RadioAstron reveals a spine-sheath jet structure in 3C 273
  We present Space-VLBI RadioAstron observations at 1.6 GHz and 4.8 GHz of the
flat spectrum radio quasar 3C 273, with detections on baselines up to 4.5 and
3.3 Earth Diameters, respectively. Achieving the best angular resolution at 1.6
GHz to date, we have imaged 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. Future dedicated numerical simulations will allow the
determination of which combination of physical parameters are needed to
reproduce the spine/sheath structure observed by Space-VLBI with RadioAstron in
3C 273
