With the Event Horizon Telescope and future Very Long Baseline Interferometry arrays poised to image supermassive black holes, there is a strong motivation to understand the dynamic aspects of the accretion flow near the black hole. Interestingly, in such highly relativistic regime, the finite light-travel time should be taken into account to correctly simulate the images, known as the slow-light effect. This paper investigates the impact of the slow-light effect on the observational signatures of hot spots corotating with the Keplerian disks. It is found that the magnification can be modulated by the hot spot’s orbital velocity. Specifically, the corotating hot spots at the maxima of magnification are located at the image positions shifted relative to the positions behind the black hole. This subsequently causes the peak of the magnification profile to shift toward alignment with the peak of the redshift factor profile, ultimately leading to flux enhancement relative to the results of previous studies. This enhancement becomes particularly pronounced for corotating hot spots in the strong-field regime at large inclination angle, indicating that the slow-light effect is essential for accurately modeling high-energy emission in the vicinity of the black hole.

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• Classical black holes
• Gravitational lenses
• Accretion disk & black-hole plasma