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

TL;DR

This study presents detailed optical-near-infrared photopolarimetric observations of blazar 3C 454.3 during its 2007 outburst, revealing complex polarization behavior, color-magnitude trends, and estimating magnetic field strength.

Contribution

First detailed multi-band photopolarimetric monitoring of 3C 454.3 during an outburst, uncovering polarization rotations and thermal emission contributions.

Findings

Detected two polarization rotation episodes with opposite directions.

Observed 'bluer-when-brighter' and 'redder-when-brighter' trends in different states.

Estimated magnetic field strength upper limit of 0.2 G.

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 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.