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The Event Horizon Telescope's image of the M87 black hole provides anexciting opportunity to study black hole physics. Since a black hole's eventhorizon absorbs all electromagnetic waves, it is difficult to actively probethe horizon's existence. However, with the help of a family of extremelycompact, horizon-less objects, named gravastars", whose external space-timesnearly identical to those of black holes, one can test the absence of eventhorizons: absences of additional features that arise due to the existence ofthe gravastar, or its surface, can be used as quantitative evidence for blackholes. We apply Gralla et al.'s approach to studying black hole images to studythe images of two types of gravastars: transparent ones and reflective ones. Inboth cases, the transmission of rays through gravastars, or their reflectionson their surfaces, lead to more rings in their images. For simple emissionmodels, where the redshifted emissivity of the disk is peaked at a particularradius r-peak, the position of a series of rings can be related in a simplemanner to light ray propagation: a ring shows up around impact parameter bwhenever rays incident from infinity at b intersects the disk at r-peak. Weshow that additional rings will appear in the images of transparent andreflective gravastars. In particular, one of the additional rings for thereflective gravastar is due to the prompt reflection of light on the gravastarsurface and appears to be well separated from the others. This can be a robustfeature, which may be reliably used to constrain the reflectivity of the blackhole's horizon.

Multiple Rings in the Shadow of Extremely Compact Objects