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 $<O(0.1\%)$. 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 $\theta \sim 60°$ in a disk with aspect ratio $H/a\sim 0.01$ (i.e., disk height $H$ at radius $a$) and gas density $\rho \sim {10}^{-10}\mathrm{g}{\mathrm{cm}}^{-3}$. 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 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 $\sim 1.6\mathrm{yr}(M_{\mathrm{SMBH}}/{10}^8{M}_{\odot })(a/{10}^3{r}_g{)}^{3/2}$.

中文翻译：

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二进制黑洞合并引力波事件S190521g的候选电磁对垒。

我们报告了（候选）二进制黑洞合并的第一个可能的光学电磁对等物。由Zwicky瞬变设施检测到的电磁耀斑与活跃银河系原子核吸积盘中被踢的二进制黑洞合并的预期一致[B. McKernan，K.E.S.Ford，I.Bartos等。，天体。J.莱特 884，L50（2019）]，不太可能[$<Ø（0.01％）$）]。缺乏颜色演变意味着它不是超新星，而是强烈暗示有恒定的温度冲击。其他虚假事件，例如微透镜或潮汐破坏事件，被排除或限制为$<Ø（0.1％）$。如果耀斑与S190521g相关，我们发现总质量的合理值${\mathrm{中号}}_{\mathrm{BBH}}〜100{\mathrm{中号}}_{\odot }$，踢速度 $v_{ķ}〜200\mathrm{公里}{\mathrm{s}}^{-\mathrm{1个}}$ 在 $\theta 〜60°$ 在具有高宽比的磁盘中 $H/\mathrm{一种}〜0.01$ （即磁盘高度 $H$ 在半径 $\mathrm{一种}$）和气体密度 $\rho 〜{10}^{-10}\mathrm{G}{\mathrm{厘米}}^{-3}$。合并可能发生在磁盘迁移陷阱处（$\mathrm{一种}〜700{\mathrm{[R}}_G$; ${\mathrm{[R}}_G\equiv G{\mathrm{中号}}_{\mathrm{中小企业}}/C^2$，在哪里 ${\mathrm{中号}}_{\mathrm{中小企业}}$是活跃的银河核超大质量黑洞的质量。参数的组合意味着S190521g中至少一个黑洞有明显的旋转。我们的光谱学时机可以防止由于偏心耀斑而造成的对宽线不对称性的有用约束。由于重新遇到磁盘，我们预计此源将重复出现耀斑$〜1.6年（{\mathrm{中号}}_{\mathrm{中小企业}}/{10}^8{\mathrm{中号}}_{\odot }）（\mathrm{一种}/{10}^3{\mathrm{[R}}_G{）}^{3/2}$。