Speaker
Description
The images the supermassive black holes at the centers of the galaxies M87 and the Milky Way, obtained by the Event Horizon Telescope (EHT) Collaboration, show a dark region known as the shadow surrounded by a bright ring. The trajectories of light rays are strongly deflected in the close vicinity of black holes and part of photons are trapped, resulting in the presence of the shadow in the observer sky. The shadow is circular in the case of a static black hole, while for a rotating one is deformed in an asymmetric form which depends on the spin of the compact object and the angle of observation. The contour of the shadow can be obtained analitically with a separation of variables in the Hamilton-Jacobi equation for photons. This method, firstly intruduced for the Kerr spacetime, has then been extended to a large class of geometries, which include rotating black holes resulting from the modified Newman-Janis algorithm. The procedure has been recently extended to black holes surrounded by a plasma which obeys a suitable separability condition. The observation of the shadow can be a tool for the study of models supermassive black holes and to find possible deviations from the Kerr solution. These deviations requiere improved capabilities with respect to the current EHT array. In this talk, a review of the current analytical research is presented along with the prospects of the next-generation Event Horizon Telescope (ngEHT).
