsand flat-spectrum radio quasars. We report on a fast TeV gamma-ray flarefrom the blazarBL~ Lacertaeobserved by VERITAS, with a rise time of $\sim$2.3~hr and a decay time of $\sim$36~min. The peak flux above 200 GeV is $(4.2 \pm 0.6) \times 10^{-6} \;\text{photon} \;\text{m}^{-2}\; \text{s}^{-1}$ measured with a 4-minute-binned light curve, corresponding to $\sim$180\% of the flux which is observed from the Crab Nebulaabove the same energy threshold. Variability contemporaneous with the TeV gamma-ray flarewas observed in GeV gamma-ray, X-ray, and optical flux, as well as in optical and radio polarization. Additionally, a possible moving emission feature with superluminal apparent velocity was identified in VLBA observations at 43 GHz, potentially passing the radio core of the jet around the time of the gamma-ray flare. We discuss the constraints on the size, Lorentz factor, and location of the emitting region of the flare, and the interpretations with several theoretical models which invoke relativistic plasmapassing stationary shocks.