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.