Helical magnetic field structure in 3C 273

A Faraday rotation analysis using multi-frequency polarimetric VLBA data

¹ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
² Korea Astronomy and Space Science Institute, Daedeok-daero 776, Yuseong-gu, Daejeon 34055, Republic of Korea
³ Department of Astronomy, Yonsei University, Yonsei-ro 50, Seodaemun-gu Seoul 03722, Republic of Korea
⁴ Institut de Radioastronomie Millimétrique, Avenida Divina Pastora, 7m Loca 20, E-18012 Granada, Spain
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁶ Orchideenweg 8, D-53123 Bonn, Germany

^⋆ Corresponding author: ttoscano@iaa.es

Received: 20 December 2024
Accepted: 15 April 2025

Abstract

We present a study on rotation measure (RM) of the quasar 3C 273. This analysis aims to discern the magnetic field structure and its temporal evolution. The quasar 3C 273 is one of the most studied active galactic nuclei due to its high brightness, strong polarisation, and proximity, which enables the resolving of the transverse structure of its jet in detail. We used polarised data from 2014, collected at six frequencies (5, 8, 15, 22, 43, 86 GHz) with the Very Long Baseline Array, to produce total and linear polarisation-intensity images, as well as RM maps. Our analysis reveals a well-defined transverse RM gradient across the jet, indicating a helical, ordered magnetic field that threads the jet and likely contributes to its collimation. Furthermore, we identified temporal variations in the RM magnitude when compared with prior observations. These temporal variations show that the environment around the jet is dynamic, with changes in the density and magnetic field strength of the sheath that are possibly caused by interactions with the surrounding medium.