Optical polarimetric study of gamma-ray emitting blazars

CELLONE, S. A.

Strasbourg

Workshop; The 2nd COST workshop on Polarization and Active Galactic Nuclei; 2015

Observatoire astronomique de Strasbourg

We present results from an optical polarimetric study of four blazars, looking for short time-scale fluctuations (i.e., microvariability) either/both in their polarisation percent or position angle in the B and R bands. The observations were obtained with the CAFOS instrument at the 2.2-m telescope of CAHA (Calar Alto, Spain), attaining a temporal resolution of ~45 min along 3 to 6 (quasi-)consecutive nights.Low-amplitude (>~1%) variations were detected at inter-night time-scales, while intra-night variations remained below approx. 1%. On the other hand, for each blazar the value of the electric vector position angle (EVPA) remained nearly constant during our observations, except for one object which showed a mild (10 deg) EVPA rotation.This study is part of an effort to characterise the photo-polarimetric variability behaviour of gamma-ray emitting blazars. Most TeV-detected blazars are relatively nearby objects, because gamma photons are efficiently absorbed by the extragalactic background light. Thus, their host galaxies are relatively bright and resolved, introducing a depolarising effect, and, at the same time, leading to systematic errors in the photo-polarimetric light-curves when seeing conditions vary with time. We study this effect in detail, quantitatively evaluating host-galaxy effects, and systematic errors introduced by uncertainties in their structural parameters.