Modeling the Spectral Energy Distribution and Variability of 3C 66A during the WEBT Campaign of 2003–2004

Abstract

The BL Lac object 3C 66A was observed in an extensive multiwavelength monitoring campaign (the Whole Earth Blazar Telescope [WEBT] campaign) from 2003 July until 2004 April. The spectral energy distribution (SED) was measured over the entire electromagnetic spectrum, with flux measurements from radio to X-ray frequencies and upper limits in the very high energy (VHE) γ-ray regime. Here we use a time-dependent leptonic jet model to reproduce the SED and optical spectral variability observed during our multiwavelength campaign. Our model simulations could successfully reproduce the observed SED and optical light curves and predict an intrinsic cutoff value for the VHE γ-ray emission at ~4 GeV. The effect of the optical depth due to the intergalactic infrared background radiation (IIBR) on the peak of the high-energy component of 3C 66A was found to be negligible. Also, the presence of a broad-line region (BLR) in the case of 3C 66A may play an important role in the emission of γ-ray photons when the emission region is very close to the central engine, but farther out, the production mechanism of hard X-ray and γ-ray photons becomes rapidly dominated by synchrotron self-Compton emission. We further discuss the possibility of an observable X-ray spectral variability pattern. The simulated results do not predict observable hysteresis patterns in the optical or soft X-ray regimes for major flares on multiday timescales.