Date of Award
12-2025
Document Type
Dissertation
Degree Name
Candidate in Philosophy
Department
Physics and Astronomy
Committee Chair/Advisor
Jonathan Zrake
Committee Member
Marco Ajello
Committee Member
Jeffrey Fung
Committee Member
Joan Marler
Abstract
In 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) measured the first gravitational waves (GWs) from the coalescence of two stellar-mass black holes. This detection marked a historic milestone in astrophysics, confirming general relativity and establishing the existence of gravitationally bound black holes. Since then, researchers have extended the search to even more massive black hole mergers. Recently, the International Pulsar Timing Array (IPTA), including the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), detected a nanohertz GW background from supermassive black hole binaries (SMBHBs), black holes with masses M ≃10^7−10^9 M⊙ orbiting throughout the universe. However, a single SMBHB has yet to be individually resolved. Additionally, there remains an observational gap between stellar-mass black hole binaries and SMBHBs, systems with two intermediate mass black holes, which are called massive black hole binaries (MBHBs). The upcoming space-based GW detector, the Laser Interferometer Space Antenna (LISA) will be capable of observing the GWs from MBHB inspirals and mergers in the millihertz frequency range, targeting systems with total binary masses between M ≃10^4−10^7 M⊙.
These sought after MBHBs are expected to form from the central black holes of merging galaxies and thus should reside within the gas accretion disks of active galactic nuclei (AGN). The scientific potential of LISA GW detections will be significantly enhanced if the host galaxies of these mergers can also be identified. Multi-messenger detections may be essential given that LISA’s localization error volumes could encompass hundreds of host galaxies, specifically for sources at z > 1. This motivates the need to understand the potential for electromagnetic (EM) signatures produced by AGN hosting MBHB inspirals and mergers to guide follow-up observations and provide host-galaxy identifications.
While a few prior studies have investigated the EM counterparts of MBHBs, most assume equal-mass binaries. Whether these predicted results hold for systems with unequal masses remains an open question, specifically for so-called intermediate mass ratio inspirals (IMRIs; q≡M2/M1 ≃ 10^−4 − 10^−2). This dissertation addresses the aforementioned gap by investigating how unequal mass binaries with low mass ratios (q = 0.001−1.0) influence accretion dynamics and, in turn, the EM signatures of MBHB inspirals and mergers. Additionally, the question of whether the circumbinary disk (CBD) torque in binaries of low mass ratio produces gas-assisted inspiral and merger is addressed. Across three distinct projects, we present both analytical and numerical predictions that provide EM templates for future LISA observations while deepening our understanding of how unequal mass binaries influence accretion dynamics and their role in the broader MBHB population.
The first project in Ch. 2 presents a theoretical study of GW-driven inspirals of MBHBs with total mass M = 10^7 M⊙ and mass ratios q = 10^−3 − 10^−1. Using an analytic toy model spectrum and grid-based hydrodynamics simulations (Sailfish), we show that the GW-driven inspiral produces detectable spectral trends in AGN disk emission years to decades before merger. The EM signatures of unequal-mass MBHBs include a gradual UV brightening and X-ray dimming in the years to decades leading up to merger, an X-ray disappearance hours to days before coalescence, and a re-brightening phase as the accretion disk relaxes and flows to the remnant black hole. These timescales are largely insensitive to the normalization of the disk’s kinematic viscosity. The disk’s quasi-thermal spectrum shows dual peaks in the UV and X-ray bands, corresponding to emission from the outer CBD and circum-secondary disk, respectively. The inner disk around the primary black hole is suppressed, as the secondary consumes most of the inflowing gas. We also discuss the implications for real-time and archival EM follow-up of LISA detections. This project was published in Clyburn and Zrake (2025).
Building on these findings, the second project in Ch. 3 explores how the predicted accretion rate trends of MBHBs depend on gas temperature, disk viscosity prescriptions, and binary mass ratio. Using simulations and an analytic model describing the evolution of the disk surface density during inspiral, we identify two sub-types of long-term secular variability in AGN emission: Type-A events, where the disk dims before merger and brightens afterward (as seen in the results from Ch. 2), and Type-B events, which brighten before merger and dim afterward. These sub-types correspond to the sign of the net angular momentum transfer from the CBD to the binary, positive binary torque for Type-A and negative for Type-B. We show that these brightness trends result from the weakening of the disk-binary torque as the binary orbit decays due to GW emission. Our derived self-similar disk solutions capture this effect and provide a framework to interpret secular changes in AGN light curves, including possible explanations for changing-look (CL) AGN. This project was published in Zrake et al. (2025).
The third and final project in Ch. 4 further develops the above studies by examining the circumbinary accretion dynamics and binary evolution of unequal mass MBHBs embedded in thin,cold disks. We simulate binaries with mass ratios q= 0.05−1.0 and disk Mach numbers M≃10−52 (corresponding to disk aspect ratios h/r ≡1/M≃0.02−0.1). Using two-dimensional grid-based hydrodynamic simulations, we investigate how increasing the Mach number and decreasing the mass ratio affect the torques exerted by the CBD on the binary and, consequently, the binary evolution. For nearly equal mass systems, the torque becomes negative at moderate Mach numbers (M≃25) and lowering q shifts this transition to higher M. In contrast, for the lowest mass ratio tested (q = 0.05), the transition to negative torque does not occur or requires larger Mach numbers than were feasible for this study (i.e., very thin and cold disks). These results suggest that intermediate-q binaries may outspiral, potentially stalling coalescence. We find that the fate of low mass ratio binaries (q ≲ 0.05) depends on the normalization of kinematic viscosity. Lower viscosity disks are found to effectively torque the binary to inspiral at sufficiently high Mach numbers. Therefore, our results suggest that intermediate mass ratio inspirals (IMRIs) in CBDs may be suppressed, as the eventual coalescence of these systems are sensitive to the complex interplay between mass ratio, disk temperature, and viscosity. This project is pending submission.
Recommended Citation
Clyburn, Madeline, "Numerical Calculations of the Electromagnetic and Gravitational Wave Signatures of Unequal Mass Black Hole Binary Inspirals" (2025). All Dissertations. 4190.
https://open.clemson.edu/all_dissertations/4190