Multiband Photopolarimetric Monitoring of the Outburst of the Blazar
  3C~454.3 in 2007
  We report on optical-near-infrared photopolarimetric observations of a blazar
3C 454.3 over 200 d. The object experienced an optical outburst in July 2007.
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 is a "bluer-when-brighter"
trend in the outburst state, and the other is a "redder-when-brighter" trend in
the faint state. These two types of features suggest a contribution of a
thermal emission to the observed flux, as suspected in previous studies. Our
polarimetric observation detected two episodes of the 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, and it 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.
