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 calculations in which the simulated fluid evolves while light rays are propagating through it—in general relativistic magnetohydrodynamic 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°), a slow-light treatment is important for accurate modeling of 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 conventional fast-light images. We find that the radius at which the plasma transitions from subrelativistic to relativistic velocities is imprinted on slow-light images via a transition from loop-dominated at small distances from the black hole (BH) to edge-dominated at a larger distance, with the loop-edge transition occurring at larger distances for lower BH spins. The jet image dynamics also vary with BH 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 both because slow-light effects are enhanced as the plasma velocity becomes more relativistic and because the plasma acceleration is itself a strong function of the spin.