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The time-dependent distribution of optical polarization angle changes in blazars

Kiehlmann, S. and Blinov, D. and Liodakis, I. and Pavlidou, V. and Readhead, A. C. S. and Angelakis, E. and Casadio, C. and Hovatta, T. and Kylafis, N. and Mahabal, A. and Mandarakas, N. and Myserlis, I. and Panopoulou, G. V. and Pearson, T. J. and Ramaprakash, A. and Reig, P. and Skalidis, R. and Słowikowska, A. and Tassis, K. and Zensus, J. A. (2021) The time-dependent distribution of optical polarization angle changes in blazars. Monthly Notices of the Royal Astronomical Society, 507 (1). pp. 225-243. ISSN 0035-8711. doi:10.1093/mnras/stab2055.

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At optical wavelengths, blazar Electric Vector Position Angle (EVPA) rotations linked with gamma-ray activity have been the subject of intense interest and systematic investigation for over a decade. One difficulty in the interpretation of EVPA rotations is the inherent 180° ambiguity in the measurements. It is therefore essential, when studying EVPA rotations, to ensure that the typical time-interval between successive observations – i.e. the cadence – is short enough to ensure that the correct modulo 180° value is selected. This optimal cadence depends on the maximum intrinsic EVPA rotation speed in blazars, which is currently not known. In this paper, we address the following questions for the RoboPol sample: What range of rotation speeds for rotations greater than 90° can we expect? What observation cadence is required to detect such rotations? Have rapid rotations been missed in EVPA rotation studies thus far? What fraction of data is affected by the ambiguity? And how likely are detected rotations affected by the ambiguity? We answer these questions with three seasons of optical polarimetric observations of a statistical sample of blazars sampled weekly with the RoboPol instrument and an additional season with daily observations. We model the distribution of EVPA changes on time-scales from 1–30 d and estimate the fraction of changes exceeding 90°. We show that at least daily observations are necessary to measure >96 per cent of optical EVPA variability in the RoboPol sample of blazars correctly and that intraday observations are needed to measure the fastest rotations that have been seen thus far.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Kiehlmann, S.0000-0001-6314-9177
Blinov, D.0000-0003-0611-5784
Liodakis, I.0000-0001-9200-4006
Pavlidou, V.0000-0002-0870-1368
Readhead, A. C. S.0000-0001-9152-961X
Angelakis, E.0000-0001-7327-5441
Casadio, C.0000-0003-1117-2863
Hovatta, T.0000-0002-2024-8199
Mahabal, A.0000-0003-2242-0244
Myserlis, I.0000-0003-3025-9497
Panopoulou, G. V.0000-0001-7482-5759
Pearson, T. J.0000-0001-5213-6231
Ramaprakash, A.0000-0001-5707-4965
Reig, P.0000-0002-6446-3050
Słowikowska, A.0000-0003-4525-3178
Tassis, K.0000-0002-8831-2038
Zensus, J. A.0000-0001-7470-3321
Alternate Title:The Distribution of Rotation Speeds in Optical Polarization Position Angle Rotations in Blazars
Additional Information:© 2021 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2021 July 14. Received 2021 July 13; in original form 2021 March 22. Published: 26 July 2021. The authors thank the anonymous referee for the positive and constructive response that helped to improve this manuscript. The authors acknowledge the contributions of O. G. King, A. Kus and E. Pazderski to the RoboPol project. The RoboPol project is a collaboration between Caltech in the USA, Max-Planck-Institute for Radio Astronomy in Germany, Toruń Centre for Astronomy in Poland, the University of Crete/FORTH in Greece, and IUCAA in India. This research was supported in part by NASA grant NNX11A043G and NSF grant AST-1109911, and by the Polish National Science Centre, grant numbers 2011/01/B/ST9/04618 and 2017/25/B/ST9/02805. DB, CC, SK, NM, RS, and KT acknowledge support from the European Research Council under the European Union’s Horizon 2020 research and innovation programme, grant agreement No 771282. VP acknowledges support from the Foundation of Research and Technology – Hellas Synergy Grants Program through project MagMASim, jointly implemented by the Institute of Astrophysics and the Institute of Applied and Computational Mathematics and by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the ‘First Call for H.F.R.I. Research Projects to support Faculty members and Researchers and the procurement of high-cost research equipment grant’ (Project 1552 CIRCE). ANR, GVP, and ACSR acknowledge support from the National Science Foundation, under grant number AST-1611547. GVP acknowledges support by NASA through the NASA Hubble Fellowship grant # HST-HF2-51444.001-A awarded by the SpaceTelescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. TJP acknowledges support from NASA grant NNX16AR31G. TH was supported by the Academy of Finland projects 317383, 320085, and 322535. ANR acknowledges support through a grant from the Infosys Foundation. This research made use of Stan,, through the PYSTAN interface,, NUMPY (Harris et al. 2020), SCIPY (Virtanen et al. 2020), STATSMODELS (Seabold & Perktold 2010), MATPLOTLIB (Hunter 2007), and CMASHER (van der Velden 2020). Data Availability: The data underlying this article are available in ‘RoboPol: AGN polarimetric monitoring data’, at
Group:Astronomy Department
Funding AgencyGrant Number
National Science Centre (Poland)2011/01/B/ST9/04618
National Science Centre (Poland)2017/25/B/ST9/02805
European Research Council (ERC)771282
Foundation of Research and Technology-Hellas Synergy Grants ProgramUNSPECIFIED
Institute of AstrophysicsUNSPECIFIED
Institute of Applied and Computational MathematicsUNSPECIFIED
Hellenic Foundation for Research and Innovation (HFRI)1552 CIRCE
NASA Hubble FellowshipHST-HF2-51444.001-A
Academy of Finland317383
Academy of Finland320085
Academy of Finland322535
Infosys FoundationUNSPECIFIED
Subject Keywords:polarization – galaxies: active – galaxies: jets – galaxies: nuclei
Issue or Number:1
Record Number:CaltechAUTHORS:20210412-133818771
Persistent URL:
Official Citation:S Kiehlmann, D Blinov, I Liodakis, V Pavlidou, A C S Readhead, E Angelakis, C Casadio, T Hovatta, N Kylafis, A Mahabal, N Mandarakas, I Myserlis, G V Panopoulou, T J Pearson, A Ramaprakash, P Reig, R Skalidis, A Słowikowska, K Tassis, J A Zensus, The time-dependent distribution of optical polarization angle changes in blazars, Monthly Notices of the Royal Astronomical Society, Volume 507, Issue 1, October 2021, Pages 225–243,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:108702
Deposited By: Tony Diaz
Deposited On:13 Apr 2021 21:40
Last Modified:04 Oct 2021 21:36

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