Core Shift and Jet Structure in Quasar 3C 279 from Multi-Frequency VLBI Observations

Jets from active galactic nuclei (AGN) trace relativistic outflows emerging from the vicinity of ac creting supermassive black holes (SMBHs), and offer a direct probe of magnetized plasma dynamics under extreme conditions. In this work, we investigate the frequency-dependent position shift of the core in the highly luminous quasar 3C 279 (z = 0.538), using Very Long Baseline Interferometry (VLBI) aiming to constrain the magnetic field strength and opacity profile near the jet base. We analyze eight epochs of quasi-simultaneous VLBI observations at 15, 43, and 86 GHz, reaching an angular resolution of 0.16 milliarcseconds (∼0.9 parsecs). The source structure is modeled through 2D Gaussian components fitted directly to the interferometric visibilities, enabling the identification of the core and innermost downstream jet components at all three frequencies. By aligning optically thin features across frequencies, we determine the frequency-dependent shifts in the apparent core position caused by synchrotron self-absorption. These shifts are used to derive the radial depen dence of the magnetic field. We study the jet geometry and estimate the magnetic field strength near its base together with the evolution of parameters of the jet plasma. These estimates will explain if the magnetic field is what makes the jet of 3C 279 so bright.