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In the current paradigm, it is believed that the compact VLBI radio core ofradio-loud AGNs represents the innermost upstream regions of relativisticoutflows. These regions of AGN jets have generally been modeled by a conicaloutflow with roughly constant opening angle and flow speed. Nonetheless, someworks suggest that a parabolic geometry would be more appropriate to fit thehigh energy spectral energy distribution properties and it has been recentlyfound that, at least in some nearby radio-galaxies, the geometry of theinnermost regions of the jet is parabolic. We compile here multi-frequency coresizes of archival data to investigate the typically unresolved upstream regionsof the jet geometry of a sample of 56 radio-loud AGNs. Data combined from thesources considered here is not consistent with the classic picture of a conicaljet starting in the vicinity of the super-massive black hole (SMBH), and mayexclude a pure parabolic outflow solution, but rather suggest an intermediatesolution with quasi-parabolic streams, which are frequently seen in numericalsimulations. Inspection of the large opening angles near the SMBH and the rangeof the Lorentz factors derived from our results support our analyses. Ourresult suggests that the conical jet paradigm in AGNs needs to be re-examinedby mm/sub-mm VLBI observations.

Resolving the Geometry of the Innermost Relativistic Jets in Active Galactic Nuclei