Relativistic Signatures of Flux Eruption Events near Black Holes

Images of supermassive black holes produced using very long baseline interferometry provide a pathway to directly observing effects of a highly curved spacetime, such as a bright photon ring that arises from strongly lensed emission. In addition, the emission near supermassive black holes is highly variable, with bright high-energy flares regularly observed. We demonstrate that intrinsic variability can introduce prominent associated changes in the relative brightness of the photon ring. We analyze both semianalytic toy models and GRMHD simulations with magnetic flux eruption events, showing that they each exhibit a characteristic loop in the space of relative photon ring brightness versus total flux density. For black holes viewed at high inclination, the relative photon ring brightness can change by an order of magnitude, even with variations in total flux density that are comparatively mild. We show that gravitational lensing, Doppler boosting, and magnetic field structure all significantly affect this feature, and we discuss the prospects for observing it in observations of M87* and Sgr A* with the next-generation Event Horizon Telescope.

Automated summary

Supermassive black holes can be observed using very long baseline interferometry to directly study the effects of highly curved spacetime. This includes observing a bright photon ring generated from strongly lensed emission. The emission near these black holes is highly variable, and changes in relative brightness of the photon ring can be caused by intrinsic variability. Analyzing toy models and simulations, we observe a characteristic loop in the relationship between relative photon ring brightness and total flux density. Gravitational lensing, Doppler boosting, and magnetic field structure play significant roles in this feature, and its observation in M87* and Sgr A* with the next-generation Event Horizon Telescope is discussed.