2009ApJ...694..174B

Prompted by a high optical state in 2007 September, the Whole Earth Blazar Telescope consortium organized an intensive optical, near-IR (JHK) and radio observing campaign on the intermediate BL Lac object 3C 66A throughout the fall and winter of 2007-2008. In this paper, we present data from 28 observatories in 12 countries, covering the observing season from late 2007 July through 2008 February. The source remained in a high optical state throughout the observing period and exhibited several bright flares on timescales of ∼10 days. This included an exceptional outburst around 2007 September 15-20, reaching a peak brightness at R∼ 13.4. Our campaign revealed microvariability with flux changes up to|dR/dt|∼ 0.02 mag/hr. Our observations do not reveal evidence for systematic spectral variability in the overall high state covered by our campaign, in agreement with previous results. In particular, we do not find evidence for spectral hysteresis in 3C 66A for which hints were found in an earlier campaign in a somewhat lower flux state. We also did not find any evidence for spectral lags in the discrete correlation functions between different optical bands. We infer a value of the magnetic field in the emission region of B∼ 19 e ^2/7_Bτ^–6/7_h D ^13/7₁ G, where e_B is the magnetic field equipartition fraction, τ_his the shortest observed variability timescale in units of hours, and D₁ is the Doppler factor in units of 10. From the lack of systematic spectral variability, we can derive an upper limit on the Doppler factor, D ≤ 28 τ^–1/8_h e ^3/16_B. This is in perfect agreement with superluminal motion measurements with the VLBI/VLBA of β_app≤ 27 and argues against models with very high Lorentz factors of Γ ≳ 50, required for a one-zone synchrotron-self-Compton interpretation of some high-frequency-peaked BL Lac objects detected at TeV γ-ray energies.