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The jet cores in blazars are resolved and found to harbour an edge brightenedstructure where the jet base appears extended at sides compared to itspropagation axis. This peculiar phenomenon invites various explanations. Weshow that the photosphere of an optically thick jet base in Active GalacticNuclei (AGNs) is observed edge brightened if the jet Lorentz factor harbours anangular dependence. The jet assumes a higher Lorentz factor along the jet axisand decreases following a power law along its polar angle. For an observer nearthe jet axis, the jet has a lower optical depth along its propagation axiscompared to off axis regions. Higher optical depths at the outer region makesthe jet photosphere appear to extend to larger radii compared to a deeperphotosphere along its propagation axis. We tackle the problem both analyticallyand numerically, confirming the edge brightening through Monte Carlosimulations. Other than the edge brightening, the outcomes are significant asthey provide a unique tool to determine the jet structure and associatedparameters by their resolved observed cores. The study paves way to explore thespectral properties of optically thick cores with structured Lorentz factors inthe future.

Optically thick jet base and explanation of edge brightening in AGN jets