1. H. Falcke, J. Phys.: Conf. Ser. 942, 012001 (2017).
2. J.-P. Luminet, arXiv: 1804.03909v1 (2018).
3. S. Nampalliwar and C. Bambi, Tutorial Guide to X-Ray and Gamma-Ray Astronomy: Data Reduction and Analysis, Ed. by C. Bambi (Springer, Singapore, 2019); arXiv: 1810.07041v3.
4. K. Akiyama, A. Alberdi, W. Alef, K. Asada, et al. (Event Horizon Telescope Collab.), Astrophys. J. 875, L1 (2019).
5. K. Akiyama, A. Alberdi, W. Alef, K. Asada, et al. (Event Horizon Telescope Collab.), Astrophys. J. 875, L2 (2019).