Abstract Quasi-simultaneous VLBI observations at 15-86 GHz have shown that in the classical superluminal radio source 3C 273, the spectral index α (Sν ∝ να) has a systematic variation along the jet. For epoch 1995.15, a spectral reversal was observed at core distance of ∼1.5 mas, where the superluminal knot C12 located. Similarly, for epoch 1997.18, two spectral reversals were observed at core distances of ∼1.8 mas and ∼ 4.2 mas, where superluminal knots C11 and C14 were, respectively. These spectral reversals are associated with local maxima of the jet width. We suggest that this phenomenon may be related to a stratification of the jet structure, i.e., its physical parameters (flow velocity, Doppler factor, electron density and energy, magnetic field strength, etc.) are substantially dependent on the distance from the jet axis. These properties may be naturally formed through gasdynamic processes when the jet expands into a lower pressure ambient medium.

Keywords galaxies: active --- galaxies: compact --- quasars: individual: 3C 273 --- galaxies: jets --- radio continuum: galaxies --- radiation mechanisms: non-thermal

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