Characterisation of mechanical loss in fused silica ribbons for use in gravitational wave detector suspensions

The majority of work contained in this thesis involves characterisation of the mechanical losses in fused silica ribbon fibres to determine their potential for use in suspending the 40kg test masses for Advanced LIGO. The design of fibres is discussed here, demonstrating the advantages of rectangular cross sections over the circular cross sections already used in GEO600, with experimental work used to show the viability of this suspension scheme. The losses of a number of modes of oscillation of fibres were investigated using different suspension designs to reduce excess loss mechanisms. Measurements made of the material loss of the fused silica, using cantilever bending modes of a fibre held at one end, gave values slightly higher than those used in the design of noise curves for Advanced LIGO. The measurements also showed a reduced thermoelastic damping effect from that theoretically calculated from which an altered value for the Young’s modulus of the fibres was found compared to the value for bulk fused silica. Measurements performed using the violin modes and pendulum modes of the fibres showed that, while excess loss mechanisms were characterised and in the case of the violin mode measurement shown to be negligible, the level of dilution of loss calculated theoretically was not achieved. The source of increased loss is thought to be due to the energy being concentrated closer to lossy welded regions of the fibre. The losses measured for the linear pendulum were the lowest ever measured. Measurements of the vertical bounce mode of a small mass suspended between two fibres has shown clear evidence that there is no intrinsic stress dependence of the material loss of fused silica and has given further evidence that the majority of loss in the fibres comes from a thin highly dissipative layer on the surface. The strength of ribbon fibres has been shown to be sufficient to carry the working load of the Advanced LIGO masses, with a 20kg test suspension being created, however there was a wide variation in measured fibre breaking strengths thought to be due to bending in the fibre coupling longitudinal force into shear stress. Issues regarding thermal stress at welds are discussed with suggested solutions for construction of Advanced LIGO suspensions

Identification and mitigation of narrow spectral artifacts that degrade searches for persistent gravitational waves in the first two observing runs of Advanced LIGO

International audienceSearches are under way in Advanced LIGO and Virgo data for persistent gravitational waves from continuous sources, e.g. rapidly rotating galactic neutron stars, and stochastic sources, e.g. relic gravitational waves from the Big Bang or superposition of distant astrophysical events such as mergers of black holes or neutron stars. These searches can be degraded by the presence of narrow spectral artifacts (lines) due to instrumental or environmental disturbances. We describe a variety of methods used for finding, identifying and mitigating these artifacts, illustrated with particular examples. Results are provided in the form of lists of line artifacts that can safely be treated as non-astrophysical. Such lists are used to improve the efficiencies and sensitivities of continuous and stochastic gravitational wave searches by allowing vetoes of false outliers and permitting data cleaning

Search for intermediate mass black hole binaries in the first observing run of Advanced LIGO

International audienceDuring their first observational run, the two Advanced LIGO detectors attained an unprecedented sensitivity, resulting in the first direct detections of gravitational-wave signals produced by stellar-mass binary black hole systems. This paper reports on an all-sky search for gravitational waves (GWs) from merging intermediate mass black hole binaries (IMBHBs). The combined results from two independent search techniques were used in this study: the first employs a matched-filter algorithm that uses a bank of filters covering the GW signal parameter space, while the second is a generic search for GW transients (bursts). No GWs from IMBHBs were detected; therefore, we constrain the rate of several classes of IMBHB mergers. The most stringent limit is obtained for black holes of individual mass 100  M⊙, with spins aligned with the binary orbital angular momentum. For such systems, the merger rate is constrained to be less than 0.93  Gpc−3 yr−1 in comoving units at the 90% confidence level, an improvement of nearly 2 orders of magnitude over previous upper limits

First low-frequency Einstein@Home all-sky search for continuous gravitational waves in Advanced LIGO data

International audienceWe report results of a deep all-sky search for periodic gravitational waves from isolated neutron stars in data from the first Advanced LIGO observing run. This search investigates the low frequency range of Advanced LIGO data, between 20 and 100 Hz, much of which was not explored in initial LIGO. The search was made possible by the computing power provided by the volunteers of the Einstein@Home project. We find no significant signal candidate and set the most stringent upper limits to date on the amplitude of gravitational wave signals from the target population, corresponding to a sensitivity depth of 48.7  [1/Hz]. At the frequency of best strain sensitivity, near 100 Hz, we set 90% confidence upper limits of 1.8×10-25. At the low end of our frequency range, 20 Hz, we achieve upper limits of 3.9×10-24. At 55 Hz we can exclude sources with ellipticities greater than 10-5 within 100 pc of Earth with fiducial value of the principal moment of inertia of 1038  kg m2

First narrow-band search for continuous gravitational waves from known pulsars in advanced detector data

International audienceSpinning neutron stars asymmetric with respect to their rotation axis are potential sources of continuous gravitational waves for ground-based interferometric detectors. In the case of known pulsars a fully coherent search, based on matched filtering, which uses the position and rotational parameters obtained from electromagnetic observations, can be carried out. Matched filtering maximizes the signal-to-noise (SNR) ratio, but a large sensitivity loss is expected in case of even a very small mismatch between the assumed and the true signal parameters. For this reason, narrow-band analysis methods have been developed, allowing a fully coherent search for gravitational waves from known pulsars over a fraction of a hertz and several spin-down values. In this paper we describe a narrow-band search of 11 pulsars using data from Advanced LIGO’s first observing run. Although we have found several initial outliers, further studies show no significant evidence for the presence of a gravitational wave signal. Finally, we have placed upper limits on the signal strain amplitude lower than the spin-down limit for 5 of the 11 targets over the bands searched; in the case of J1813-1749 the spin-down limit has been beaten for the first time. For an additional 3 targets, the median upper limit across the search bands is below the spin-down limit. This is the most sensitive narrow-band search for continuous gravitational waves carried out so far