M. Mazzoni

The Virgo interferometer

The Virgo gravitational wave detector is an interferometer with 3 km long arms in construction near Pisa to be commissioned in the year 2000. Virgo has been designed to achieve a strain sensitivity of a few times at 200 Hz. A large effort has gone into the conception of the mirror suspension system, which is expected to reduce noise to the level of at 10 Hz. The expected signals and main sources of noise are briefly discussed; the choices made are illustrated together with the present status of the experiment.

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.

An inverted pendulum preisolator stage for the VIRGO suspension system

The design of a new preisolator stage for the VIRGO superattenuator is presented. The device is essentially a 6 m high inverted pendulum with horizontal resonant frequency of 30 mHz. An isolation of 65 dB at 1 Hz has been achieved. Very low forces are needed to move the whole superattenuator acting on the inverted pendulum. For this reason, the system is a suitable platform for the active control of the mirror suspension.

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.

The VIRGO interferometer for gravitational wave detection.

The Virgo gravitational wave detector is an interferometer with 3 km long arms in construction near Pisa in Italy. The accessible sources at the design sensitivity and main noises are reviewed. Virgo has devoted a significant effort to extend sensitivity to low frequency reaching the strain level h = 10−21 Hz−1/2 at 10 Hz while at 200 Hz h = 3 · 10−23 Hz−1/2. Design choices and status of construction are presented.

Excitation of the inner 4d shell of neutral iodine

Abstract The absorption spectrum of neutral atomic iodine has been photographed in the EUV region and three strong autoionized resonances have been identified. A broad absorption feature has been observed and is ascribed to a collective excitation of the 4d inner shell.

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.

Inner shell transitions of Br I in the EUV

Abstract The EUV line spectrum originating from transitions of the inner 3d shell of neutral atomic bromine has been observed in absorption. Fano parameters have been derived for the three autoionized resonances n d 10 (n + 1) s 2 (n + 1) p 5 2 P −n d 9 (n + 1) s 2 (n + 1) p 6 2 D observed in both bromine ( n = 3) and iodine ( n = 4) spectra.

Monocrystalline fibres for low thermal noise suspension in advanced gravitational wave detectors

Thermal noise in mirror suspension will be the most severe fundamental limit to the low-frequency sensitivity of future interferometric gravitational wave detectors. We propose a new type of materials to realize low thermal noise suspension in such detectors. Monocrystalline suspension fibres are good candidates both for cryogenic and for ambient temperature interferometers. Material characteristics and a production facility are described in this paper.