Unlocking Time-domain & Multimessenger Astrophysics and the Discovery of the First Optical Counterpart to a Gravitational Wave Source

The modern study of astrophysics is being transformed by advances across multiple fronts. New fast, wide, deep, and multi-color surveys are pushing into novel parameter spaces and generating an exponentially growing volume of data. Simultaneously, the first direct detection of gravitational waves (GWs) in 2015 has created a global race to search for their luminous counterparts. Here I present three major results that reside at the intersection of this new science and the methods, algorithms, and technology that enable it. The first is {\tt YSE-PZ}, a transient survey management platform that enables three major transient surveys: the Young Supernova Experiment, the Keck Infrared Transient Survey, and the Swope Supernova Survey. Second, I describe the discovery of the first optical counterpart to gravitational wave source, the kilonova (KN) SSS17a/AT 2017gfo. Discovering the KN and localizing it to the galaxy NGC~4993 paved the way for all the science that followed: it confirmed that there are electromagnetic (EM) counterparts to GWs, it allowed a redshift of NGC 4993 to be combined with the GW-derived luminosity distance to GW170817 and enabled the first standard siren measurement of H$_{0}$, and lead to the ability for rapid follow-up observations to confirm that binary neutron star (BNS) mergers are prolific astrophysical sites for the synthesis of the $r$-process elements. Finally, I present a new ultraviolet, optical, and infrared search for the electromagnetic (EM) counterpart to GW190425, the second-ever BNS merger discovered by the LIGO-Virgo-KAGRA Collaboration (LVK). Intrinsic properties and observational limitations meant that any counterpart to GW190425 would be much harder to discover than SSS17a was, and neither our search, nor the broader EM community's search, discovered a credible counterpart. I contrast this speculative counterpart to SSS17a and discuss how KN diversity complicates our picture of heavy-element nucleosynthesis. I perform a combined analysis of the EM community's search for GW190425 using a new GW search and analysis tool called {\tt Teglon}. Through {\tt Teglon}, I calculate the most comprehensive upper limits on this potential EM counterpart to GW190425, as well as discuss opportunities for enhancing the community's coordination for the next BNS GW event in the LVK's fourth observing run and beyond