Spectral reversal and stratification of the jet in 3C 273

Quasi-simultaneous VLBI observations at 15-86 GHz have shown that in the classical superluminal radio source 3C 273, the spectral index a (S-nu alpha nu (alpha)) has a systematic variation along the jet. For epoch 1995.15, a spectral reversal was observed at core distance similar to1.5 mas, where the superluminal knot C12 located. Similarly, for epoch 1997.18, two spectral reversals were observed at core distances of similar to1.8 mas and similar to4.2 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.