Black hole shadows of α′ -corrected black holes

In this paper we study the qualitative features induced by corrections to general relativity (GR) coming from string theory, on the shadows of rotating black holes. We research the slowly rotating black hole solutions up to order O(a³), to first order in α^', also including the dilaton. We provide a detailed characterization of the geometry, as well as the innermost stable circular orbit (ISCO) and photon ring; then, we proceed to obtain the black hole images within the relativistic thin-disk model. We characterize the images by computing the diameter, displacement, and asymmetry. A comparison with the Kerr case indicates that all these quantities grow due to the α^' correction and that the departure from GR for different observables is enhanced depending on the angle of view; namely, for the diameter, the maximum departure is obtained when the system is face-on, while for the displacement and asymmetry, the departure from GR is maximized for the edge-on point of view.