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 $k_r = 0.73^{+0.12}_{-0.19}$, 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.

Figures (3)

• Figure 1: Measured relative core shift as a function of frequency for 1803+784. The total shift vector, along with its components in right ascension and declination, are shown. The error bars represent 3$\sigma$ uncertainties. The best-fit model is shown as a dashed green line.
• Figure 2: Fit of the polarization angle as a function of $\lambda^2$ for the core and jet regions of 1803+784. The shaded regions represent the 3$\sigma$ confidence intervals of the model fits. Band D data have been averaged because of the low S/N in this band.
• Figure 3: Multi-frequency polarimetric image of 1803+784. Panels (a)--(c) show the total intensity maps at bands A, C, and D in logarithmic scale, normalized to their peak. The dashed vertical line indicates the core position at band D, marking the core shift. Panel (e) displays all contours from the four bands superposed, along with the core and jet Gaussian models. Panel (f) shows the spectral index map ($\alpha$). Panel (g) presents the rotation measure (RM) map. Only regions above 0.8% of the peak intensity are included.