## Jul 28, 2023

Prof. Galin Gyulchev

**Affiliation:** University of Sofia, Bulgaria

**Title:** Image of the shadow and thin accretion disk in gravity with a minimally coupled scalar field

**Abstract:** This talk will discuss the possible observable relativistic images of a black hole and naked singularity appearing in a rotating geometry in Einstein's gravity, minimally coupled to a scalar field. Particular attention will be paid to a Kerr-like (KL) alternative to the rotating Fisher-Janis-Newman-Winicour solution. The study includes analytical and numerical calculations of equatorial circular orbits, shadow images, and radiation from thin accretion disks for various values of the object's angular momentum and scalar charge. It is found that the KL solution cannot be ruled out by the observations for small values of the scalar charge either. As the scalar charge increases, the optical properties change dramatically. The photon region does not hide the singularity, so it should be classified as a strong singularity. The compact object's shadow and accretion disk images can become multiply connected and strongly oblate. This qualitatively new feature can be used to distinguish observationally black holes from naked singularities via contemporary Very Long Baseline Interferometry experiments at short wavelengths, possible for implementation via the global radio telescope array such as Event Horizon Telescope.