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

Abstract

We report the first plausible optical electromagnetic (EM) counterpart to a (candidate) binary black hole (BBH) merger. Detected by the Zwicky Transient Facility (ZTF), the EM flare is consistent with expectations for a kicked BBH merger in the accretion disk of an active galactic nucleus (AGN), 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 microlensing or a tidal disruption event, are ruled out or constrained to be $<O(0.1\%$). If the flare is associated with S190521g, we find plausible values of: total mass $M_{\mathrm{BBH}}\sim 100M_{\odot }$, kick velocity $v_k\sim 200\mathrm{km}{\mathrm{s}}^{-1}$ at $\hat{I}¸\sim 60^{\circ }$ in a disk with aspect ratio $H/a\sim 0.01$ (i.e., disk height $H$ at radius $a$) and gas density $\ddot{I}\sim 10^{-10}\mathrm{g}{\mathrm{cm}}^{-3}$. The merger could have occurred at a disk migration trap ($a\sim 700r_g$; $r_g\equiv G{M}_{\mathrm{SMBH}}/c^2$, where $M_{\mathrm{SMBH}}$ is the mass of the AGN 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 re-encountering with the disk in $\sim 1.6\mathrm{yr}(M_{\mathrm{SMBH}}/10^8{M}_{\odot })(a/10^3{r}_g{)}^{3/2}$.

9 pages, 4 figures, accepted for publication in Physical Review Letters (June 25, 2020)