The multifrequency campaign on 3C 279 in January 2006

摘要

Context. The prominent blazar 3C?279 is known for its large-amplitude variability throughout the electromagnetic spectrum and its often γ-ray-dominated spectral energy distribution. However, the?characterization of its broadband spectral variability still lacks a consistent picture, and the origin of its high-energy emission is still unclear. Aims. We intend to characterize the spectral energy distribution (SED) and spectral variability of 3C?279 in its optical high state. Methods. Prompted by an optical high state of 3C?279, we organized an extensive multiwavelength campaign with coverage from radio to hard X-ray energies. The core components of the campaign were INTEGRAL and Chandra ToO observations in January?2006, augmented by X-ray data from Swift and RXTE as well as radio through optical coverage. Results. The blazar was observed at a moderately high optical state. A?well-covered multifrequency spectrum from radio to hard X-ray energies could be derived. During the flare, the radio spectrum was inverted, with a prominent spectral peak near 100?GHz, which propagated in time toward lower frequencies. The SED shows the typical two-bump shape, the?signature of non-thermal emission from a relativistic jet. As?a?result of the long exposure times of INTEGRAL and Chandra, the?high-energy spectrum (0.3?100?keV) was precisely measured, showing?– for the first time?– a possible downward curvature. A?comparison of this SED from 2006 to the one observed in?2003, also centered on an INTEGRAL observation, but during an optical low-state, revealed the surprising fact that?– despite a significant change of the high-frequency synchrotron emission (near-IR/optical/UV)?– the low-energy end of the high-energy component (X-ray energies) remained virtually unchanged compared to?2003. Conclusions. Our results prove that the two emission components do not vary simultaneously. This provides strong constraints on the modeling of the overall emission of 3C?279. When interpreted with a steady-state leptonic model, the?variability among the SEDs displaying almost identical X-ray spectra at low flux levels, but drastically different IR/optical/UV fluxes, can be reproduced by a change solely of the low-energy cutoff of the relativistic electron spectrum. In?an internal shock model for blazar emission, such?a change could be achieved through a varying relative Lorentz factor of colliding shells producing internal shocks in the jet, and/or?the efficiency of generating turbulent magnetic fields (e.g.,?through the Weibel instability) needed for efficient energy transfer from protons to electrons behind the?shock.