Photon ring bounds of scalar hairy charged black holes (2401.08200v2)

Abstract: We study photon rings of constant scalar hairy black holes with mass $m$, charge $Q$, and scalar hair $S$ obtained from the Einstein-Maxwell-conformally coupled scalar theory. These black holes are classified as scalar hairy Reissner-Nordstr\"{o}m (SHRN) black hole, scalar hairy charged black hole with $S> -Q^2$, and mutated-RN black hole. The first two respect both dominant and strong energy conditions and have positive ADM mass and entropy, while the last does not respect two energy conditions and has negative ADM mass and entropy. We find that all of these black holes respect the lower bound ($r_\gamma\ge 1.2 r_+$) of photon rings. We obtain all real bounds of photon rings for these black holes and discuss physical and observational properties of real bounds. It is shown that the observationally favored region based on the shadow radius includes SHRN black hole, scalar hairy charged black hole, and mutated-RN black hole.