Multiband Photopolarimetric Monitoring of an Outburst of the Blazar 3C 454.3 in 2007

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Abstract We report on optical—near-infrared photopolarimetric observations of a blazar 3C 454.3 over 200 d. The object experienced an optical outburst in 2007 July. This outburst was followed by a short state fainter than $V =$ 15.2 mag, lasting $\sim $25 d. The object then entered an active state during which we observed short flares having a timescale of 3–10 d. The object showed two types of features in the color–magnitude relationship. One was a “bluer-when-brighter” trend in the outburst state, and the other was a “redder-when-brighter” trend in the faint state. These two types of features suggest a contribution of thermal emission to the observed flux, as suspected in previous studies. Our polarimetric observation detected two episodes of rotation of the polarization vector. The first one was a counterclockwise rotation in the $QU$ plane during the outburst state. After this rotation event of the polarization vector, the object entered a rapidly fading stage. The second one was seen in a series of flares during the active state. Each flare had a specific position angle of polarization, which apparently rotated clockwise from the first to the last flares. Thus, the object exhibited rotations of the polarization vector in opposite directions. We estimated a decay timescale of the short flares during the active state, and then calculated an upper limit of the strength of the magnetic field, $B =$ 0.2 G, assuming a typical beaming factor of blazars, $\delta =$ 20. This upper limit of $B$ is smaller than those previously estimated from spectral analysis.