On 2019 August 14 the Laser Interferometer Gravitational Wave Observatory (LIGO) and the Virgo gravitational wave interferometer announced the detection of a binary merger, S190814bv, with a low false alarm rate (FAR) of about 1 in $1.6times 10^{25}$ years, a distance of $267pm 52$ Mpc, a 90% (50%) localization region of about 23 (5) deg$^2$, and a probability of being a neutron star--black hole (NS-BH) merger of $>99%$. The LIGO/Virgo Collaboration (LVC) defines NS-BH such that the lighter binary member has a mass of $<3$ M$_odot$ and the more massive one has $>5$ M$_odot$, and this classification is in principle consistent with a BH-BH merger depending on the actual upper mass cut-off for neutron stars. Additionally, the LVC designated a probability that the merger led to matter outside the final BH remnant of $<1%$, suggesting that an electromagnetic (EM) counterpart is unlikely. Here we report our optical follow-up observations of S190814bv using the Magellan Baade 6.5 m telescope to target all 96 galaxies in the GLADE catalog within the 50% localization volume (representing about 70% of the integrated luminosity within this region). No counterpart was identified to a median $3sigma$ limiting magnitude of $i=22.2$ ($M_iapprox -14.9$ mag), comparable to the brightness of the optical counterpart of the binary neutron star merger GW170817 at the distance of S190814bv; similarly, we can rule out an on-axis jet typical of short GRBs. However, we cannot rule out other realistic models, such as a kilonova with only $sim 0.01$ M$_odot$ of lanthanide-rich material, or an off-axis jet with a viewing angle of $theta_{rm obs}gtrsim 15^circ$.
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