The Multimessenger Study of Neutron Stars with Gravitational Waves and Gamma Rays 24 views

Neutron stars are fascinating objects, being associated with the most catastrophic events in our universe. When massive stars run out of fuel, their core collapses and explodes as a supernova, leaving behind a very compact object that can have densities $\sim 10^{15}$ g/cm$^{3}$ in their interior. Binary systems containing neutron stars have been observed not only in radio and x-ray, but also in gravitational waves and gamma rays. These observations can lead to constraints on neutron-star properties and their equation of state in a multimessenger picture. In this Thesis, we studied different aspects of neutron stars across different astrophysical stages and observational channels. We focused mainly on neutron-star oscillation modes, their correlations with other observables through universal relations, and their imprint on gravitational waves and gamma-rays that can be emitted in the stars' environment. We also obtained novel constraints on neutron-star observables and on their equation of state through these observations. We first study protoneutron stars and investigate the time evolution of universal relations for these systems; we found that such relations can have negligible deviations from the conventional relations that are firmly established for cold neutron stars and thus could be used to estimate protoneutron star parameters in a future detection of gravitational waves from a core-collapse supernova. We then investigated how current gravitational-wave data can constrain the parameter space of anisotropic stars through universal relations; using a phenomenological model for anisotropy, we obtained an anisotropy-dependent universal relation, that was then used to obtain equation-of-state-independent bounds on the anisotropy, showing how universal relations can be used in practice to test fundamental physics. Next, we introduced a new technique to constrain the equation of state of neutron-star matter using gamma-ray data; under the assumption that the kilohertz quasi-periodic oscillations recently discovered in short gamma-ray bursts are produced by oscillation modes of binary neutron-star merger remnants, we obtained novel bounds on the equation of state and a tight constraint of the neutron-star radius. Finally, we studied current and future prospects for the detection of such kilohertz quasi-periodic oscillations in short gamma-ray bursts; while current detectors present limited prospects, future detectors such as StarBurst show promising detection capabilities for binary neutron-star merger remnants signatures. The results of this Thesis show that universal relations provide a powerful framework to connect neutron-star oscillations with gravitational-wave and gamma-ray observations, enabling new constraints on neutron-star properties and on the equation of state of dense matter.

PHD (Doctor of Philosophy)

All rights reserved by the author (no additional license for public reuse)

2026-07-29