Slow-Light Effect in the Jet-Launching Region of M87

Abstract

We explore the impact of "slow-light" radiative transfer - i.e., general relativistic radiative transfer (GRRT) calculations in which the simulated fluid evolves while light rays are propagating through it - in general relativistic magnetohydrodynamic (GRMHD) models of the M87 jet. Because the plasma in the jet-launching region is accelerated to relativistic velocities, and because the jet in M87 is nearly aligned with the line of sight (offset by ~17 degrees), a slow-light treatment is important for accurately modeling the observable structure. While fast-light images exhibit prominent helical or loop-shaped features in the jet - which we associate with narrow bundles of magnetic field lines - these features become stretched and smoothed-out in slow-light images. Our slow-light images instead exhibit a double-edged, cone-like morphology that is more consistent with observations of M87 than corresponding fast-light images. We find that the radius at which the plasma transitions from sub-relativistic to relativistic velocities is imprinted on slow-light images via a transition from loop-dominated at small distances from the black hole to edge-dominated at a larger distance, with the loop-edge transition occurring at larger distances for lower black hole spins. The jet image dynamics also vary with black hole spin, with low-spin models producing jets that exhibit substantial "wobbling", while high-spin models produce jets that are straighter and more stable in time. The spin-dependent jet morphology and variability are revealed by slow-light imaging because slow-light effects become more enhanced as the plasma velocity becomes more relativistic, and because the plasma acceleration is itself a strong function of the spin.