The WEBT Campaign on the Intermediate BL Lac Object 3C66A in 2007-2008

TL;DR

This paper reports on an intensive multi-wavelength observational campaign of the BL Lac object 3C 66A during 2007-2008, revealing high activity, microvariability, and constraining physical parameters like magnetic field and Doppler factor.

Contribution

It provides the first detailed multi-wavelength campaign data on 3C 66A, constrains its magnetic field and Doppler factor, and challenges models requiring very high Lorentz factors.

Findings

Detected bright flares and microvariability in 3C 66A.

Derived magnetic field strength of approximately 19 G.

Set an upper limit on the Doppler factor at 28.

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 e_B^{2/7} \tau_h^{-6/7} D_1^{13/7} G. From the lack of systematic spectral variability, we can derive an upper limit on the Doppler factor, D <= 28 \tau_h^{-1/8} e_B^{3/16}. This is in agreement with superluminal motion measurements of \beta_{app} \le 27 and argues against models with very high Lorentz factors of \Gamma > 50, required for a one-zone SSC interpretation of some high-frequency-peaked BL Lac objects detected at TeV gamma-ray energies.