Black hole shadows: analytical calculations and future observational prospects (presentación oral, expositor)

JAVIER BADÍA; ERNESTO F. EIROA

Córdoba

Conferencia; Grav24; 2024

FAMAF, Universidad de Córdoba, Argentina

In recent years, the Event Horizon Telescope (EHT) Collaboration published the images of two supermassive black holes, located at the centers of the galaxies M87 and the Milky Way. These images show a dark region, known as the shadow, surrounded by a bright ring. A black hole strongly deflects the trajectories of light rays in its vicinity and part of photons coming from the region behind it are trapped, resulting in the presence of the shadow in the observer sky. For a static black hole the shadow is circular, while for a rotating one is deformed in an asymmetric form that depends on the angle of observation and the spin of the compact object. The standard analytical method for obtaining the contour of the shadow is based on the separation of variables in the Hamilton-Jacobi equation, which was introduced for the Kerr geometry many years ago. This method has then been extended to a large class of spacetimes; in particular, it was done for rotating black holes obtained by using the modified Newman-Janis algorithm. Recently, the formalism has been also adopted for black holes surrounded by a plasma satisfying a certain separability condition. The observation of the shadow is a useful tool to obtain the physical properties of supermassive black holes and to parametrize possible deviations from the Kerr solution in the strong field scenario. However, this kind of detailed studies exceed the current capabilities of the EHT. The next-generation Event Horizon Telescope (ngEHT) will greatly improve the existing EHT array, thus allowing to open new avenues of research. This talk presents a review of the analytical research and the prospects of the ngEHT.