Jet Archaeology and Forecasting: Image Variability and Magnetic Field Configuration

TL;DR

This paper explores how magnetic field variations near black holes influence jet morphology and variability, proposing that jet observations can reveal horizon-scale magnetic dynamics and black hole spin characteristics.

Contribution

It introduces a method to connect horizon-scale magnetic flux variations with observable jet features, aiding in testing black hole accretion models and estimating black hole spin.

Findings

Jet width variability reflects magnetic flux changes near the horizon.

Jet acceleration and morphology are influenced by black hole spin and magnetic field configuration.

Time-averaged images show features dependent on black hole spin and magnetic field structure.

Abstract

We investigate how magnetic field variations around accreting black holes on event horizon scales affect the morphology of magnetically-driven jet 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 black hole 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 black hole. Furthermore, the pattern of jet width variations shows acceleration up to the relativistic regime as it moves away from the black hole, 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 black hole spin, shape diverse features through relativistic beaming and aberration. Higher black hole 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 black hole spin through observations bridging horizon and jet-launching scales.