The Distribution of Rotation Speeds in Optical Polarization Position Angle Rotations in Blazars. (arXiv:2104.02622v1 [astro-ph.HE])

The Distribution of Rotation Speeds in Optical Polarization Position Angle Rotations in Blazars. (arXiv:2104.02622v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kiehlmann_S/0/1/0/all/0/1">S. Kiehlmann</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Blinov_D/0/1/0/all/0/1">D. Blinov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Liodakis_I/0/1/0/all/0/1">I. Liodakis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pavlidou_V/0/1/0/all/0/1">V. Pavlidou</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Readhead_A/0/1/0/all/0/1">A. C. S. Readhead</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Angelakis_E/0/1/0/all/0/1">E. Angelakis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Casadio_C/0/1/0/all/0/1">C. Casadio</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hovatta_T/0/1/0/all/0/1">T. Hovatta</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kylafis_N/0/1/0/all/0/1">N. Kylafis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mahabal_A/0/1/0/all/0/1">A. Mahabal</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mandarakas_N/0/1/0/all/0/1">N. Mandarakas</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Myserlis_I/0/1/0/all/0/1">I. Myserlis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Panopoulou_G/0/1/0/all/0/1">G. V. Panopoulou</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pearson_T/0/1/0/all/0/1">T. J. Pearson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ramaprakash_A/0/1/0/all/0/1">A. Ramaprakash</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Reig_P/0/1/0/all/0/1">P. Reig</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Skalidis_R/0/1/0/all/0/1">R. Skalidis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Slowikowska_A/0/1/0/all/0/1">A. Slowikowska</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tassis_K/0/1/0/all/0/1">K. Tassis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zensus_J/0/1/0/all/0/1">J. A. Zensus</a>

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{deg} 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{deg} 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 questions of (i) the upper limit of rotation speeds for rotations
greater than 90{deg}, (ii) the observation cadence required to detect such
rotations, (iii) whether rapid rotations have been missed in EVPA rotation
studies thus far, (iv) what fraction of data is affected by the ambiguity, and
(v) how likely detected rotations are 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 1-3 days and estimate the fraction of changes
exceeding 90{deg}. We show that daily observations are necessary to measure
>96% of optical EVPA variability in blazars correctly and that intra-day
observations are needed to measure the fastest rotations that have been seen
thus far.

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