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On 2020 February 24, during their third observing run ("O3"), the LaserInterferometer Gravitational-wave Observatory and Virgo Collaboration (LVC)detected S200224ca: a candidate gravitational wave (GW) event produced by abinary black hole (BBH) merger. This event was one of the best-localizedcompact binary coalescences detected in O3 (with 50%/90% error regions of 13/72deg$^2$), and so the Neil Gehrels Swift Observatory performed rapidnear-UV/X-ray follow-up observations. Swift-XRT and UVOT covered approximately79.2% and 62.4% (respectively) of the GW error region, making S200224ca the BBHevent most thoroughly followed-up in near-UV (u-band) and X-ray to date. Nolikely EM counterparts to the GW event were found by the Swift BAT, XRT, orUVOT, nor by other observatories. Here we report on the results of our searchesfor an EM counterpart, both in the BAT data near the time of the merger, and infollow-up UVOT/XRT observations. We also discuss the upper limits we can placeon EM radiation from S200224ca, and the implications these limits have on thephysics of BBH mergers. Namely, we place a shallow upper limit on thedimensionless BH charge, $\hat{q} < 1.4 \times10^{-4}$, and an upper limit onthe isotropic-equivalent energy of a blast wave $E < 4.1\times10^{51}$ erg(assuming typical GRB parameters).

Swift Multiwavelength Follow-up of LVC S200224ca and the Implications for Binary Black Hole Mergers