Multifrequency polarimetry of high-synchrotron peaked blazars probes the shape of their jets

¹ DiSAT, Università 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

Received: 15 April 2024
Accepted: 27 July 2024

Abstract

Multifrequency polarimetry is emerging as a powerful probe of blazar jets, especially with the advent of the Imaging X-ray Polarimetry Explorer (IXPE) space observatory. We studied the polarization of high-synchrotron peaked (HSP) blazars, for which both optical and X-ray emission can be attributed to synchrotron radiation from a population of nonthermal electrons. We adopted an axisymmetric stationary force-free jet model in which the electromagnetic fields are determined by the jet shape. When the jet is nearly parabolic, the X-ray polarization degree is Π_X ∼ 15–50%, and the optical polarization degree is Π_O ∼ 5–25%. The polarization degree is strongly chromatic: Π_X/Π_O ∼ 2–9. This chromaticity is due to the softening of the electron distribution at high energies, and is much stronger than for a uniform magnetic field. The electric vector position angle (EVPA) is aligned with the projection of the jet axis on the plane of the sky. These results compare very well with multifrequency polarimetric observations of HSP blazars. When the jet is instead nearly cylindrical, the polarization degree is large and weakly chromatic (we find Π_X ∼ 70% and Π_O ∼ 60%, close to the expected values for a uniform magnetic field). The EVPA is perpendicular to the projection of the jet axis on the plane of the sky. A cylindrical geometry is therefore practically ruled out by current observations. The polarization degree and the EVPA may be less sensitive to the specific particle acceleration process (e.g., magnetic reconnection or shocks) than previously thought.