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It has become commonplace is astronomy to describe the transverse coarsestructure of jets in loosely defined terms such as "sheath" and "spine" basedon discussions of parsec scale properties. But, the applicability, dimensionand prominence of these features on sub-lt-yr scales has previously beenunconstrained by observation. The first direct evidence of jet structure nearthe source in M\,87 is extreme limb brightening (a double-rail morphology), 0.3- 0.6 mas from the source, that is prominent in observations with highresolution and sensitivity. Intensity cross-cuts of these images provide threestrong, interdependent constraints on the geometry responsible for thedouble-rail morphology: the rail to rail separation, the peak to troughintensity ratio and the rail widths. Analyzing these constraints indicates thathalf or more of the jet volume resides in a thick-walled, tubular, mildlyrelativistic, protonic jet only $\sim 0.25$ lt-yr (or $\sim 300$ M, where M isthe central black hole mass in geometrized units) from the source. By contrast,the Event Horizon Telescope Collaboration interprets their observations withthe aid of general relativistic magnetohydrodynamic simulations that produce aninvisible (by construction) jet with a surrounding luminous, thin sheath. Yet,it is shown that synthetic images of simulated jets are center brightened 0.3 -0.6 mas from the source. This serious disconnection with observation occurs ina region previously claimed in the literature to be well represented by thesimulations. The limb brightening analysis motivates a discussion of possiblesimulation modifications to improve conformance with observations.

The Details of Limb Brightening Reveal the Structure of the Base of the Jet in M\,87 for the First Time