RadioAstron reveals a spine/sheath jet structure in 3C273

Space-VLBI RadioAstron observations at 1.6 GHz of the flat spectrum radio quasar 3C 273, performed in the framework of the AGN polarization KSP, have achieved the best angular resolution at 1.6 GHz to date, successfully imaging a limb-brightening in the jet not previously detected in this source. In contrast, at 4.8 GHz, we detected emission from a central stream of plasma, with a spatial distribution complementary to the limb-brightened emission, indicating an origin in the spine of the jet. While a stratification across the jet width in the flow density, internal energy, magnetic field, or bulk flow velocity are usually invoked to explain the limb-brightening, the different jet structure detected at the two frequencies probably requires a stratification in the emitting electron energy distribution. Ongoing dedicated numerical simulations will allow the determination of which combination of physical parameters are needed to reproduce the spine/sheath structure observed by RadioAstron in 3C 273.