Probing the extreme physics of neutron stars: magneto-elastic oscillations of magnetars and tidal resonances in binary mergers

Abstract

Neutron stars host some of the most extreme physical conditions in the universe, including supranuclear densities, ultra strong magnetic fields, and significant relativistic compactness. These conditions give rise to distinctive astrophysical phenomena, such as giant flares from magnetars and gravitational waves from merging neutron stars. Observations of these events provide opportunities to probe physical regimes where nuclear physics, magnetohydrodynamics, and general relativity intersect. The first part of this dissertation develops a tangled magnetic field model for magnetars to investigate the origin of the quasi periodic oscillations observed in the tails of giant flares. A highly tangled magnetic field supports a discrete spectrum of magneto elastic modes which can persist for the duration of a flare. The geometry and location of the energy released during a flare determine which modes are activated. Using the giant flare from the Soft Gamma Repeater 1900+14, we place constraints on the energetics and location of the triggering mechanism. Our results indicate that neither purely internal nor purely external energy release can simultaneously account for the observed rise time and the amplitudes of the detected oscillations. Instead, a substantial release of energy both inside the star--necessary to excite magneto elastic modes to observable amplitudes--and outside the star--required to match the rapid onset of the flare--is needed. A localized internal deposition of energy excites enough modes to account for the observed quasi periodic oscillations. The second part of the dissertation examines corrections to tidal resonance in inspiraling binary neutron stars and their impact on inferring the behavior of matter at supranuclear densities. Tidal interactions are resonantly amplified when the tidal forcing frequency (same as the gravitational wave frequency) is near the neutron star's fundamental oscillation frequency, which is sensitive to nonlinear hydrodynamics, background spin, and relativistic effects. These corrections are not captured by the linear universal relations commonly used in current gravitational wave models. Using Hamiltonian Monte Carlo simulations, we quantify the systematic bias in inferred tidal deformability that arises when each correction is omitted, demonstrating the importance of incorporating these effects in future waveform modeling efforts.