Resolving the Innermost Geometry of Relativistic Jets in Active Galactic
  Nuclei
  In the current paradigm, it is believed that the compact VLBI radio core of
radio-loud active galactic nuclei (AGNi) represents the innermost upstream
regions of relativistic outflows. These regions of AGN jets have generally been
modeled by a conical outflow with a roughly constant opening angle and flow
speed. Nonetheless, some works suggest that a parabolic geometry would be more
appropriate to fit the high energy spectral distribution properties and it has
been recently found that, at least in some nearby radio galaxies, the geometry
of the innermost regions of the jet is parabolic. We compile here
multi-frequency core sizes of archival data to investigate the typically
unresolved upstream regions of the jet geometry of a sample of 56 radio-loud
AGNi. Data combined from the sources considered here are not consistent with
the classic picture of a conical jet starting in the vicinity of the
super-massive black hole (SMBH), and may exclude a pure parabolic outflow
solution, but rather suggest an intermediate solution with quasi-parabolic
streams, which are frequently seen in numerical simulations. Inspection of the
large opening angles near the SMBH and the range of the Lorentz factors derived
from our results support our analyses. Our result suggests that the conical jet
paradigm in AGNi needs to be re-examined by millimeter/sub-millimeter VLBI
observations.
