The WEBT Campaign on the Intermediate BL Lac Object 3C66A in 2007-2008
Abstract
Prompted by a high optical state in September 2007, the Whole Earth Blazar Telescope (WEBT) 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. The source remained in a high optical state throughout the observing period and exhibited several bright flares on time scales of ~ 10 days. This included an exceptional outburst around September 15 - 20, 2007, 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 or spectral lags. We infer a value of the magnetic field in the emission region of B ~ 19 eB2/7 τh-6/7 D113/7 G. From the lack of systematic spectral variability, we can derive an upper limit on the Doppler factor, D <= 28 τh-1/8 eB3/16. This is in agreement with superluminal motion measurements of βapp 27 and argues against models with very high Lorentz factors of > 50, required for a one-zone SSC interpretation of some high-frequency-peaked BL Lac objects detected at TeV gamma-ray energies.