Temporal variability of polarization in blazars

¹ DiSAT, Université dell’Insubria Via Valleggio 11 I-22100 Como, Italy
² INAF – Osservatorio Astronomico di Brera Via E. Bianchi 46 I-23807 Merate, Italy
³ Gran Sasso Science Institute Viale F. Crispi 7 I-67100 L’Aquila, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 30 September 2025
Accepted: 10 January 2026

Abstract

We investigated the temporal variability of the polarization of synchrotron radiation from blazar jets. Multiwavelength observations reveal high-amplitude rotations of the electric vector position angle (EVPA), both in the optical and in the X-rays. The polarization degree and the EVPA often show seemingly erratic variability. To interpret these observations, we present a geometric and deterministic model in which off-axis, compact emitting features (i.e., blobs) propagate along the jet at the local flow velocity. The jet’s electromagnetic fields determine the dynamics of the blobs, which we calculated self-consistently using an analytical model of magnetically dominated outflows. The jet is axisymmetric, and its electromagnetic fields lack a turbulent component. We show that the observed polarization is sensitive to the blobs’ initial spatial configurations. For the same jet structure, we observe several remarkably complex polarization patterns, including large EVPA rotations of 180° or more in both directions and more erratic fluctuations. Simultaneous high-amplitude variations of the polarization degree and the EVPA can coincide with peaks in the observed luminosity. However, seemingly uncorrelated variations are also possible. We discuss the feasibility of constraining the particle acceleration mechanism from multifrequency polarimetric observations.