Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational-Wave Event S190521g. - Graham MJ; Ford KES; McKernan B; Ross NP; Stern D; Burdge K; Coughlin M; Djorgovski SG; Drake AJ; Duev D; Kasliwal M; Mahabal AA; et al. (Physical review letters, 2020)