The dependence of optical polarisation of blazars on the synchrotron peak frequency

Abstract

The RoboPol instrument and the relevant program was developed in order to conduct a systematic study of the optical polarisation variability of blazars. Driven by the discovery that long smooth rotations of the optical polarisation plane can be associated with the activity in other bands and especially in gamma rays, the program was meant to investigate the physical mechanisms causing them and quantify the optical polarisation behaviour in blazars. Over the first three nominal observing seasons (2013, 2014 and 2015) RoboPol detected 40 rotations in 24 blazars by observing a gamma–ray-loud and gamma–ray-quite unbiassed sample of blazars, providing a reliable set of events for exploring the phenomenon. The obtain datasets provided the ground for a systematic quantification of the variability of the optical polarisation in such systems. In the following after a brief review of the discoveries that relate to the gamma-ray loudness of the sources we move on to discuss a simple jet model that explains the observed dichotomy in terms of polarisation between gamma–ray-loud and quite sources and the dependence of polarisation and the stability of the polarisation angle on the synchrotron peak frequency.

Additional Information

Copyright owned by the author(s) under the term of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. The RoboPol program is a collaboration between: University of Crete (Heraklion, Greece), Max-Planck-Institut für Radioastronomie (Germany), California Institute of Technology (USA), Inter-University Centre for Astronomy and Astrophysics (India) and the Torun Centre for Astronomy, Nicolaus Copernicus University (Poland) . This research has made use of data from the MOJAVE database that is maintained by the MOJAVE team [14]. We wish to thank the internal MPIfR referee Dr C. Casadio for the careful reading and the thorough comments.

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