An enhanced sensitivity procedure for continuous gravitational wave detection: targeting the Galactic Center

The recent announcement by the LIGO and Virgo Collaborations of the direct detection of gravitational waves started the era of gravitational wave astrophysics. Each of the GW events detected so far, shed light on multiple aspects of gravity. These last two years of great scientific discoveries would not have been possible without the constant work of generations of scientists all around the world. Commissioning and detector characterization activities required a lot of effort and manpower to reach the sensitivity level and stability needed for the detections. In fact, detector characterization activities continue also during data taking, providing important data quality information to data analysis groups. Although in few years several important results have been obtained, this is just the beginning. Indeed there are several other potential sources of gravitational waves not yet detected. In particular, the search for continuous gravitational waves, which are very weak but long and persistent signals, is a very active field. The most probable sources of continuous waves signals are rapidly rotating asymmetric neutron stars, both isolated or in binary systems. In this thesis I will summarize my 3 years PhD work done in the Rome Virgo group. The main subject is the search for gravitational waves signal emitted by isolated non-axisymmetric rotating neutron stars. After a short introduction to gravitational waves and to the principles of detection (Chapters 1 and 2), in Chapter 3 I will talk about my contribution to detector characterization activities, performed during Virgo commissioning and science runs. I will describe the role played by a spectral lines monitoring tool, called NoEMi (Noise Event Miner), developed by the Rome group in 2010, which I have been responsible for, during these 3 years. NoEMi has been used through O1 and O2 Observational runs and in the commissioning phase of LIGO and Virgo detectors. It has been also used for Virgo data validation of the two gravitational wave events GW170814 and GW170817 and it is currently used for the post-commissioning identification of instrumental lines in both LIGO and Virgo data. The second part of the Thesis is dedicated to the new data analysis framework I have developed in the context of continuous gravitational wave searches. It consists of a novel organization of the data, the so-called Band Sampled Data collection, and of several functions needed to efficiently operate on the data itself. This framework dramatically improves the flexibility in data handling, allowing the user to select and manipulate data in a very efficient way, by properly taking into account the characteristics and the needs of the specific type of search she/he is doing. Overall it results in better computational performance (which, at fixed available computing resources, means better search sensitivity) and immediate adaptability to different kinds of search or, even, to different portions of the same multi-step analysis pipeline. To test the capability of this new framework, a complete pipeline for directed searches of continuous waves signals has been developed using the BSD framework. The pipeline has been applied to a real gravitational wave search (Part III), pointing to the Milky Way central region for which a large number of unknown neutron stars are expected to exist. The results of this search, done using the last observational run (O2) of the LIGO detectors, didn’t show any evidence of the presence of continuous wave emission from the few inner parsecs of our Galaxy. Interesting limits on the minimum detectable strain and ellipticity of the sources have been placed. This is the first directed search for continuous waves signals performed within the Virgo Collaboration and the first LIGO-Virgo O2 directed search toward the Galactic center. The BSD framework developed in this thesis project will become the core of all CW searches of the Rome Virgo group. Furthermore, it represents a great starting point for the development of different types of searches, like that for long transient signals that could be emitted by the post-merger remnant of GW170817

LIGO detector characterization in the second and third observing runs

The characterization of the Advanced LIGO detectors in the second and third observing runs has increased the sensitivity of the instruments, allowing for a higher number of detectable gravitational-wave signals, and provided confirmation of all observed gravitational-wave events. In this work, we present the methods used to characterize the LIGO detectors and curate the publicly available datasets, including the LIGO strain data and data quality products. We describe the essential role of these datasets in LIGO–Virgo Collaboration analyses of gravitational-waves from both transient and persistent sources and include details on the provenance of these datasets in order to support analyses of LIGO data by the broader community. Finally, we explain anticipated changes in the role of detector characterization and current efforts to prepare for the high rate of gravitational-wave alerts and events in future observing runs

DEVELOPING AND IMPLEMENTING A PRACTICAL MODEL OF REAL-TIME REDESIGN AND PROBLEM SOLVING FOR FRONTLINE HEALTHCARE PROFESSIONALS

This research develops and implements a practical model of real-time redesign and problem solving for front line healthcare professionals using systems thinking methodologies. Healthcare quality, safety and service issues have been well-documented and lamented, calling into question the current approaches for addressing these issues. The work environment for healthcare professionals has become overburdened with time pressure, workarounds, waste, and failure to learn from the small events which occur on a frequent basis at the front-line. Desensitization may occur until sentinel events stimulate an organizational reaction. Other industries have developed system engineering methodologies, including the Toyota production system, theory of constraints, six sigma and others, to address manufacturing quality, service and safety issues. Many of these concepts were developed within the context of a linear manufacturing environment, with solutions often derived "off-line" by external experts. Healthcare reality is considered more complex and requires adaptive approaches, suggesting that modifications based on complex adaptive systems theory may be necessary. The development of the model evolved based on key systems thinking principles adapted to meet the needs of the healthcare experience and introduced to front-line healthcare workers using on-line problem solving. This research includes real-time understanding of what is working or not working in the current condition as it occurs, the ideas of the staff to improve the patient experience, including asset-based problem-solving and introduction of system thinking and design principles using ideas from various systems engineering methodologies in a healthcare worker friendly way. The research focuses on the deep systems of the organization (or clinical microsystem) and ability of front line teams to redesign processes in real-time using rapid cycle mini-experiments and the results of the redesign. Using case study and action research design, the research analyzes the experiences of an intact work group of a clinical microsystem to test the implementation of a model, labeled an Excellence Makeover. The researcher acts as a participant-observer of the emergent experience and solutions from the staff. The model will then be analyzed and additional refinements will be suggested for additional research

