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

TL;DR

This study investigates the innermost regions of AGN jets, revealing that their geometry is likely intermediate between conical and parabolic, challenging the traditional conical jet model and suggesting the need for further high-resolution observations.

Contribution

The paper provides observational evidence that AGN jet geometry is intermediate, not purely conical or parabolic, prompting a re-evaluation of jet models.

Findings

Jet geometry is inconsistent with a purely conical model.

Evidence supports an intermediate, quasi-parabolic jet structure.

Results highlight the need for mm/sub-mm VLBI observations.

Abstract

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.