Probing the disk-jet coupling in M 87

Context. Recent GMVA observations of M 87 at event horizon scales revealed a ring-like structure that is ∼50% larger at 86 GHz than the ring observed by the Event Horizon Telescope at 230 GHz. Aims. We studied a possible origin of the increased ring size at 86 GHz and the role the nonthermal electron population plays in the observed event horizon scales. Methods. We carried out 3D general relativistic magnetohydrodynamic simulations followed by radiative transfer calculations. We incorporated synchrotron emission from both thermal and nonthermal electrons into the calculations. To better compare our results to observations, we generated synthetic interferometric data adjusted to the properties of the observing arrays. We fit geometrical models to these data in Fourier space through Bayesian analysis to monitor the variable ring size and width over the simulated time span. Results. We find that the 86 GHz ring is always larger than the 230 GHz ring, which can be explained by the increased synchrotron self-absorption at 86 GHz and the mixed emission from both the accretion disk and the jet footpoints, as well as flux arcs ejected from a magnetized disk. We find agreement with the observations, particularly within the error range of the observational value of M/D for M 87. Conclusions. We show that state-of-the art 3D general relativistic magnetohydrodynamic simulations combined with thermal and nonthermal emitting particles can explain the observed frequency-dependent ring size in M 87. Importantly, we find that MAD events triggered in the accretion disk can significantly increase the lower-frequency ring sizes.