The time-dependent distribution of optical polarization angle changes in blazars
Funding Information: 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, Tor?n 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 Astrophysicsand the Institute of Applied and Computational Mathematics and by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the 'First Call forH.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, https://mc-stan.org/, through the PYSTAN interface, https://pystan.readthedocs.io/, NUMPY (Harris et al. 2020), SCIPY (Virtanen et al. 2020), STATSMODELS (Seabold & Perktold 2010), MATPLOTLIB (Hunter 2007), and CMASHER (van der Velden 2020). Publisher Copyright: © 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. ; 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 degrees 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 degrees 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 degrees 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 degrees. 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. ; Peer reviewed