Abstract

(EM) radiation, with their observable signatures shaped by the complex interplay between orbital dynamics, gas dynamics, and radiative processes. The evolution of the surrounding accretion flow is governed by general relativistic magnetohydrodynamics (GRMHD) within a strongly curved, timedependent spacetime, making the prediction of observable EM signals a challenging computational problem. This dissertation presents the development of a general relativistic ray-tracing framework that post-processes numerical relativity GRMHD simulations of accreting SMBHB systems. The framework enables the calculation of predicted images, spectra, and light curves throughout the inspiral, merger, and early post-merger phases. In parallel, improvements are made to an analytical model of SMBHB accretion flows. The analytical model is designed to reproduce the large-scale behavior observed in GRMHD simulations while remaining computationally efficient. Application of the ray-tracing framework to numerical relativity simulations reveals substantial changes in the structure and radiative properties of the accretion flow during the final stages of binary evolution. As the binary approaches merger, the individual mini-disks become increasingly disrupted before disappearing entirely. Simultaneously, the system becomes optically thick over nearly the entire computational domain, causing the hot coronal emission to be effectively buried beneath the photosphere. Consequently, the X-ray-producing corona is strongly suppressed, whereas thermal emission from the optically thick gas dominates the emergent radiation. This transition produces a pronounced softening of the spectrum, characterized by the disappearance of the high-energy spectral component and a substantial enhancement of the ultraviolet and soft X-ray emission immediately prior to merger. The analytical model is used to investigate the impact of finite light-travel time on observable variability by comparing calculations performed with and without the fast-light approximation. While the fast-light approximation reproduces the overall morphology of the light curves, neglecting the orbital motion of the black holes during photon propagation introduces a measurable shift in the timing and duration of periodic self-lensing flares. These differences become increasingly important for accurately modeling rapidly evolving binary systems and demonstrate that finite light-traveltime effects should be included when predicting precision electromagnetic signatures for future multimessenger observations. i

Publication Date

8-17-2026

Document Type

Dissertation

Student Type

Graduate

Degree Name

Astrophysical Sciences and Technology (Ph.D.)

Department, Program, or Center

Physics and Astronomy, School of

College

College of Science

Advisor

Manuela Campanelli

Advisor/Committee Member

Scott Noble

Advisor/Committee Member

Andrew Robinson

Campus

RIT – Main Campus

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