G. Calamai

Measurement of the VIRGO superattenuator performance for seismic noise suppression

Below a few tens of hertz interferometric detection of gravitational waves is masked by seismic vibrations of the optical components. In order to isolate the mirrors of the VIRGO interferometer, a sophisticated suspension system, called superattenuator, has been developed. Its working principle is based on a multistage pendulum acting on seismic vibrations as a chain of second order mechanical low-pass filters. A complete superattenuator has been built and tested. This apparatus allows extending the VIRGO detection band down to a few Hz. A detailed description of the attenuation system and its performance are presented in this article.

The linear variable differential transformer (LVDT) position sensor for gravitational wave interferometer low-frequency controls

Low-power, ultra-high-vacuum compatible, non-contacting position sensors with nanometer resolution and centimeter dynamic range have been developed, built and tested. They have been designed at Virgo as the sensors for low-frequency modal damping of Seismic Attenuation System chains in Gravitational Wave interferometers and sub-micron absolute mirror positioning. One type of these linear variable differential transformers (LVDTs) has been designed to be also insensitive to transversal displacement thus allowing 3D movement of the sensor head while still precisely reading its position along the sensitivity axis. A second LVDT geometry has been designed to measure the displacement of the vertical seismic attenuation filters from their nominal position. Unlike the commercial LVDTs, mostly based on magnetic cores, the LVDTs described here exert no force on the measured structure.

Inertial control of the mirror suspensions of the VIRGO interferometer for gravitational wave detection

In order to achieve full detection sensitivity at low frequencies, the mirrors of interferometric gravitational wave detectors must be isolated from seismic noise. The VIRGO vibration isolator, called the superattenuator, is fully effective at frequencies above 4 Hz. But the residual motion of the mirror at the mechanical resonant frequencies of the system is too large for the interferometer locking system and must be damped. A multidimensional feedback system, using inertial sensors and digital processing, has been designed for this purpose. An experimental procedure for determining the feedback control of the system has been defined. In this article a full description of the system is given and experimental results are presented.

On-line power spectra identification and whitening for the noise in interferometric gravitational wave detectors

The knowledge of the noise power spectral density of an interferometric detector of gravitational waves is fundamental for detection algorithms and for the analysis of the data. In this paper we address both the problem of identifying the noise power spectral density of interferometric detectors by parametric techniques and the problem of the whitening procedure of the sequence of data. We will concentrate the study on a power spectral density like that of the Italian–French detector VIRGO and we show that with a reasonable number of parameters we succeed in modelling a spectrum like the theoretical one of VIRGO, reproducing all of its features. We also propose the use of adaptive techniques to identify and to whiten the data of interferometric detectors on-line. We analyse the behaviour of the adaptive techniques in the field of stochastic gradient and in the least-squares filters. As a result, we find that the least-squares lattice filter is the best among those we have analysed. It succeeds optimally in following all the peaks of the noise power spectrum, and one of its outputs is the whitened part of the spectrum. Besides, the fast convergence of this algorithm, it lets us follow the slow non-stationarity of the noise. These procedures could be used to whiten the overall power spectrum or only some region of it. The advantage of the techniques we propose is that they do not require a priori knowledge of the noise power spectrum to be analysed. Moreover, the adaptive techniques let us identify and remove the spectral line, without building any physical model of the source that produced it.

Noise parametric identification and whitening for LIGO 40-m interferometer data

One of the goals of gravitational data wave analysis is the knowledge and accurate estimation of the noise power spectral density of the data taken by the detector, this being necessary in the detection algorithms. In this paper we show how it is possible to estimate the noise power spectral density of gravitational wave detectors using modern parametric techniques and how it is possible to whiten the noise data before they pass to the algorithms for gravitational wave detection. We report the analysis we made of data taken by the Caltech 40-m prototype interferometer to identify the noise power spectral density and to whiten the sequence of noise. We concentrate our study on data taken in November 1994; in particular, we analyze two frames of data: the 18nov94.2.frame and the 19nov94.2.frame. We show that it is possible to whiten these data, to a good degree of whiteness, using a high order whitening filter. Moreover, we can choose to whiten only a restricted band of frequencies around the region we are interested in, obtaining a higher level of whiteness.

Status of the low frequency facility experiment

The low frequency facility is a VIRGO R&D experiment having the goal of performing a direct measurement of the thermal noise of the VIRGO suspensions by means of a two-mirror Fabry–Perot cavity suspended to the last stage of the attenuating chain. The present status of advancement of this experiment is reported: the apparatus, including mechanical and optical parts, has been completely built and put into operation. Vacuum facilities and the first control loops are active. First measurements on the suspended cavity are in progress.

Testing the performance of a blind burst statistic

In this work, we estimate the performance of a method for the detection of burst events in the data produced by interferometric gravitational wave detectors. We compute the receiver operating characteristics in the specific case of a simulated noise having the spectral density expected for Virgo, using test signals taken from a library of possible waveforms emitted during the collapse of the core of type II supernovae.