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Published December 2016 | Published + Supplemental Material + Accepted Version
Journal Article Open

RoboPol: the optical polarization of gamma-ray-loud and gamma-ray-quiet blazars


We present average R-band optopolarimetric data, as well as variability parameters, from the first and second RoboPol observing season. We investigate whether gamma-ray-loud and gamma-ray-quiet blazars exhibit systematic differences in their optical polarization properties. We find that gamma-ray-loud blazars have a systematically higher polarization fraction (0.092) than gamma-ray-quiet blazars (0.031), with the hypothesis of the two samples being drawn from the same distribution of polarization fractions being rejected at the 3σ level. We have not found any evidence that this discrepancy is related to differences in the redshift distribution, rest-frame R-band luminosity density, or the source classification. The median polarization fraction versus synchrotron-peak-frequency plot shows an envelope implying that high-synchrotron-peaked sources have a smaller range of median polarization fractions concentrated around lower values. Our gamma-ray-quiet sources show similar median polarization fractions although they are all low-synchrotron-peaked. We also find that the randomness of the polarization angle depends on the synchrotron peak frequency. For high-synchrotron-peaked sources, it tends to concentrate around preferred directions while for low-synchrotron-peaked sources, it is more variable and less likely to have a preferred direction. We propose a scenario which mediates efficient particle acceleration in shocks and increases the helical B-field component immediately downstream of the shock.

Additional Information

© 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2016 September 1. Received 2016 September 1; in original form 2016 April 27. Published: 05 September 2016. The RoboPol project is a collaboration between Caltech in the USA, MPIfR in Germany, Toruń Centre for Astronomy in Poland, the University of Crete/FORTH in Greece, and IUCAA in India. The University of Crete group acknowledges support by the 'RoboPol' project, which is implemented under the 'Aristeia' action of the 'Operational Programme Education and Lifelong Learning' and is co-funded by the European Social Fund (ESF), Greek National Resources, the European Commission Seventh Framework Program (FP7) through grants PCIG10-GA-2011-304001 'JetPop' and PIRSES-GA-2012-31578 'EuroCal'. This research was also supported in part by NASA grant NNX11A043G and NSF grant AST-1109911, the Polish National Science Centre, grant number 2011/01/B/ST9/04618 and the COST Action MP1104 'Polarization as a tool to study the Solar System and beyond'. KT acknowledges support by the European Commission Seventh Framework Program (FP7) through the Marie Curie Career Integration Grant PCIG-GA-2011-293531 'SFOnset'. IM and SK were funded by the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne. MBa acknowledges support from the International Fulbright Science and Technology Award. TH was supported in part by the Academy of Finland project number 267324. We would also like to acknowledge partial support from the EU FP7 Grant PIRSES- GA-2012-316788. The work of MBo is supported by the South African Research Chair Initiative of the Department of Science and Technology and the National Research Foundation of South Africa. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. It also made use of ASTROPY, http://www.astropy.org, a community-developed core PYTHON package for Astronomy (Astropy Collaboration 2013). EA wishes to thank Dr R. Porcas and Dr F. Mantovani of the MPIfR for the fruitful discussions on the statistical behaviour of the polarization parameters, and the MPIfR internal referee Dr B. Boccardi for the constructive comments on the manuscript. Finally, EA wants to thank the anonymous referee for the careful reading and numerous insightful comments of the first manuscript.

Attached Files

Accepted Version - 1609.00640.pdf

Published - stw2217.pdf

Supplemental Material - stw2217_supp.zip


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August 19, 2023
August 19, 2023