Searching for the Astrophysical Gravitational-Wave Background and Prompt Radio Emission from Compact Binaries

Abstract

Gravitational-wave astronomy is now a reality. During my time at Caltech, the Advanced LIGO and Virgo observatories have detected gravitational waves from dozens of compact binary coalescences. All of these gravitational-wave events occurred in the relatively local Universe. In the first part of this thesis, I will instead look towards the remote Universe, investigating what LIGO and Virgo may be able to learn about cosmologically-distant compact binaries via observation of the stochastic gravitational-wave background. The stochastic gravitational-wave background is composed of the incoherent superposition of all distant, individually-unresolvable gravitational-wave sources. I explore what we learn from study of the gravitational-wave background, both about the astrophysics of compact binaries and the fundamental nature of gravitational waves. Of course, before we can study the gravitational-wave background we must first detect it. I therefore present searches for the gravitational-wave background using data from Advanced LIGO's first two observing runs, obtaining the most stringent upper limits to date on strength of the stochastic background. Finally, I consider how one might validate an apparent detection of the gravitational-wave background, confidently distinguishing a true astrophysical signal from spurious terrestrial artifacts. The second part of this thesis concerns the search for electromagnetic counterparts to gravitational-wave events. The binary neutron star merger GW170817 was accompanied by a rich set of electromagnetic counterparts spanning nearly the entire electromagnetic spectrum. Beyond these counterparts, compact binaries may additionally generate powerful radio transients at or near their time of merger. First, I consider whether there is a plausible connection between this so-called radio and fast radio bursts — enigmatic radio transients of unknown origin. Next, I present the first direct search for prompt radio emission from a compact binary merger using the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA). While no plausible candidates are identified, this effort successfully demonstrates the prompt radio follow-up of a gravitational-wave source, providing a blueprint for LIGO and Virgo follow-up in their O3 observing run and beyond.