Direct observation of black holes is one of the grand challenges in astronomy. If there are super-compact objects which possess unstable circular orbits of photons, however, it may be difficult to distinguish them from black holes by observing photons. As a model of super-compact objects, we consider a gravastar (gravitational-vacuum-star) which was originally proposed by Mazur and Mottola. For definiteness, we adopt a spherical thin-shell model of a gravastar developed by Visser and Wiltshire, which connects interior de-Sitter geometry and exterior Schwarzschild geometry. We find that unstable circular orbits of photons can appear around the gravastar. Then, we investigate the optical images of the gravastar possessing unstable circular orbits, with assuming the optically transparent surface of it and two types of optical sources behind the gravastar: (i) an infinite optical plane and (ii) a companion star. The main feature of the image of (i) is that a bright disk and a dark thick ring surrounding the disk appear in the center of the region which would be completely dark if the compact object was not the gravastar but Schwarzschild black hole. Also in the case (ii), a small disk and arcs around the disk appear in the region which would be completely dark for the lensing image by Schwarzschild black hole. Because characteristic images appear inside the gravastar in both cases, we could tell the difference between a black hole and a gravastar with high-resolution VLBI observations near future.
Multiple Rings in the Shadow of Extremely Compact Objects
事件视界望远镜拍摄的M87黑洞图像为研究黑洞物理提供了一个令人兴奋的机会。由于黑洞的事件视界吸收了所有的电磁波，因此很难主动探测视界的存在。然而，借助于一类极其紧凑的、无视界的天体，即引力星，其外部时空与黑洞的时空几乎相同，人们可以检验事件视界的缺失：由于引力星或其表面的存在而产生的附加特征的缺失，可以用作黑洞的定量证据。我们应用Gralla et al.的方法来研究黑洞图像研究的两种类型的引力星的图像：透明的和反射的。在这两种情况下，射线通过重力仪的传输，或者它们在其表面的反射，导致它们的图像中有更多的环。对于简单的辐射模型，其中红移的辐射率的磁盘是在一个特定的半径r峰达到峰值，一系列的环的位置可以在一个简单的方式与光线传播：一个环出现在周围的碰撞参量b时，从无限的光线入射在b相交的磁盘r峰。我们表明，额外的环将出现在透明和反射重力星的图像。特别地，反射重力仪的附加环之一是由于重力仪表面上的光的迅速光的反射，并且看起来与其他环很好地分离。这可以是一个鲁棒的特征，可以可靠地用于约束黑洞视界的反射率。
Dynamical gravastars may evade no-go results for exotic compact objects, together with further analytical and numerical results for the dynamical gravastar model
Observational Signatures of Rotating Ayón-Beato-García Black Holes: Shadows, Accretion Disks and Images
College of Physics Science and Technology, Hebei University（河北大学物理科学与技术学院）；Hebei Key Lab of Optic-Electronic Information and Materials, Hebei University（河北大学河北省光电信息材料重点实验室）；National-Local Joint Engineering Laboratory of New Energy Photoelectric Devices, Hebei University（河北大学新能源光电器件省部共建国家重点实验室培育基地）；Key Laboratory of High-precision Computation and Application of Quantum Field Theory of Hebei Province, Hebei University（河北大学量子场论高精度计算与应用重点实验室）
Reshaping the inner shadow of a Kerr black hole by a torn accretion disk
School of Mathematics and Physics, University of South China（南华大学数学与计算学院）；School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）；Department of Mechanics, Chongqing Jiaotong University（重庆交通大学力学系）
Imprints of Black Hole Shadows and Polarization Patterns of Various Thick Disks: Bumblebee gravity
University of Management and Technology（管理技术大学）；Princess Nourah bint Abdulrahman University（公主努拉·宾特·阿卜杜勒·拉赫曼大学）；Chongqing Jiaotong University（重庆交通大学）；Chongqing Normal University（重庆师范大学）
Imprints of Dark Matter on the Shadow and Polarization Images of a Black Hole Illuminated by Various Thick Disks
University of Management and Technology（管理技术大学）；COMSATS University Islamabad, Lahore Campus（科摩萨茨伊斯兰堡大学拉合尔校区）；Chongqing Jiaotong University（重庆交通大学）；Chongqing Normal University（重庆师范大学）
Shadows and Thin-Disk Images of Kerr-Newman Black Holes in a Bertotti-Robinson Magnetic Field
Institute of Fundamental Physics and Quantum Technology, and School of Physical Science and Technology, Ningbo University（宁波大学基础物理与量子技术研究所、物理科学与技术学院）；School of Physics, and Center for High Energy Physics, Peking University（北京大学物理学院、高能物理中心）
Shadows and Polarimetric Signatures of Rotating Simpson-Visser Black Holes with Thick Disk Illumination
Sichuan Minzu College（四川民族学院）；Xihua University（西华大学）；Chongqing Normal University（重庆师范大学）；University of Management and Technology（管理技术大学）；COMSATS University Islamabad, Lahore-Campus（科迈茨伊斯兰堡大学拉合尔校区）；Princess Nourah bint Abdulrahman University（公主努拉·宾特·阿卜杜勒拉赫曼大学）
Novel extended inner shadow in images of Johannsen-Psaltis black holes with thin accretion disks
Beijing Normal University（北京师范大学）；Key Laboratory of Multiscale Spin Physics (Beijing Normal University), Ministry of Education（多尺度自旋物理教育部重点实验室（北京师范大学））；Beijing Information Science and Technology University（北京信息科技大学）；Faculty of Arts and Sciences, Beijing Normal University, Zhuhai（北京师范大学珠海校区文理学院）
Visual Characteristics of a Rotating Black Hole in $4$D Einstein-Gauss-Bonnet Gravity with Thin Accretion Disk Under EHT Constraints
University of Management and Technology（管理技术大学）；University of Okara（奥卡拉大学）；Chongqing Normal University（重庆师范大学）；Imam Mohammad Ibn Saud Islamic University (IMSIU)（伊玛目穆罕默德·本·沙特伊斯兰大学）
