The Gravity Collective: A Search for the Electromagnetic Counterpart to the Neutron Star–Black Hole Merger GW190814
Authors: Kilpatrick Charles D. , Coulter David A. , Arcavi Iair , Brink Thomas G. , Dimitriadis Georgios , Filippenko Alexei V. , Foley Ryan J. , Howell D. Andrew , Jones David O. , Kasen Daniel , Makler Martin , Piro Anthony L. , Rojas-Bravo César , Sand David J. , Swift Jonathan J. , Tucker Douglas , Zheng WeiKang , Allam Sahar S. , Annis James T. , Antilen Juanita , Bachmann Tristan G. , Bloom Joshua S. , Bom Clecio R. , Bostroem K. Azalee , Brout Dillon , Burke Jamison , Butler Robert E. , Butner Melissa , Campillay Abdo , Clever Karoli E. , Conselice Christopher J. , Cooke Jeff , Dage Kristen C. , de Carvalho Reinaldo R. , de Jaeger Thomas , Desai Shantanu , Garcia Alyssa , Garcia-Bellido Juan , Gill Mandeep S. S. , Girish Nachiket , Hallakoun Na’ama , Herner Kenneth , Hiramatsu Daichi , Holz Daniel E. , Huber Grace , Kawash Adam M. , McCully Curtis , Medallon Sophia A. , Metzger Brian D. , Modak Shaunak , Morgan Robert , Muñoz Ricardo R. , Muñoz-Elgueta Nahir , Murakami Yukei S. , E. Felipe Olivares , Palmese Antonella , Patra Kishore C. , Pereira Maria E. S. , Pessi Thallis L. , Pineda-Garcia J. , Quirola-Vásquez Jonathan , Ramirez-Ruiz Enrico , Rembold Sandro Barboza , Rest Armin , Rodríguez Ósmar , Santana-Silva Luidhy , Sherman Nora F. , Siebert Matthew R. , Smith Carli , Smith J. Allyn , Soares-Santos Marcelle , Stacey Holland , Stahl Benjamin E. , Strader Jay , Strasburger Erika , Sunseri James , Tinyanont Samaporn , Tucker Brad E. , Ulloa Natalie , Valenti Stefano , Vasylyev Sergiy S. , Wiesner Matthew P. , Zhang Keto D. 
Published: December 2021

Abstract
We present optical follow-up imaging obtained with the Katzman Automatic Imaging Telescope, Las Cumbres Observatory Global Telescope Network, Nickel Telescope, Swope Telescope, and Thacher Telescope of the LIGO/Virgo gravitational wave (GW) signal from the neutron star–black hole (NSBH) merger GW190814. We searched the GW190814 localization region (19 deg2 for the 90th percentile best localization), covering a total of 51 deg2 and 94.6% of the two-dimensional localization region. Analyzing the properties of 189 transients that we consider as candidate counterparts to the NSBH merger, including their localizations, discovery times from merger, optical spectra, likely host galaxy redshifts, and photometric evolution, we conclude that none of these objects are likely to be associated with GW190814. Based on this finding, we consider the likely optical properties of an electromagnetic counterpart to GW190814, including possible kilonovae and short gamma-ray burst afterglows. Using the joint limits from our follow-up imaging, we conclude that a counterpart with an r-band decline rate of 0.68 mag day−1, similar to the kilonova AT 2017gfo, could peak at an absolute magnitude of at most −17.8 mag (50% confidence). Our data are not constraining for “red” kilonovae and rule out “blue” kilonovae with M > 0.5 M
⊙ (30% confidence). We strongly rule out all known types of short gamma-ray burst afterglows with viewing angles <17° assuming an initial jet opening angle of ∼5.°2 and explosion energies and circumburst densities similar to afterglows explored in the literature. Finally, we explore the possibility that GW190814 merged in the disk of an active galactic nucleus, of which we find four in the localization region, but we do not find any candidate counterparts among these sources.