The multifrequency campaign on 3C 279 in January 2006

Abstract

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.