The jet cores in active galactic nuclei (AGNs) are resolved and found to harbor an edge-brightened structure where the jet base appears extended at the sides compared to its propagation axis. This peculiar phenomenon invites various explanations. We show that the photosphere of an optically thick jet base in AGNs is observed edge brightened if the jet Lorentz factor harbors an angular dependence. The jet assumes a higher Lorentz factor along the jet axis and decreases following a power law along its polar angle. For an observer near the jet axis, the jet has a lower optical depth along its propagation axis compared to off-axis regions. Higher optical depth at the outer region makes the jet photosphere appear to extend to larger radii compared to a deeper photosphere along its propagation axis. We tackle the problem both analytically and numerically, confirming the edge brightening through Monte Carlo simulations. Other than the edge brightening, the outcomes are significant as they provide a unique tool to determine the jet structure and associated parameters by their resolved observed cores. The study paves the way to explore the spectral properties of optically thick cores with structured Lorentz factors in the future.

中文翻译：

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活动星系核喷流的光学厚喷流基底和边缘增亮的解释


活动星系核（AGN）中的喷流核心被解析并发现其具有边缘增亮的结构，其中喷流底部与其传播轴相比似乎在侧面延伸。这种奇特的现象引起了各种解释。我们表明，如果射流洛伦兹因子具有角度依赖性，那么活动星系核中光学厚射流基底的光球层就会被观察到边缘变亮。射流沿射流轴呈现较高的洛伦兹因子，并沿其极角遵循幂律减小。对于射流轴附近的观察者来说，与离轴区域相比，射流沿其传播轴具有较低的光学深度。与沿其传播轴更深的光球相比，外部区域较高的光学深度使得射流光球看起来延伸到更大的半径。我们通过分析和数值方法解决了这个问题，通过蒙特卡罗模拟确认了边缘增亮。除了边缘增亮之外，结果也很重要，因为它们提供了一种独特的工具，可以通过解析的观察到的核心来确定射流结构和相关参数。该研究为未来探索具有结构化洛伦兹因子的光学厚核的光谱特性铺平了道路。