J. Alberto Lobo

Coincidence experiments between interferometric and resonant bar detectors of gravitational waves

Gravitational wave coincidence experiments between bars and interferometers may be an attractive option once the new generation of full scale interferometers begins taking data. We discuss various ways in which these disparate types of data can be compared in searches for bursts (from supernovae, for example), for pulsar signals, and for a stochastic background. Comparison of broadband interferometer data with narrowband bar data is appropriate in most searches for bursts, but in many cases the results---especially null results (upper limits)---are difficult to interpret. By narrowbanding the interferometer data to the bandwidth of the bar detector, one produces data sets that may give much clearer information in certain burst searches and that are appropriate for searches for a stochastic background of gravitational waves. We suggest, in fact, that there are circumstances where searches for a stochastic background could be more efficiently performed between a bar and an interferometer than between two interferometers. We examine, in some detail, the effect of narrowbanding the interferometer data. We apply this method to a real interferometer and bar data and assess its signal-to-noise performance for different classes of gravitational wave signals.

Multiple mode gravitational wave detection with a spherical antenna

Apart from omnidirectional, a solid elastic sphere is a natural multi-mode and multi-frequency device for the detection of Gravitational Waves (GW). Motion sensing in a spherical GW detector thus requires a multiple set of transducers attached to it at suitable locations. If these are resonant then they exert a significant back action on the larger sphere and, as a consequence, the joint dynamics of the entire system must be properly understood before reliable conclusions can be drawn from its readout. In this paper, I present and develop an analytic approach to study such dynamics which generalises currently existing ones and clarifies their actual range of validity. In addition, the new formalism shows that there actually exist resonator layouts alternative to the highly symmetric TIGA, potentially having interesting properties. One of these (I will call it PHC), which only requires five resonators per quadrupole mode sensed, and has mode channels , will be described in detail. Also, the perturbative nature of the proposed approach makes it very well adapted to systematically assess the consequences of realistic mistunings in the device parameters by robust analytic methods. In order to check the real value of the mathematical model, its predictions have been confronted with experimental data from the LSU prototype detector TIGA, and agreement between both is found to consistently reach a satisfactory precision of four decimal places.

Hollow sphere: a flexible multimode gravitational wave antenna

Hollow spheres have the same theoretical capabilities as the usual solid ones, since they share identical symmetries. The hollow sphere is, however, more flexible, as thickness is an additional parameter one can vary to approach given specifications. I will briefly discuss the more relevant properties of the hollow sphere as a GW detector (frequencies, cross-sections), and suggest some scenarios where it can generate significant astrophysical information.

The detection of gravitational waves

This chapter is concerned with the question: how do gravitational waves (GWs) interact with their detectors? It is intended to be a theoretical review of the fundamental concepts involved in interferometric and acoustic (Weber bar) GW antennas. In particular, the type of signal the GW deposits in the detector in each case will be assessed, as well as its intensity and deconvolution. Brief reference will also be made to detector sensitivity characterisation, including very summary data on current state of the art of GW detectors.

Errors on the inverse problem solution for a noisy spherical gravitational wave antenna

A single spherical antenna is capable of measuring the direction and polarization of a gravitational wave. It is possible to solve the inverse problem using only linear algebra even in the presence of noise. The simplicity of this solution enables one to explore the error on the solution using standard techniques. In this paper we derive the error on the direction and polarization measurements of a gravitational wave. We show that the solid angle error and the uncertainty on the wave amplitude are direction independent. We also discuss the possibility of determining the polarization amplitudes with isotropic sensitivity for any given gravitational wave source.

Acoustic resonance widening in GW detectors: detailed modelling of the dissipation processes

Internal friction effects are responsible for line widening of the acoustic resonance frequencies in mechanical oscillators and result in damped oscillations of its eigenmodes with a decay time Q/ω. We study the solutions to the equations of motion for the case of spherical oscillators, to be used as the next generation of acoustic gravitational wave detectors, based on various different assumptions about the materials constituent equations. Quality factor dependence on mode frequency is determined in each case, and a discussion of its applicability to actual gravitational wave detectors is made on the basis of available experimental evidence.

Stochastic backgrounds of gravitational waves and spherical detectors

The analysis of how a stochastic background of gravitational radiation interacts with a spherical detector is given in detail, which leads to explicit expressions for the system response functions, as well as for the cross-correlation matrix of different readout channels. It is shown that distinctive features of GW-induced random detector excitations, relative to locally generated noise, are, in practice, insufficient for separating the signal from the noise by means of a single sphere.

Errors on the inverse problem solution in a noisy spherical GW antenna

The inverse problem can be solved using only linear algebra in the case of a spherical GW detector, even in the presence of noise. The simplicity of such solution enables one to explore the error on the solution using standard techniques. We derive the error on the direction and polarization measurements of a gravitational wave, and show that the solid angle error and the uncertainty on the wave amplitude are direction independent. We show that the polarization amplitudes can too be determined with isotropic sensitivity for any given gravitational wave source.

Spherical detectors of gravitational waves

Resonant mass detectors of GWs of spherical shape constitute the fourth generation of such kind of antennae, and are scheduled to start operation in the near future. In this communication I present a general description of the fundamental principles underlying the physics of this kind of detector, as well as of the motion sensor set suitable to retrieve the information generated by the incidence of a GW on the antenna.

New ideas for a transducer layout in a spherical GW antenna

We discuss a general procedure to find the coupled motion of a set of resonant transducers attached to a spherical GW detector, and present some preliminary results. A new specific proposal for a polyhedric, quasispherical antenna is also presented.