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 $\varphi$ and the electromagnetic field strength $F$ of the form $\mu (\varphi )F$, we investigate the existence of exotic compact objects (ECOs) and their observational signatures in photon and massive-particle dynamics. For $\mu (\varphi )$ 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.

16 pages, 8 figures

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