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A recent article on high-resolution 86 GHz observations with the Global
Millimeter VLBI Array, the phased Atacama Large Millimeter/submillimeter Array,
and the Greenland Telescope describes the detection of a limb-brightened
cylindrical jet, $25 \mu\rm{as}< z< 100 \mu\rm{as}$, where $z$ is the axial
displacement from the supermassive black hole in the sky plane. It was shown to
be much wider and much more collimated than 2D simulations of electromagnetic
(Blandford-Znajek) jets from the event horizon predicted. This was an
unanticipated discovery. The claimed detection of a jet connected to the
accretion flow provides a direct observational constraint on the geometry and
physics of the jet launching region for the first time in any black hole jetted
system. This landmark detection warrants further analysis. This Letter focuses
on the most rudimentary properties, the shape and size of the source of the
detected jet emission, the determination of which is not trivial due to
line-of-sight effects. Simple thick-walled cylindrical shell models for the
source were analyzed to constrain the thickness of the jet wall. The analysis
indicates a tubular jet source with a radius $R\approx 144 \mu\rm{as}\approx
38M$ and that the tubular jet walls have a width $W \approx 36\mu\rm{as}
\approx 9.5 M$, where $M$ is the geometrized mass of the black hole (a volume
comparable to that of the interior cavity). The observed cylindrical jet
connects continuously to the highly limb-brightened jet (previously described
as a thick-walled tubular jet) that extends to $z> 0.65$ mas, and the two are
likely in fact the same outflow (i.e., from the same central engine).
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