Characterization of the Hough all-sky search for continuous gravitational wave signals using LIGO data

Abstract

[eng] For the first time, scientists have observed ripples in the fabric of space-time called gravitational waves, arriving at Earth from a cataclysmic event in the distant Universe. These gravitational waves were detected on September 14, 2015 at 9:51 a.m. UTC by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors. This confirms a major prediction of Albert Einstein’s 1915 theory of general relativity and opens an unprecedented new window onto the cosmos. Isolated spinning neutron stars in our galaxy are also among the targets of the ground-based interferometric gravitational wave detectors. If these stars are not perfectly symmetric about their axis of rotation, e.g. if they have a “mountain” on their surface, they are expected to emit continuous gravitational waves (CW). This thesis is devoted to the characterization of a search method for continuous gravitational wave signals from unknown sources - neutron stars that do not beam a radio signal in Earth’s direction - using the Hough transform. Unlike searches for gravitational waves from pulsars (whose locations, gravitational wave emission frequencies, and spin-down rates are well known), searches for electromagnetically quiet sources require algorithms which look at vastly larger parameter spaces: all sky directions, all frequencies, and all spin-down rates. In addition, the algorithms have to account for “rapid” modulation of the signal due to Earth’s rotation (both Doppler modulation of the frequency and amplitude modulation due to the diurnal change in detector antenna pattern) and the slower modulation due to Earth’s orbit around the sun. Unfortunately, this is a computationally intractable problem: there is not enough computing power available to search such a large and essentially continuous parameter space in sky position, frequency, and spin-down rate as well as in gravitational wave polarization. Using optimal search methods, the UIB Relativity and Gravitation group efforts focus on making all-sky CW searches computationally manageable, that is on the development of effective computational methods using limited computing power by taking a first pass at the data using computationally inexpensive methods, for identifying interesting candidates or regions in parameter space and then performing follow-up searches with much more precise (and computationally expensive) methods over a much more restricted region. Although other methods exist, the UIB group has devised a clever technique based on the Hough transform that partially immunizes the computationally cheap search against instrumental artefacts that naturally pollute the experimental interferometer data. These methods have served as the basis for a number of continuous wave searches during initial LIGO. Currently I am involved in a number of refinements using this Hough transform method that will allow to follow weaker signals without increasing the computational cost, and I have contributed to the Continuous Wave Mock Data Challenge - a chance to explore the capabilities of the search algorithms within the LIGO-Virgo Continuous Wave working group - as well as to the analysis of Advanced LIGO O1 data.