1. Abbott, B.P., et al.: LIGO: the laser interferometer gravitational-wave observatory. Rep. Progress Phys. 72, 076901 (2009)
2. Event Horizon Telescope Collaboration, Akiyama, K., et al., First M87 event horizon telescope results. I. The shadow of the supermassive black hole. Astrophys. J. 875(1), L1 (2019). arXiv:1906.11238
3. Cardoso, V., Pani, P.: The observational evidence for horizons: from echoes to precision gravitational-wave physics. arXiv:1707.03021
4. Cardoso, V., Pani, P.: Tests for the existence of black holes through gravitational wave echoes. Nat. Astron. 1(9), 586–591 (2017). arXiv:1709.01525
5. Cardoso, V., Pani, P.: Testing the nature of dark compact objects: a status report. Liv. Rev. Rel. 22(1), 4 (2019). arXiv:1904.05363