发表机构
University of Valladolid UVA; Universidade Federal do Pará; Universidade Federal do Sul e Sudeste do Pará; Universidade Federal do Espírito Santo; Universidade Federal do Ceará（巴利亚多利德大学; 帕拉联邦大学; 南和东南帕拉联邦大学; 圣埃斯皮里图联邦大学; 塞阿拉联邦大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本文研究广义弦云环绕黑洞的光学特征，发现弦参数改变光子球与阴影，并约束Sgr A*和M87*参数，且弦云非均匀修改透镜图像。
AI 中文摘要
本文研究了由Letelier-Alencar时空描述的广义弦云环绕黑洞的光学特征。我们首先分析零测地线，表明弦参数相对于史瓦西值可以增大或减小光子球半径和临界碰撞参数。由于时空不是渐近平坦的，临界碰撞参数并不直接等于物理阴影半径。我们利用这一区别，从Sgr A*和M87*的阴影半径界限推导出模型参数的约束。在弱场区域，我们使用Gauss-Bonnet方法和零测地线方程的微扰展开计算光偏转角，获得一致的结果，其中包含渐近锥形几何的贡献以及局部引力偏转。我们还研究了Shapiro时间延迟，并识别出与非闵可夫斯基渐近背景相关的距离依赖贡献。最后，我们研究了几何薄且光学薄的吸积盘的光学外观，并通过反向光线追踪构建天球图像。传递函数和透镜图像表明，广义弦云以非均匀方式修改零测地线映射，而不是简单缩放史瓦西图像。特别是，阴影较小的配置仍可能导致主爱因斯坦环图像向外位移，表明不同的光学观测量探测了该几何中光子传播的不同方面。
英文摘要
In this work, we investigate the optical signatures of black holes surrounded by a generalized cloud of strings, described by the Letelier--Alencar spacetime. We first analyze null geodesics, showing that the string parameters can either increase or decrease the photon sphere radius and the critical impact parameter relative to their Schwarzschild values. Since the spacetime is not asymptotically flat, the critical impact parameter does not directly coincide with the physical shadow radius. We use this distinction to derive constraints on the model parameters from the shadow radius bounds of Sgr A* and M87*. In the weak-field regime, we calculate the light deflection angle using both the Gauss--Bonnet method and a perturbative expansion of the null geodesic equations, obtaining consistent results that contain a contribution from the asymptotically conical geometry in addition to the local gravitational deflection. We also examine the Shapiro time delay and identify a distance dependent contribution associated with the non Minkowskian asymptotic background. Finally, we study the optical appearance of geometrically and optically thin accretion disks and construct celestial sphere images through backward ray tracing. The transfer functions and lensing images show that the generalized cloud of strings modifies the null geodesic mapping in a nonuniform manner rather than producing a simple rescaling of the Schwarzschild image. In particular, configurations with a smaller shadow may still produce an outward displacement of the main Einstein ring image, demonstrating that different optical observables probe distinct aspects of photon propagation in this geometry.
Comments24 pages, 13 figures. Comments are welcome. V2: Some references added
