Program: GN-2020B-Q-228

Jets from active galactic nuclei play a key role in galactic evolution. While some of the basic dynamical properties of jets are understood, the location and mechanism of particle acceleration are unknown. As one consequence, the ~10% error in the mass determination for M87 by the Event Horizon Telescope is dominated by such uncertainties. We propose novel observations exploiting jet variability at millimeter and near infrared wavelengths to discriminate between the three primary scenarios: 1) thermal particles dominating the inner jets, 2) nonthermal particles dominating the inner jets, and 3) thermal particles at the base, then nonthermally accelerated perhaps 100 gravitational radii from the black hole. By monitoring the inner jet emission of M87 with Gemini/NIRI at 3.7 microns and the jet base emission with the SMA at 270 GHz, we will determine if there is correlated variability, and if so, whether there is a time lag. Whatever mechanism we find for M87 likely applies to many AGN jets. To achieve statistical significance will need several years, and we therefore request long-term status.