Evidence of a non-equipartition energy regime in 1803+784 Core-shift and Faraday rotation measurements from simultaneous multi-frequency polarimetric VGOS observations

TL;DR

This study uses simultaneous multi-frequency polarimetric VLBI observations of the blazar 1803+784 to reveal a deviation from equipartition in the jet core, impacting our understanding of AGN jet physics and VLBI measurements.

Contribution

The paper provides the first direct measurement of core shift and Faraday rotation across a broad frequency range, demonstrating non-equipartition conditions in an AGN jet core.

Findings

Core-shift power-law index deviates from equipartition value

Internal and external Faraday rotation components identified

Highlights importance of non-equipartition effects in jet physics

Abstract

Context. Compact jets from active galactic nuclei (AGN) are commonly assumed to be in equipartition between particle and magnetic-field energy densities at the regions where the radio emission dominates at centimetre wavelengths. This assumption has significant implications for both jet physics and the accuracy of VLBI-based astrometry and geodesy. Aims. We tested the validity of the energy equipartition hypothesis in AGN cores at centimetre wavelengths by analysing the blazar 1803+784 using simultaneous broadband full-polarization observations with the VLBI Global Observing System (VGOS). Methods. We present VGOS observations of the blazar 1803+784 covering the 3-11 GHz frequency range. The data were processed using a dedicated calibration pipeline, followed by model fitting and multi-frequency imaging analysis. We measured the frequency-dependent core shift and mapped the spectral index and rotation measure (RM) across the source. Results. We find a core-shift power-law index of $k_r = 0.73^{+0.12}_{-0.19}$, significantly deviating from the expected equipartition value of $k_r = 1$. This indicates that either the equipartition condition or the conical jet geometry, or both, are not fulfilled in the centimetre-wavelength core region. The wide frequency coverage of VGOS also allows us to decouple the Faraday rotation of the core into an internal component (${\rm RM}_I = 121 \pm 8$ rad m$^{-2}$, produced in the core region) and an external component (${\rm RM}_E = -44 \pm 9$ rad m$^{-2}$, associated with a distant, extended medium that may also affect the polarization in downstream regions of the jet at larger scales). Conclusions. These results demonstrate the power of VGOS for high-fidelity simultaneous multi-frequency polarimetric studies of compact AGN jets, and underline the need to account for non-equipartition effects in both jet astrophysics and geodetic VLBI.