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 $<O(0.1\%)$. 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 $\sim 1.6\mathrm{yr}(M_{\mathrm{SMBH}}/{10}^8{M}_{\odot })(a/{10}^3{r}_g{)}^{3/2}$.

Figure 1

Left panel: A Mollweide projection of the 50% and 90% LIGO localization regions for S190521g (with 44%/56% in the northern/southern hemisphere) and the location of ZTF19abanrhr (within the 78% contour). ZTF covered 48% of the 90% region and contours at declination $<-30°$ indicate southern hemisphere regions not covered by ZTF. Right panel: The marginal luminosity distance distribution integrated over the sky (dotted blue line) for S190521g as well as the conditional distance distribution (black line) at the position of ZTF19abanrhr. The red line corresponds to the luminosity distance of ZTF19abanrhr, assuming a Planck15 cosmology [34].

Figure 2

ZTF $g$-band photometry, $r$-band photometry, and $g-r$ color for $\mathrm{J}1249+3449$ over the past 25 months. The flare beginning $\mathrm{MJD}\sim 58650$ represents a $5\sigma$ departure from the ZTF baseline for this source. The flare emission is fit according to the model described in the text and assuming a linear model for the source continuum behavior over time. The dashed vertical line corresponds to the S190521g trigger time.

Figure 3

Lightcurve for $\mathrm{J}1249+3449$, including an additional decade of CRTS photometry (binned at 15 day intervals). ZTF data is binned in three day intervals, with $g$- and $r$-band data corrected to the CRTS photometric system using median offsets of 0.52 mag for $g$ band and 0.34 mag for $r$ band.

Figure 4

Spectra of $\mathrm{J}124942.3+344929$, the AGN associated with ZTF19abanrhr from SDSS (UT 2006 January 30) and Keck (UT 2020 January 25). Other than fading by $\sim 30\%$, there are no strong spectral changes over the intervening decade (rest frame).