Date of Award
8-2025
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Physics and Astronomy
Committee Chair/Advisor
Dr. Jonathan Zrake
Committee Member
Dr. Dieter H. Hartmann
Committee Member
Dr. Bradley Meyer
Committee Member
Dr. Joan Marler
Abstract
Binary neutron star (BNS) mergers are some of the most energetic events in the universe, producing both short gamma-ray bursts (GRBs) and kilonovae. GRBs are brief, bright flashes of gamma rays, powered by ultra-relativistic jets launched during the merger. These jets produce a prompt burst of gamma rays, followed by a broadband afterglow generated by external shocks as the outflow interacts with the surrounding medium. Kilonovae are optical-infrared transients from the radioactive decay of heavy, neutron-rich nuclei formed through the rapid neutron capture process (r-process) in the merger ejecta. Together, GRBs and kilonovae provide unique laboratories for probing fundamental physics under extreme conditions, as well as studying the synthesis and transport of heavy chemical elements in the universe. This dissertation investigates the aftermath of BNS mergers through a combination of analytical modeling and numerical simulation, focusing on three interconnected aspects: the galactic-scale transport of r-process material, the long-term detectability of kilonova remnants, and the early-time dynamics of relativistic outflows in the context of GRBs.
Chapter 2 examines the chemical evolution consequences of BNS mergers occurring in the galactic halo. Developing a toy model for Rayleigh-Taylor-unstable ejecta, we demonstrate that the resulting r-process-enriched clouds rapidly cool, fragment, and are ultimately destroyed by Kelvin-Helmholtz instabilities during their descent toward the galactic disk. As a result, direct enrichment of star-forming regions is unlikely; instead, the ejecta becomes assimilated into the halo medium and can only contribute to chemical evolution via large-scale accretion flows or turbulent diffusion.
Chapter 3 explores the prospects for detecting long-lived gamma-ray line emission from kilonova remnants (KNRs), which persist for up to ~106 years following the merger. By combining binary population synthesis, galactic orbital modeling, and nucleosynthesis yields, we quantify the distribution of KNRs in the Milky Way and predict their gamma-ray fluxes. Our results show that while current gamma-ray telescopes lack the sensitivity to detect KNRs, next-generation instruments with moderate improvements could open a new observational window into late-time r-process decay signatures.
Chapter 4 focuses on the hydrodynamics of the early GRB afterglow phase. We re-derive a semi-analytic two-zone model for the shocked ejecta and circumburst medium, capturing the structure between the forward and reverse shocks, and test the accuracy of this model against detailed special relativistic hydrodynamic (SRHD) simulations. Comparisons reveal that the two-zone model systematically overestimates thermal energy in the reverse shock region when the reverse shock itself is Newtonian. We identify and clarify the relationships among key timescales - such as the deceleration time and the reverse shock crossing time - and demonstrate that standard approximations can mischaracterize the dynamics and emission of early GRB afterglows, particularly when the reverse shock is weak.
As summarized in Chapter 5, together, these studies provide a comprehensive picture of the physical and observable consequences of BNS mergers, spanning from the earliest moments of jet medium interaction to the gradual diffusion of heavy elements into the galactic halo, and culminating in the prospects for detecting the long-lived signatures of radioactive decay. Future work will include extensions to the project outlined in Chapter 4, as well as the study of tidal disruption events (TDEs) using both semi-analytic and numerical tools.
Recommended Citation
Amend, Benjamin, "Chemical & Electromagnetic Signatures of Binary Neutron Star Mergers" (2025). All Dissertations. 3972.
https://open.clemson.edu/all_dissertations/3972
Author ORCID Identifier
0000-0002-9930-3591
Included in
Other Astrophysics and Astronomy Commons, Physical Processes Commons, Stars, Interstellar Medium and the Galaxy Commons