Author ORCID Identifier

https://orcid.org/0000-0002-3360-9299

Semester

Summer

Date of Graduation

2025

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Sarah Burke-Spolaor

Committee Member

Justin Ellis

Committee Member

Emmanuel Fonseca

Committee Member

Katherine Goodrich

Abstract

Supermassive black hole binaries (SMBHBs) are a natural consequence of hierarchical galaxy formation and evolution. When galaxies merge, their central supermassive black holes (SMBHs) are expected to form bound pairs that evolve from kiloparsec to sub-parsec separations under the influence of dynamical friction, stellar scattering, and gas dynamics. Ultimately, these SMBHs reach the regime where gravitational wave emission dominates and drives the final inspiral and coalescence. Understanding the formation, evolution, and observational signatures of SMBHBs is critical to advancing multi-messenger astrophysics, particularly in the era of pulsar timing arrays (PTAs) and the upcoming Laser Interferometer Space Antenna (LISA), which opens new windows into gravitational wave astronomy.

This dissertation explores the diverse range of electromagnetic signatures associated with SMBHBs, particularly in active galactic nuclei (AGN). These signatures include periodic photometric variability due to Doppler boosting or disk perturbations, dual or offset AGN activity, Doppler-shifted broad emission lines indicating orbital motion, precessing jets, and features tied to binary-induced accretion dynamics. These electromagnetic tracers serve as vital tools for identifying SMBHB candidates and correlating them with gravitational wave detections.

A key contribution of this work is the creation of a curated database of published peer-reviewed SMBHB candidate papers, which compiles observational evidence, classification criteria, and binary parameters for each binary system. This database serves as a resource for researchers to assess the robustness of SMBHB claims, identify trends in detection methods, and prioritize targets for multi-wavelength and gravitational wave follow-up. It provides a foundation for future population studies, model validation, and coordinated observational campaigns. This dissertation highlights the use of such a database through the exploration of gravitational waves for a subset of candidates from the database. Furthermore, this dissertation incorporates fast radio burst (FRB) X-ray observations to estimate the black hole mass in a candidate AGN progenitor. These observations underscore the growing importance of transient and high-energy astrophysics in informing studies of SMBHBs.

This work seeks to enhance our ability to detect, interpret, and contextualize SMBHBs using both electromagnetic and gravitational wave observations. Gaining deeper insight into SMBHBs is key to unraveling the mysteries of black hole formation and long-term evolution. This work aims to help lay critical groundwork for the next generation of multi-messenger discoveries.

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