A Galaxy-targeted Search for the Optical Counterpart of the Candidate NS–BH Merger S190814bv with Magellan

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 of about 1 in 1.6 × 1025 yr, a distance of 267 ± 52 Mpc, a 90% (50%) localization region of about 23 (5) deg2, 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⊙ and the more massive one has >5 M⊙, and this classification is in principle consistent with a BH–BH merger depending on the actual upper mass cutoff 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 Galaxy List for the Advanced Detector Era catalog within the 50% localization volume (representing about 70% of the integrated luminosity within this region). No counterpart was identified to a median 3σ limiting magnitude of i = 22.2 (Mi ≈ −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 ∼0.01 M⊙ of lanthanide-rich material, or an off-axis jet with a viewing angle of θobs ≳ 15°.