A peculiar multiwavelength flare in the blazar 3C 454.3

The blazar 3C 454.3 exhibited a strong flare seen in gamma-rays, X-rays and optical/near-infrared bands during 2009 December 3-12. Emission in the V and J bands rose more gradually than did the gamma-rays and soft X-rays, though all peaked at nearly the same time. Optical polarization measurements showed dramatic changes during the flare, with a strong anticorrelation between optical flux and degree of polarization (which rose from similar to 3 to similar to 20 per cent) during the declining phase of the flare. The flare was accompanied by large rapid swings in polarization angle of similar to 170.. This combination of behaviours appears to be unique. We have cm-band radio data during the same period but they show no correlation with variations at higher frequencies. Such peculiar behaviour may be explained using jet models incorporating fully relativistic effects with a dominant source region moving along a helical path or by a shock-in-jet model incorporating three-dimensional radiation transfer if there is a dominant helical magnetic field. We find that spectral energy distributions at different times during the flare can be fit using modified one-zone models where only the magnetic field strength and particle break frequencies and normalizations need change. An optical spectrum taken at nearly the same time provides an estimate for the central black hole mass of similar to 2.3 x 10(9) M-circle dot. We also consider two weaker flares seen during the similar to 200 d span over which multiband data are available. In one of them, the V and J bands appear to lead the gamma-ray and X-ray bands by a few days; in the other, all variations are simultaneous.