Polarimetry imprints of exotic compact objects: relativistic fluid spheres and gravastars

Abstract: Recent observations of polarimetric parameters of active galactic nuclei motivate the study of polarization in the spacetime of exotic compact objects which can mimic the features of black holes in the strong field regime of gravity. In this work, we study the properties of two models for ultra-compact objects containing light rings - relativistic fluid spheres and gravastars. We have simulated the orbit of a hot spot around the considered objects in the polarization ray-tracing software GYOTO, and extracted observables, namely integrated images of the Stokes parameters I, Q, U; their evolution during the orbit in the QU-plane, and the electric vector position angle (EVPA). Our models resemble the black hole qualitatively, with slight additional imprints which provide a useful tool to constrain the spacetime metric of supermassive compact objects with current and future observations, and probe the fundamental properties of these objects. We have found that one fluid star model with a pressure singularity resembles the black hole entirely, while another gravastar features notable differences in the EVPA curve in the low-inclination case. Since differences between the models become more pronounced for a higher inclination, our results could potentially restrict the adequateness of ECO classes with future high-inclination observations.