Ray tracing ultracompact boson stars: visibility modulations and incomplete photon rings

Abstract

Modified black holes and black hole mimickers can give rise to additional photon rings in ray-traced images. With the photon ring structure of Kerr well understood, the question arises how well future versions of the Event Horizon Telescope images would be able to constrain the presence of non-Kerr photon rings. In this work, we investigate how likely additional photon rings are to present themselves through low-frequency modulations of the visibility amplitude, i.e. in Fourier space where the data live. To this end, we generate ray-traced images of stable and unstable ultracompact, solitonic boson stars, both in spherical symmetry and with rotation. We find that clear modulations in the associated visibility amplitude are only present in spherical symmetry, for a restricted range of observer inclinations, hence suggesting that these modulations can not be a generic expectation of ultracompact black-hole mimickers. Additionally, we discuss how the photon rings are incomplete for the rotating boson stars considered, and how an unstable prograde light ring may be needed to remedy this.