From Silence to Signal: Composing the Science of Neutron Star-Black Hole Mergers

open_access
Open Access
Author:
Gupta, Ish
Graduate Program:
Physics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 12, 2025
Committee Members:
Mauricio Terrones, Program Head/Chair
Bangalore Sathyaprakash, Chair & Dissertation Advisor
David Radice, Major Field Member
Eugenio Bianchi, Major Field Member
Derek Fox, Outside Unit & Field Member
Keywords:
gravitational waves
black holes
neutron stars
multi-messenger
astronomy
cosmology
astrophysics
next generation
cosmic explorer
einstein telescope
Abstract:
Gravitational-wave (GW) detections have significantly enhanced our understanding of the universe, opening a new observational window onto extreme astrophysical phenomena. The majority of these signals have been from binary black hole mergers, providing unprecedented opportunities to test general relativity in strong gravity and highly dynamic regimes. The groundbreaking observation of the first binary neutron star merger, GW170817, initiated multi-messenger astronomy with GWs, offering insights into heavy-element production through r-process nucleosynthesis and constraining the nuclear equation of state. Neutron star–black hole (NSBH) binaries, consisting of a neutron star orbiting a black hole, further expand this multi-messenger potential. In addition, the pronounced mass asymmetry in NSBH systems activates higher-order harmonics in the GW signal, significantly enhancing the estimation of binary parameters compared to more symmetric binary systems. This dissertation demonstrates how these unique characteristics position NSBH mergers as critical sources for multi-messenger astronomy, inferring astrophysical properties and cosmological parameters. We first evaluate the detectability and parameter estimation precision of NSBH mergers, highlighting how improvements in detector sensitivity and network configurations will enable unprecedented measurements of the properties of these compact binaries. Next, we delve deeper into the multi-messenger implications by examining prospects for detecting electromagnetic counterparts, specifically kilonovae, accompanying GW detections. Our findings illustrate that next-generation GW observatories will facilitate timely follow-up of kilonovae from these mergers, and that these kilonovae will be detectable by advanced telescopes such as the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope. Precise estimation of binary properties also allows us to constrain the formation and evolution of NSBH mergers. We discuss methods to distinguish between different formation scenarios using GW observables and population synthesis modeling. Demonstrating their applicability, we apply these methods to GW230529 and effectively rule out certain formation channels discussed in the literature. Moreover, we highlight how precise spin measurements of neutron stars in NSBH binaries can provide further insights into their formation histories. Finally, we quantify the cosmological potential of NSBH mergers, particularly as "gray sirens"---sources combining elements of dark and bright sirens. We project the ability of NSBH binaries detected by future detector networks in resolving the Hubble tension, showing that these systems can help constrain Hubble constant with better than 2% precision within the next ten years. This underscores the substantial impact of NSBH mergers as powerful complementary tools for cosmological measurements, leveraging both precise distance measurements and enhanced localizability enabled by electromagnetic counterparts. Overall, this dissertation establishes NSBH mergers as pivotal astrophysical and cosmological tools, highlighting their unique contributions to GW astronomy, multi-messenger astrophysics, and precision cosmology, and setting a foundation for future theoretical and observational exploration.