[Paper Review] Multi-frequency linear and circular radio polarization monitoring of jet emission elements in $Fermi$ blazars

I. Myserlis, E. Angelakis+6 more|arXiv (Cornell University)|Jan 9, 2014

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This study presents multi-frequency, high-cadence radio polarization monitoring of PKS 1510−089 within the F-GAMMA program, revealing a complex polarization evolution during a major flaring event. The observed 90° electric vector position angle (EVPA) swings around MJD 55900 are interpreted as radiative swings due to opacity transitions, providing direct evidence of optically thick to thin regime evolution in relativistic jet outflows.

ABSTRACT

Radio emission in blazars -- the aligned subset of Active Galactic Nuclei (AGN) -- is produced by synchrotron electrons moving relativistically in their jet's magnetic field. Under the assumption of some degree of uniformity of the field, the emission can be highly polarized -- linearly and circularly. In the radio regime, the observed variability is in most of the cases attributed to flaring events undergoing opacity evolution, i.e. transitions from optically thick to thin emission (or vice versa). These transistions have a specific signature in the polarization parameter space (angle and magnitude) which can be traced with high cadence polarization monitoring and provide us with a unique probe of the microphysics of the emitting region. Here we present the full Stokes analysis of radio emission from blazars observed in the framework of the F-GAMMA program and discuss the case study of PKS\,1510$-$089 which has shown a prominent polarization event around MJD 55900.

Motivation & Objective

• To investigate the microphysical properties of relativistic jets in Fermi blazars using high-cadence multi-frequency radio polarization monitoring.
• To distinguish between geometric and radiative mechanisms driving electric vector position angle (EVPA) swings in polarized jet emission.
• To analyze the full Stokes parameters (I, Q, U, V) to trace opacity evolution and magnetic field topology in jet emission elements.
• To identify and characterize polarization events linked to spectral evolution and flaring activity in PKS 1510−089.
• To determine whether observed EVPA rotations are due to radiative swings (opacity transitions) or geometric motion of emitting plasma.

Proposed method

• Conducted high-cadence, multi-frequency (5–23 GHz) polarimetric observations using the 100-m Effelsberg telescope as part of the F-GAMMA program.
• Performed full Stokes parameter analysis (I, Q, U, V) to derive linear polarization degree, EVPA, and circular polarization from calibrated data.
• Tracked spectral index evolution (α) across bands (2.6–8.4 GHz and 10.5–23.1 GHz) to infer optical depth (τ) and distinguish between optically thick and thin regimes.
• Modeled EVPA evolution using two competing mechanisms: geometric rotation from helical jet motion and radiative swing from opacity transitions (τ ≈ 1).
• Used the theoretical prediction that EVPA rotates by exactly 90° during radiative swings when τ ≈ 1, with polarization degree dropping to zero.
• Compared observed rotation rates (1.2°/day at 5 GHz, 0.5–0.8°/day at 10 GHz) with theoretical expectations to infer the nature of the EVPA swings.

Experimental results

Research questions

• RQ1 What causes the observed 90° EVPA swings in PKS 1510−089 around MJD 55900, and is it due to radiative or geometric mechanisms?
• RQ2 How do the polarization degree and spectral index evolve during the flaring event, and what do they reveal about the optical depth of the emitting region?
• RQ3 Can the observed polarization behavior be explained by a single emission element undergoing opacity evolution, or are multiple components required?
• RQ4 What is the role of circular polarization in tracing the magnetic field topology and viewing angle in relativistic jets?
• RQ5 How do the observed rotation rates of the EVPA compare with theoretical predictions for radiative swings?

Key findings

• The EVPA in PKS 1510−089 rotated by approximately 125° from ~65° to −60° between MJD 55600 and 56170, with a rate of 1.2°/day at 5 GHz and 0.5°/day at 10 GHz.
• A second, opposing EVPA rotation of ~100° (from −60° to 40°) occurred after MJD 56170, with a rate of 1.2°/day at 5 GHz and 0.8°/day at 10 GHz.
• The polarization degree remained stable at ~2–3% throughout the flaring event, indicating no opacity transition occurred during the main flare, consistent with optically thick emission (α > 0).
• The two fast EVPA rotations around MJD 56170 are interpreted as radiative swings due to opacity transitions (τ ≈ 1), each causing a 90° rotation and a dip in polarization degree.
• The observed rotation rates and spectral behavior support a model in which a second emission element, initially optically thick, undergoes a transition to optically thin regime, causing the double EVPA swing.
• The absence of polarization degree drop during the main flare suggests that the initial emission element remained optically thick, while the radiative swings were driven by a subsequent, distinct emission component.