Searches for novel gravitational-wave sources with ground and space-based detectors

Abstract: 

Gravitational waves (GWs) probe the fundamental nature of neutron stars (NSs) and black holes (BHs). Observations of GWs by ground-based interferometric detectors, like the Laser Interferometer Gravitational-wave Observatory (LIGO), have yielded key insights into the formation channels of compact binaries and the physics of ultra-dense NS matter. The planned space-based Laser Interferometer Space Antenna (LISA) will detect entirely new GW sources inaccessible with present-day interferometers. In this thesis, I present new data analysis methods for ground and space-based detectors that enable future discoveries of novel GW sources.

Spinning, non-axisymmetric NSs can emit weak continuous gravitational waves (CWs). Most CW searches assume a specific phase model for the signal, but are less sensitive to sources that deviate from this model, such as NSs in binary systems. In this thesis, I describe an end-to-end CW search pipeline that is robust to a wider range of signal morphologies, combining semi-coherent matched filtering techniques with a hidden Markov model (HMM) frequency tracking scheme. Using Advanced LIGO data from the third observing run, I applied this pipeline to analyze candidate signals reported by a previous radiometer-style GW search. No credible CW signals were detected. By recovering simulated signals into detector data, I show that our approach can detect CW signals with amplitudes h ~ 9e-26 in the most sensitive frequency band (~200 Hz) of the detectors. I also apply this pipeline in a search for CWs from the Vela pulsar following a spin-up glitch.

In the second part of this thesis, I characterized the ability of LISA to detect hierarchical triple systems, consisting of a stellar-mass BH binary (BHB) orbiting a supermassive black hole (SMBH). The stellar-mass BHB component may undergo high-amplitude eccentricity oscillations due to gravitational torques exerted by the SMBH, emitting repeated GW bursts detectable by LISA. Focusing on potential BHB-SMBH triples in the Galactic centre, I used simulated LISA data to demonstrate that an unmodelled wavelet decomposition of the data recovers the time-frequency properties of each burst, and further show how this approach can be used to study the eccentricity evolution of the perturbed BHB and the dynamics of BHB-SMBH triples.