Paper

{
  "raw_xml": "<entry>\n    <id>http://arxiv.org/abs/2602.20746v1</id>\n    <title>Evidence of a non-equipartition energy regime in 1803+784 Core-shift and Faraday rotation measurements from simultaneous multi-frequency polarimetric VGOS observations</title>\n    <updated>2026-02-24T10:21:12Z</updated>\n    <link href='https://arxiv.org/abs/2602.20746v1' rel='alternate' type='text/html'/>\n    <link href='https://arxiv.org/pdf/2602.20746v1' rel='related' title='pdf' type='application/pdf'/>\n    <summary>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.\n  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).\n  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.\n  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).\n  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.</summary>\n    <category scheme='http://arxiv.org/schemas/atom' term='astro-ph.HE'/>\n    <category scheme='http://arxiv.org/schemas/atom' term='astro-ph.IM'/>\n    <published>2026-02-24T10:21:12Z</published>\n    <arxiv:comment>Published in Astron. Astrophys. (A&amp;A) Letters, 700, L22 (2025). 6 pages, 3 figures. Published online: 20 Aug 2025</arxiv:comment>\n    <arxiv:primary_category term='astro-ph.HE'/>\n    <arxiv:journal_ref>Astron. Astrophys., 700, L22 (2025)</arxiv:journal_ref>\n    <author>\n      <name>V. Pérez-Díez</name>\n    </author>\n    <author>\n      <name>I. Martí-Vidal</name>\n    </author>\n    <author>\n      <name>E. Albentosa-Ruiz</name>\n    </author>\n    <author>\n      <name>R. Bachiller</name>\n    </author>\n    <arxiv:doi>10.1051/0004-6361/202555706</arxiv:doi>\n    <link href='https://doi.org/10.1051/0004-6361/202555706' rel='related' title='doi'/>\n  </entry>"
}