Scalarized Einstein-Maxwell black holes in external scalar fields

Abstract

We construct static black holes in Einstein-Maxwell-scalar theory under prescribed external scalar fields, mapping their existence domain and determining how their scalar charges vary with the external field. Focusing on solutions with positive scalar charge, we find that a positive external field induces scalar charge in Reissner-Nordström (RN) black holes below the zero-source scalarization threshold and extends the allowed charge-to-mass range. A negative external field instead introduces a finite lower existence boundary and permits distinct black holes under identical external conditions. Coupling these isolated responses through the leading companion-generated scalar fields, we find mutual scalar induction in binaries whose components are RN black holes in isolation. For binaries with scalar charges of the same sign, their scalar charges need not increase as the separation decreases, while some binaries with opposite scalar charges admit multiple equilibria and cannot continue along their original equilibrium branches to smaller separations.