Program: GN-2019A-Q-121

During the epoch of rapid supermassive black-hole growth at z=1-3, active galactic nuclei (AGN) experienced epochs of high accretion rates significantly above the Eddington limit. This high-Eddington accretion was in the form of 'slim disks' of notably different structure than standard 'thin disks'. Mrk 142 is one of very few bright, nearby AGN that is amenable to time-domain studies to empirically constrain this important accretion mode. As a result, it is the target of a large, international space- and ground-based campaign of high energy through optical photometric and spectroscopic observations to study the accretion-disk and broad-line region structure of a high-Eddington supermassive black hole for the first time. As a key element in this program, we propose 45 long-slit observations of Mrk 142 with GMOS-North for a total of 18 hrs during February and March of 2019. As the only 8-m class observatory in the campaign, Gemini's unique contribution will be to enable velocity-resolved time-lags to be measured across the broad H-beta emission line through high signal-to-noise ratio spectroscopy.