A Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational Wave Event GW190521g

We report the first plausible candidate electromagnetic (EM) counterpart to a binary black hole (BBH) merger. The EM flare is consistent with expectations for a kicked BBH merger in the accretion disk of an active galactic nucleus (AGN) [1], and is unlikely (< O(0.01%)) due to intrinsic variability of this source. The lack of color evolution implies that it is not a supernovae and instead is strongly suggestive of a constant temperature shock. Other false-positive events, such as micro- lensing or a tidal disruption event, are ruled out or constrained to be < O(0.1%). If the flare is associated with GW190521g, we find plausible combined values of: total mass MBBH ∼ 100M⊙, kick velocity vk ∼ 200kms−1 at θ ∼ 60◦ in a disk with aspect ratio H/a ∼ 0.01 and gas density ρ ∼ 10−10 gcm−3. The merger could have occurred at a disk migration trap (a ∼ 700rg). The timing of our spectroscopy prevents useful constraints on broad-line asymmetry due to an off- center flare. We predict a repeat flare in this source due to a re-encountering with the disk in ∼ 1.6yr(MSMBH/108M⊙)(a/103rg)3/2.