Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational-Wave Event S190521g

We report the first plausible optical electromagnetic counterpart to a (candidate) binary black hole merger. Detected by the Zwicky Transient Facility, the electromagnetic flare is consistent with expectations for a kicked binary black hole merger in the accretion disk of an active galactic nucleus [B. McKernan, K. E. S. Ford, I. Bartos et aL, Astrophys. J. Lett. 884, L50 (2019)] and is unlikely [< O(0.01%))] due to intrinsic variability of this source. The lack of color evolution implies that it is not a supernova and instead is strongly suggestive of a constant temperature shock. Other false-positive events, such as microlensing or a tidal disruption event, are ruled out or constrained to be < 0(0.1%). If the flare is associated with S190521g, we find plausible values of total mass M-BBH similar to 100 M-circle dot, kick velocity v(k) similar to 200 km s(-1) at theta similar to 60 degrees in a disk with aspect ratio H/a similar to 0.01 (i.e., disk height H at radius a) and gas density rho similar to 10(-10)g cm(-3). The merger could have occurred at a disk migration trap (a similar to 700r(g); r(g) (math) GM(SMBH)/c(2), where M-SMBH the mass of the active galactic nucleus supermassive black hole). The combination of parameters implies a significant spin for at least one of the black holes in S190521g. 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 reencountering with the disk in similar to 1.6 yr(M-SMBH/10(8) M-circle dot)(a/10(3) r(g))(3/2) .