Shadows of boson and Proca stars with thin accretion disks

Abstract

In this work, we obtain the shadow images of spherically symmetric scalar boson and Proca stars using analytical fittings of numerical solutions, when illuminated by a geometrically thin accretion disk. We chose a sample of four boson and four Proca stars with radii ranging from more compact configurations with $R\sim 9M$ to more dilute configurations with $R\sim 20M$, where $M$ is the total mass of the bosonic star. In these configurations, the absence of the photon sphere (the locus of unstable bound geodesics) makes the optical appearance of these stars to be dominated by a single luminous ring enclosing a central brightness depression, and no further light rings are available. We show that if one considers face-on observations and a disk model whose emission is truncated at some finite radius at which the luminosity attains its maximum value, both the size of the shadow, as well as the luminosity and depth of the bright region, are heavily influenced by the emission profile, with the choice of the type and parameters of the bosonic stars in our samples having a sub-dominant influence. These differences are nonetheless significantly magnified when one allows the accretion disk to extend close enough to the center of the star. Our results point out that even though bosonic stars are horizonless and do not have a photon sphere, some of them may be able to produce conventional black hole shadow-like images provided that their compactness is large enough, thus being potentially consistent with current and future observations.