1803+784核心位移和法拉第旋转测量中非等分能区的证据：来自同时多频极化VGOS观测
Evidence of a non-equipartition energy regime in 1803+784 Core-shift and Faraday rotation measurements from simultaneous multi-frequency polarimetric VGOS observations
AI总结：
通过VGOS观测发现1803+784核心区域能量非等分，核心位移指数偏离预期值，揭示喷流几何或等分条件未满足，需考虑非等分效应。
摘要：
背景。活动星系核（AGN）紧凑喷流通常被认为在射电波长主导区域中粒子能量密度与磁场能量密度之间处于等分状态。这一假设对喷流物理和基于VLBI的天体测量和大地测量精度有重大影响。\n目标。我们通过使用VLBI全球观测系统（VGOS）进行的同时宽带全极化观测，检验了在厘米波长下AGN核心处能量等分假设的正确性，分析了变星1803+784。\n方法。我们展示了覆盖3-11 GHz频率范围的变星1803+784的VGOS观测。数据经过专用校准流程处理，随后进行模型拟合和多频成像分析。我们测量了频率依赖的核心位移，并在源上映射了频谱指数和旋转测量（RM）。\n结果。我们发现核心位移幂律指数为$ k_r = 0.73^{+0.12}_{-0.19} $，显著偏离预期的等分值$ k_r = 1 $。这表明在厘米波长核心区域，要么等分条件，要么锥形喷流几何形状，或两者均未得到满足。VGOS的宽频率覆盖也使我们能够将核心的法拉第旋转分解为内部成分（${\rm RM}_I = 121 \pm 8$ rad m$^{-2}$，产生在核心区域）和外部成分（${\rm RM}_E = -44 \pm 9$ rad m$^{-2}$，与远处扩展介质有关，可能也影响喷流下游区域的极化。\n结论。这些结果展示了VGOS在高保真同时多频极化研究紧凑AGN喷流方面的强大能力，并强调了在喷流天体物理学和大地VLBI中考虑非等分效应的必要性。
英文摘要：
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.
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