Resolving the Geometry of the Innermost Relativistic Jets in Active
  Galactic Nuclei
  In the current paradigm, it is believed that the compact VLBI radio core of
radio-loud AGNs represents the innermost upstream regions of relativistic
outflows. These regions of AGN jets have generally been modeled by a conical
outflow with 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 energy 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 AGNs. Data combined from the
sources considered here is 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 AGNs needs to be re-examined
by mm/sub-mm VLBI observations.
