Speaker
Description
The RoboPol program has been monitoring an unbiased sample of blazars since 2013 with a cadence of less than a few days. The main drive has been to quantify their optical polarisation properties, understand its variability and gain insight in the mechanisms producing the smooth and long rotations of the polarisation angle, in a systematic and unbiased way. Here we focus on the magnitude of polarisation. We present average R-band optopolarimetric data, as well as variability parameters, from the first and second RoboPol observing season. We show that gamma-ray-loud blazars are systematically more polarised than gamma-ray-quiet ones. We however do not find any evidence that this discrepancy is related to the redshift distribution, rest-frame R-band luminosity density, or the source classification. Furthermore, we find that median polarisation fraction drops with the synchrotron-peak-frequency and so is the randomness of the polarisation angle distribution. We propose a scenario which mediates efficient particle acceleration in shocks and increases the helical B-field component immediately downstream of the shock.
