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[Abridged] Context. The Galactic Center supermassive black hole is well knownto exhibit transient peaks of flux density on a daily basis across thespectrum. Recent infrared and millimeter observations have strengthened thecase for the association between these flares and circular orbital motion inthe vicinity of the event horizon. The strongly polarized synchrotron radiationassociated with these events leads to specific observables called QU loops,that is, looping motion in the Stokes QU plane of linear polarization. Aims. Wewant to deepen the understanding of the QU loops associated with orbiting hotspots. We compute such loops in Minkowski and Schwarzschild spacetimes in orderto determine which aspects of the observed patterns are due to special- orgeneral-relativistic phenomena. Results. We show that QU loops in Minkowskispacetime at low or moderate inclination i < 45 deg share all qualitativefeatures of Schwarzschild QU loops: there exist QU loops for all setupsconsidered (including face-on view and vertical magnetic field), there may beone or two QU loops per orbital period for a vertical magnetic fieldconfiguration, there are always two QU loops in case of a toroidal magneticfield. We provide analytical formulas in Minkowski spacetime to explain thedetails of this behavior. Moreover, we analyze the flux variation of the hotspot and show that it is dictated either by the angular dependence of theradiative transfer coefficients, or by relativistic beaming. In the formercase, this can lead to extreme flux ratios even at moderate inclination.Finally, we highlight the increasing mirror asymmetry of the Schwarzschild QUtrack with increasing inclination and show that this behavior is a specificSchwarzschild feature caused by light bending.

Polarized signatures of orbiting hot spots: special relativity impact and probe of spacetime curvature