Astrophysical signatures of neutron stars in compact binaries and experimental improvements on gravitational-wave detectors

Abstract

Neutron stars (NSs) are astrophysical laboratories that allow us to probe physics at extreme conditions. The first half of this Thesis is devoted to exploring how we can connect theoretical models of NSs to observational signatures whose detections are made possible by state-of-the-art instruments. We start by exploring the dynamics of super-Eddington winds launched in type-I X-ray bursts at the surface of a NS. We show that freshly synthesized heavy elements can be exposed by the wind and will dominate the composition at the photosphere after ~ 1 s. This may create detectable absorption edges in burst spectra and explain the observed transitions from super-expansions to moderate expansions. Gravitational-wave (GW) observatories such as Advanced LIGO (aLIGO) open up a new possibility to probe deep inside the NS by examining the tidal signatures in the GW waveforms.

In this Thesis, we study the tidal excitations of g-modes in a cold, superfluid NS during the inspiral driven by gravitational radiation and their resulting phase shifts in the GW waveform. We consider both the g-modes supported by the muon-to-electron gradient in the outer core and the g-modes supported by the hyperon-to-proton gradient in the inner core. We further show that the former might be detectable by event stacking with the third generation of GW detectors. The second half of this Thesis is devoted to the experimental upgrades to a LIGO interferometers. The focus will be on the angular sensing and control system. We will cover design considerations on the system based on both stability and noise requirements. This is followed by a thorough discussion of the radiation-pressure torques, including both the Sidles-Sigg and the d[Rho]/d[theta] effects.

More importantly, we show that such optical torques can be compensated for with newly developed techniques, which is a critical step for aLIGO to reach high-power operations. Lastly, we discuss the prospects of detecting GW at 5 Hz with ground-based detectors and demonstrate that low-frequency sensitivity is crucial for both increasing the detection range for black-hole binaries and enabling timely localization of binary NS systems.