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

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 $k_r=0.73_{-0.19}^{+0.12}$, 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 (${\mathrm{RM}}_I=121\pm 8$ rad m${}^{-2}$, produced in the core region) and an external component (${\mathrm{RM}}_E=-44\pm 9$ 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.

Published in Astron. Astrophys. (A&A) Letters, 700, L22 (2025). 6 pages, 3 figures. Published online: 20 Aug 2025

References in corpus (13)

• The Gaia mission
• Gaia Data Release 3: Summary of the content and survey properties
• The third realization of the International Celestial Reference Frame by very long baseline interferometry
• MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VIII. Faraday rotation in parsec-scale AGN jets
• Opacity in compact extragalactic radio sources and its effect on astrophysical and astrometric studies
• Multiwaveband Polarimetric Observations of 15 Active Galactic Nuclei at High Frequencies: Correlated Polarization Behavior
• Jet collimation in NGC 315 and other nearby AGN
• Multi-frequency Black Hole Imaging for the Next-Generation Event Horizon Telescope
• Polarization calibration techniques for new-generation VLBI
• Exploring the disk-jet connection in NGC 315
• Evidence of the $Gaia$--VLBI position differences being related to radio source structure
• Impacts of the image alignment over frequency for VLBI Global Observing System
• Towards an astronomical use of new-generation geodetic observations. I. From the correlator to full-polarization images

Cited by in corpus (1)

• Magnetic field and plasma number density from radio and millimeter core measurements in AGN jets