2025A&A...700L..22P

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_I = 121 ± 8 rad m^–2, produced in the core region) and an external component (RM_E = -44 ± 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).
Conlcusions. 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.