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

TL;DR

This paper explores exotic compact objects in Einstein-scalar-Maxwell theories, analyzing their structure, photon rings, gravitational lensing, and stable orbits to identify potential observational signatures and constraints.

Contribution

It introduces electrically charged ECOs with shell-like structures and examines their photon rings, lensing, and stable orbits, providing new insights into their observational features.

Findings

Existence of shell-like charged ECOs with finite physical quantities

Photon rings can be absent if certain stability conditions are met

Gravitational lensing angle peaks near the ECO radius

Abstract

In Einstein-scalar-Maxwell theories with a coupling between the scalar field $\phi$ and the electromagnetic field strength $F$ of the form $\mu(\phi) F$, we investigate the existence of exotic compact objects (ECOs) and their observational signatures in photon and massive-particle dynamics. For $\mu(\phi)$ 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 photon rings and place bounds on a model parameter by requiring the absence of a linearly stable photon ring. Under this condition, photon echoes from ECOs are absent. We also compute the gravitational-lensing deflection angle $\Psi$ 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.