Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational-Wave Event S190521g
M. J. Graham, K. E. Saavik Ford, Barry McKernan, Nicholas P. Ross, Daniel Stern, Kevin B. Burdge, M. W. Coughlin, S. G. Djorgovski, A. J. Drake, Dmitry A. Duev, M. M. Kasliwal, A. Mahabal, Sjoert van Velzen, Justin Belecki, Eric C. Bellm, Rick Burruss, S. B. Cenko, Virginia Cunningham, G. Hélou, S. R. Kulkarni, Frank J. Masci, Thomas A. Prince, Dan Reiley, H. Rodriguez, B. Rusholme, Roger M. Smith, Maayane T. Soumagnac
Abstract
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)AJLEEY2041-821310.3847/2041-8213/ab4886] 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_{BBH}∼100 M_{⊙}, kick velocity v_{k}∼200 km s^{-1} at θ∼60° in a disk with aspect ratio H/a∼0.01 (i.e., disk height H at radius a) and gas density ρ∼10^{-10} g cm^{-3}. The merger could have occurred at a disk migration trap (a∼700r_{g}; r_{g}≡GM_{SMBH}/c^{2}, where M_{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 ∼1.6 yr(M_{SMBH}/10^{8} M_{⊙})(a/10^{3}r_{g})^{3/2}.