Decoding Continuous Gravitational Waves from Pulsars: A Multi-Wavelength Approach
The detection of continuous gravitational wave (CW) signals from isolated neutron stars will provide new insights into the physics of neutron stars and significantly contribute to the field of gravitational wave (GW) astronomy. CW signals are expected, for instance, from young pulsars with non-axisymmetric configurations, particularly those within pulsar wind nebulae (PWNe). However, detecting such signals poses significant challenges, requiring advanced data analysis techniques and the integration of multi-wavelength observations. The research presented in this thesis focuses on developing, applying, and refining methodologies to detect CW signals using data from ground-based GW detectors and Imaging Atmospheric Cherenkov Telescopes (IACTs). In particular, this research integrates very-high-energy (VHE) observations from the High Energy Stereoscopic System (HESS) with data from the second observing run of the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) to enhance the sensitivity of CW detection strategies. The initial chapters review the evolution of CW search techniques, emphasising the critical role of IACTs in addressing challenges in CW signal detection. Young pulsars within PWNe are highlighted as prime candidates for CW searches due to the non-axisymmetric configurations expected shortly after formation. A novel search framework is introduced and applied in two CW searches, as detailed in our peer-reviewed publications. In both cases, we employ a semi-coherent analysis method that combines the F-statistic with a hidden Markov model (HMM) to track the stochastic wandering of the CW signal frequency throughout the observation period. This approach strikes a good balance between computational efficiency and sensitivity. The first search targets ten pulsars associated with TeV-bright sources, refining the CW search parameter space by using the TeV properties of these sources. A semi-coherent targeted search reveals 5,256 candidates across 24 sub-bands, with only 12 surviving the rigorous data quality vetoes designed to differentiate between astrophysical signals and instrumental noise. A follow-up analysis suggests that these candidates are likely of non-astrophysical origin. We then present a directed search for CW signals from HESS J1427-608, an unidentified TeV source potentially harbouring a young pulsar. Multi-wavelength observations are used to refine the search parameter space. Although no CW signals are detected, this study establishes the first upper limits on the GW strain amplitude, mass ellipticity, and r-mode amplitude for HESS J1427-608. These findings improve our understanding of CW signal detection techniques and lay the groundwork for future research, demonstrating the utility of multi-wavelength observations and sophisticated data analysis techniques. Ongoing advancements in GW detectors, improvements in the HMM’s tracking capabilities, and future TeV observatories, such as the Cherenkov Telescope Array Observatory (CTAO), have the potential to significantly improve detection strategies and build upon the results presented here, thus paving the way for more sensitive and comprehensive searches for CW signals.
Continuous gravitational-wave detection, data analysis, targeted searches, directed searches, HESS
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