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 ${\gt}96{{\ \rm per\ cent}}$ 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.

中文翻译：

---

耀变体中光偏振角变化的时间依赖性分布

在光学波长上，与伽马射线活动相关的耀变体电矢量位置角 (EVPA) 旋转十多年来一直是人们强烈关注和系统研究的主题。解释 EVPA 旋转的一个困难是测量中固有的 180° 模糊性。因此，在研究 EVPA 旋转时，必须确保连续观察之间的典型时间间隔（即节奏）足够短，以确保选择正确的模 180° 值。这种最佳节奏取决于耀变体中的最大内在 EVPA 旋转速度，目前尚不清楚。在本文中，我们针对 RoboPol 样本解决了以下问题：我们可以预期旋转大于 90° 的旋转速度范围是多少？检测这种旋转需要什么观察节奏？迄今为止，在 EVPA 轮换研究中是否遗漏了快速轮换？哪一部分数据受到歧义的影响？检测到的旋转受歧义影响的可能性有多大？我们用 RoboPol 仪器每周对耀变体的统计样本进行三个季节的光学偏振观测来回答这些问题，另外一个季节每天进行观测。我们在 1-30 天的时间尺度上对 EVPA 变化的分布进行建模，并估计超过 90° 的变化比例。