  We report on the detection of a very rapid TeV gamma-ray flare from BL
Lacertae on 2011 June 28 with the Very Energetic Radiation Imaging Telescope
Array System (VERITAS). The flaring activity was observed during a 34.6-minute
exposure, when the integral flux above 200 GeV reached $(3.4\pm0.6) \times
10^{-6} \;\text{photons}\;\text{m}^{-2}\text{s}^{-1}$, roughly 125% of the Crab
Nebula flux measured by VERITAS. The light curve indicates that the
observations missed the rising phase of the flare but covered a significant
portion of the decaying phase. The exponential decay time was determined to be
$13\pm4$ minutes, making it one of the most rapid gamma-ray flares seen from a
TeV blazar. The gamma-ray spectrum of BL Lacertae during the flare was soft,
with a photon index of $3.6\pm 0.4$, which is in agreement with the measurement
made previously by MAGIC in a lower flaring state. Contemporaneous radio
observations of the source with the Very Long Baseline Array (VLBA) revealed
the emergence of a new, superluminal component from the core around the time of
the TeV gamma-ray flare, accompanied by changes in the optical polarization
angle. Changes in flux also appear to have occurred at optical, UV, and GeV
gamma-ray wavelengths at the time of the flare, although they are difficult to
quantify precisely due to sparse coverage. A strong flare was seen at radio
wavelengths roughly four months later, which might be related to the gamma-ray
flaring activities. We discuss the implications of these multiwavelength
results.
