M. Böttcher, J. Harvey, M. Joshi, et al.
(72 authors),
Astrophysical Journal, 631, 169 (2005)
The BL Lac object 3C 66A was the target of an extensive multiwavelength monitoring campaign from July 2003 through April 2004 (with a core campaign from Sept. - Dec. 2004), involving observations throughout the electromagnetic spectrum. Radio, infrared, and optical observations were carried out by the WEBT-ENIGMA collaboration. At higher energies, 3C 66A was observed in X-rays (RXTE), and at very-high-energy (VHE) gamma-rays (STACEE, VERITAS). In addition, the source has been observed with the VLBA at 9 epochs throughout the period September 2003 - December 2004, including 3 epochs contemporaneous with the core campaign.
A gradual brightening of the source over the course of the campaign was observed at all optical frequencies, culminating in a very bright maximum around Feb. 18, 2004. The WEBT campaign revealed microvariability with flux changes of ~ 5 % on time scales as short as ~ 2 hr. The source was in a relatively bright state, with several bright flares on time scales of several days. The spectral energy distribution (SED) indicates a peak in the optical regime. A weak trend of optical spectral hysteresis with a trend of spectral softening throughout both the rising and decaying phases, has been found. On longer time scales, there appears to be a weak indication of a positive hardness-intensity correlation for low optical fluxes, which does not persist at higher flux levels.
The 3 - 10 keV X-ray flux during the core campaign was historically high and its spectrum very soft, indicating that the low-frequency component of the broadband SED extends beyond ~ 10 keV. No significant X-ray flux and/or spectral variability was detected. STACEE and Whipple observations provided upper flux limits at > 150 GeV and > 390 GeV, respectively. The 22 and 43 GHz data from the 3 VLBA epochs made between September 2003 and January 2004 indicate a rather smooth jet with only very moderate internal structure. After decomposition into 6 Gaussian components, evidence for superluminal motion (8.5 +/- 5.6 h-1c) was found in only one component, while the apparent velocities of all other components are consistent with 0. The radial radio brightness profile suggests a magnetic field decay ~ r-1 and, thus, a predominantly perpendicular magnetic field orientation.