Light rings, gravitational lensing, and ISCOs of exotic compact objects in Einstein-scalar-Maxwell theories

Abstract

In Einstein-scalar-Maxwell theories with a coupling between the scalar field $ϕ$ and the electromagnetic field strength $F$ of the form $μ(ϕ) F$, we investigate the existence of exotic compact objects (ECOs) and their observational signatures in photon and massive-particle dynamics. For $μ(ϕ)$ diverging at the origin while all physical quantities remain finite, we demonstrate the existence of electrically charged ECOs with a shell-like structure whose density peaks at an intermediate radius. We compute their mass and radius, together with the scalar and vector field profiles, on a static and spherically symmetric background. We then examine the existence of light rings and place bounds on a model parameter by requiring the absence of a linearly stable light ring. Under this condition, photon echoes from ECOs are absent. We also compute the gravitational-lensing deflection angle and show that it attains a maximum for an impact parameter of the same order as the ECO radius. Finally, we study the parameter space in which innermost stable circular orbits of massive particles exist.