Jet Archaeology and Forecasting: Image Variability and Magnetic Field Configuration

Abstract We investigate how magnetic field variations around accreting black holes (BHs) on event horizon scales affect the morphology of magnetically driven jets on larger scales. By performing radiative transfer calculations on general relativistic magnetohydrodynamics simulations, we find that temporal variation in the magnetic flux on the event horizon and the jet power are imprinted on the variability of jet width up to several hundred gravitational radii. When the magnetic flux around the BH drops and then rises, the jet initially narrows or becomes truncated, then widens, creating a thin-thick pattern that propagates down the jet. This suggests that extended jet observations can provide a history record of horizon-scale magnetic field dynamics, and conversely, upcoming changes in the jet image can be predicted from direct observation of the magnetized accreting plasma near the BH. Furthermore, the pattern of jet width variations shows acceleration up to the relativistic regime as it moves away from the BH, aligning with plasma bulk motion. We also find in time-averaged images that both the bulk plasma motion and magnetic field configuration in the jet-launching region, which are sensitive to BH spin, shape diverse features through relativistic beaming and aberration. Higher BH spins result in more poloidal bulk motion and toroidal magnetic fields, leading to more symmetric jet images and linear polarization patterns. These results suggest a new method for testing the magnetically arrested disk model and the Blandford–Znajek process, and for determining the BH spin through observations bridging horizon and jet-launching scales.