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Intra-night optical polarization monitoring of blazars

Aristeidis Polychronakis, Ioannis Liodakis, Anastasia Glykopoulou, Dmitry Blinov, Ivan Agudo, Svetlana G. Jorstad, Beatriz Agis-Gonzalez, Sara Capecchiacci, Alberto Floris, Sebastian Kielhmann, John A. Kypriotakis, Dimitrios A. Langis, Nikos Mandarakas, Karan Pal, Francisco J. Aceituno, Giacomo Bonnoli, Victor Casanova, Gabriel Emery, Juan Escudero Pedrosa, Jorge Otero-Santos, Daniel Morcuende, Alfredo Sota, Vilppu Piirola

Abstract

Blazars are known for their extreme variability across the electromagnetic spectrum. Variability at very short timescales can push the boundaries between competing models offering us much needed discriminating power. This is particularly true for polarization variability that allows us to probe particle acceleration and high-energy emission models in blazars. Here we present results from the first pilot study of intra-night optical polarization monitoring conducted using RoboPol at the Skinakas Observatory and supplemented by observations from the Calar Alto, Perkins, and Sierra Nevada observatories. Our results show that while variability patterns can widely vary between sources, variability on timescales as short as minutes is prevalent in blazar jets. The amplitude of variations are typically small, a few percent for the polarization degree and less than 20 degrees for the polarization angle, pointing to a significant contribution to the optical emission from a turbulent magnetic field component, while the overall stability of the polarization angle over time points to a preferred magnetic field orientation.

Intra-night optical polarization monitoring of blazars

Abstract

Blazars are known for their extreme variability across the electromagnetic spectrum. Variability at very short timescales can push the boundaries between competing models offering us much needed discriminating power. This is particularly true for polarization variability that allows us to probe particle acceleration and high-energy emission models in blazars. Here we present results from the first pilot study of intra-night optical polarization monitoring conducted using RoboPol at the Skinakas Observatory and supplemented by observations from the Calar Alto, Perkins, and Sierra Nevada observatories. Our results show that while variability patterns can widely vary between sources, variability on timescales as short as minutes is prevalent in blazar jets. The amplitude of variations are typically small, a few percent for the polarization degree and less than 20 degrees for the polarization angle, pointing to a significant contribution to the optical emission from a turbulent magnetic field component, while the overall stability of the polarization angle over time points to a preferred magnetic field orientation.

Paper Structure

This paper contains 5 sections, 3 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Sample & Analysis
  3. Results
  4. Comparison with archival observations
  5. Conclusions

Figures (5)

  • Figure 1: Distributions of the $1/\eta$ metric for the polarization degree ($\Pi$, upper panel) and the polarization angle ($\Psi$, lower panel), across all 65 observing nights. The vertical red dashed line in each panel indicates the median of the distribution, which also serves as the threshold separating variable from non-variable nights.
  • Figure 2: Intra-night RoboPol observations of BL Lac (example of a source showing polarization variability). The top panel shows the polarization degree, and the bottom panel the polarization angle. The observations have an average cadence of 3.31 minutes.
  • Figure 3: Intra-night RoboPol observations of J1542+6129 (example of a source without polarization variability). The top panel shows the polarization degree, and the bottom panel the polarization angle. The observations have an average cadence of 2.29 minutes.
  • Figure 4: Intra-night RoboPol observations of J2148+0657 (example of a source with binned observations; two consecutive images combined). The grey points show the unbinned observations. The top panel shows the polarization degree, and the bottom panel the polarization angle. The upper limits have been omitted for clarity. The observations have an average cadence of 12.99 minutes.
  • Figure 5: Comparison of our results (intra-night monitoring) with those of the previous survey (long-term monitoring; Blinov2021). The left panel shows the polarization degree ($\Pi$), and the right panel the polarization angle ($\Psi$). Different colors indicate different sources. The black dashed line on each panel marks equality ($\rm{y = x}$) and illustrates the agreement between the long-term and intra-night monitoring observations.