MULTIMESSENGER ASTRONOMY AND TESTING GENERAL RELATIVITY WITH GRAVITATIONAL WAVES

Abstract

The detection of gravitational waves (GW) from compact binary coalescence events has revolutionized our ability to study extreme gravity using neutron stars and stellar-mass black holes. This dissertation presents techniques to accelerate parameterized tests of general relativity and to understand astrophysical sources that generate both electromagnetic and gravitational-wave emission. First, I show how to use a multiband decomposition of the likelihood to test General Relativity in the strong-field regime. The multiband decomposition significantly reduces the computational cost of parameterized tests. Our method speeds up the analysis of binary neutron star signals by a factor of $\mathcal{O}(10)$ for a low-frequency cutoff of 20 Hz, verified through both simulated and real data. This approach improves the efficiency and feasibility of long-duration signal analysis, essential for probing the deviations from general relativity with gravitational waves measured by ground and space-based interferometric gravitational-wave detectors. Next, I present a Bayesian framework to jointly analyze gravitational-wave and electromagnetic triggers and report their significance for rapid follow-up observations. I use this framework to motivate the RAVEN software that is used by the LIGO-Virgo-KAGRA Collaboration to search multimessenger sources. I present tests and preliminary results from the first part of the fourth observing run (O4a). Finally, I demonstrate how joint inference of gravitational-wave and gamma-ray data breaks degeneracies between source parameters and allows improved understanding of the progenitors. This work highlights the potential of combining gravitational-wave and electromagnetic observations to advance our understanding of the universe through astronomical observations.