Multimessenger observations of compact binary mergers

Centered on the use and development of standard open-source codebases, this thesis advances two distinct domains of gravitational physics through independent lines of research. First, it presents developments in the field of externally triggered and targeted observations of compact binary mergers using the matched filtering algorithm within {\ttfamily PyCBC}. Specifically, it provides the first end-to-end technical overview of the newly developed {\ttfamily PyCBC\_XTS} pipeline---introduced across {\ttfamily pycbc-v2.8.x} and {\ttfamily pycbc-v2.11.x}---designed to replace legacy {\ttfamily PyGRB} for multimessenger observations during the LIGO-Virgo-KAGRA fourth observing run (O4) and beyond. Second, it details the pathway to stable numerical relativity simulations of low-mass neutron star--black hole coalescences. This work encompasses new general-relativistic hydrodynamics simulations in the equal and near-equal-mass regime---and their extracted gravitational-wave emission---performed with {\ttfamily FUKA} and the {\ttfamily Einstein Toolkit}. A central theme of this manuscript is detailing the developmental roadmap required to make these advancements possible. For the data analysis component, this work emphasizes the architectural challenges of building a complex, large-scale search infrastructure entirely from scratch. After comprehensively detailing the pipeline's construction and internal logic, it showcases the operational readiness of {\ttfamily PyCBC\_XTS} through a targeted study of the historic multimessenger transient GRB 170817A / GW170817. Similarly, the numerical relativity component focuses on the unprecedented challenge of configuring and stabilizing neutron star-black hole simulations employing solely open-source frameworks, and tabulated composition-dependent equations of state, available in public databases for the neutron star matter. It documents the methodological milestones necessary to achieve physical viability in a previously underexplored region of parameter space. These years-long efforts culminate in the successful simulation and waveform extraction of two specific systems, characterized by mass ratios of $q=1.0$ and $q=0.5$. Both topics represent state-of-the-art research with a concrete pathway for future integration. As new phenomenological gravitational waveform models focus on calibrating these specific regions of the neutron star-black hole parameter space against numerical relativity data, they will enable the construction of highly accurate template banks. Ultimately, these newly calibrated banks can be directly ingested by GW templated search pipelines like the one presented here, seamlessly uniting the computational advancements of numerical relativity with the practical application of next-generation gravitational-wave searches.