RoboPol: First season rotations of optical polarization plane in blazars

TL;DR

This study reports on optical polarization swings in blazars observed by RoboPol, revealing potential links to gamma-ray activity and suggesting these rotations are unlikely due to random polarization variations.

Contribution

First systematic analysis of optical polarization rotations in blazars, linking them to gamma-ray flares and challenging the random walk model as an explanation.

Findings

Bright gamma-ray flares tend to occur near polarization rotations.

Blazars with rotations show larger and faster polarization angle variations.

Rotations are unlikely caused by random polarization vector changes.

Abstract

We present first results on polarization swings in optical emission of blazars obtained by RoboPol, a monitoring program of an unbiased sample of gamma-ray bright blazars specially designed for effective detection of such events. A possible connection of polarization swing events with periods of high activity in gamma rays is investigated using the dataset obtained during the first season of operation. It was found that the brightest gamma-ray flares tend to be located closer in time to rotation events, which may be an indication of two separate mechanisms responsible for the rotations. Blazars with detected rotations have significantly larger amplitude and faster variations of polarization angle in optical than blazars without rotations. Our simulations show that the full set of observed rotations is not a likely outcome (probability $\le 1.5 \times 10^{-2}$) of a random walk of the polarization vector simulated by a multicell model. Furthermore, it is highly unlikely ($\sim 5 \times 10^{-5}$) that none of our rotations is physically connected with an increase in gamma-ray activity.