Boosting the sensitivity of continuous gravitational waves all-sky searches using advanced filtering techniques

The work presented in this PhD thesis has been done in the context of gravitational-waves searches. Since the first detection on the 14th September 2015 by the LIGO-Virgo collaboration, a growing number of gravitational-wave events has been detected, all emitted by the coalescence of binary systems involving black holes and/or neutron stars. My work is focused on the search for continuous gravitational waves, which still miss the first detection. These signals are expected to be emitted, for instance, by spinning neutron stars with an asymmetric shape with respect to the rotation axis, and are at least five orders of magnitude weaker than the typical amplitude of detected binary coalescences. In this PhD thesis I report on the work done in four different projects, with the common purpose of increasing the sensitivity of continuous-wave searches, involving both data analysis and instrumental aspects. The first project is a contribution to the commissioning of the Virgo interferometer in view of the next observing run, O4, which will start in May 2023. My contribution has been mainly devoted to the noise hunting activity, focused on the identification and mitigation of instrumental-noise sources that can degrade the sensitivity of continuous-wave searches. The other three projects are related to data analysis. I have focused, in particular, on all-sky searches for sources without electromagnetic counterpart and long-lasting signals from rapidly evolving newly-born neutron stars. I have studied in great detail the robustness of an all-sky data analysis method in the case of overlapping signals. This is relevant for some exotic classes of continuous wave sources and, more generally, in view of third generation detectors, like Einstein Telescope. I have developed a two-dimensional filter, called triangular filter, to be applied to the search for long-lasting gravitational waves from unstable neutron stars, showing that thanks to this method an increase of the search sensitivity of about $20\%$ is achievable. Finally, I describe the first steps of a wide work to develop a new procedure for all-sky continuous-wave searches, exploiting a statistics based on the sidereal modulation, that affects astrophysical signals, due to the Earth rotation

Present and Future of Gravitational Wave Astronomy

The first detection on Earth of a gravitational wave signal from the coalescence of a binary black hole system in 2015 established a new era in astronomy, allowing the scientific community to observe the Universe with a new form of radiation for the first time. More than five years later, many more gravitational wave signals have been detected, including the first binary neutron star coalescence in coincidence with a gamma ray burst and a kilonova observation. The field of gravitational wave astronomy is rapidly evolving, making it difficult to keep up with the pace of new detector designs, discoveries, and astrophysical results. This Special Issue is, therefore, intended as a review of the current status and future directions of the field from the perspective of detector technology, data analysis, and the astrophysical implications of these discoveries. Rather than presenting new results, the articles collected in this issue will serve as a reference and an introduction to the field. This Special Issue will include reviews of the basic properties of gravitational wave signals; the detectors that are currently operating and the main sources of noise that limit their sensitivity; planned upgrades of the detectors in the short and long term; spaceborne detectors; a data analysis of the gravitational wave detector output focusing on the main classes of detected and expected signals; and implications of the current and future discoveries on our understanding of astrophysics and cosmology

Detector characterisation and searches for gravitational waves using GEO 600

We are currently on the brink of the first direct detection of gravitational waves (GWs) with a new generation of GW detectors currently being commissioned. In the period before the advanced detectors come online we must prepare techniques for detector characterisation and advanced data analysis methods to improve our sensitivity to potential sources of GWs. We begin with an outline of GWtheory, derived from Einstein’s general theory of relativity.We introduce each of the main classes of GWsignals, as distinguished by the GW community, focusing mainly on GW bursts. A short outline is also given for compact binary coalescences, continuous waves, and stochastic background. An introduction to GW interferometers is then given, focusing mainly on the GEO600 detector. The stationary noise sources that limit the frequency dependent sensitivity of current GW detectors are discussed: optical readout noise, thermal noise, and seismic noise. We discuss transient noise events (glitches) and the veto methods that are used to remove these events from the GW data channel. Details are given for glitch hunting that was performed at GEO 600 to identify and understand sources of transient noise. We demonstrate a cost-benefit analysis method that could be used for increasing the number of observable sources, by assessing the severity of detector noise sources to efficiently guide commissioning. We introduce X-PI P E L I N E, a coherent search pipeline for GW bursts associated with astrophysical transients such as gamma-ray bursts, and give details of pipeline line development that we were involved in. Results from a search for GW associated with 78 gamma-ray bursts that occurred while only GEO 600 and one of the LIGO or Virgo detectors were taking data are presented; these events have not previously been analysed. The sensitivity of searches for GW bursts is often critically limited by non-Gaussian noise fluctuations that are difficult to distinguish from real signals. We utilised the boosted decision tree multivariate analysis classifier to probe the full space of measured properties of events in an attempt to maximise the power to accurately classify events as signal or background, compared to the standard X-PI P E L I N E. While the LIGO and Virgo detectors are undergoing intense commissioning, GEO600 is the only GW detector taking observations. Therefore, we demonstrate the feasibility of performing a single detector analysis for GW bursts using GEO 600 in preparation for any exceptional astrophysical events (such as a Galactic supernova)