V. Dattilo

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.

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.

Relevance of Newtonian seismic noise for the VIRGO interferometer sensitivity

In this paper we analyse the noise level induced by changes in the mass density distribution around the Virgo interferometric antenna. These stochastic mass density fluctuations generate a gravitational field which couples directly to the mirrors of the optical apparatus, and it could be relevant if the planned final sensitivity of the Virgo interferometer is to be reached.

THE CREEP PROBLEM IN THE VIRGO SUSPENSIONS : A POSSIBLE SOLUTION USING MARAGING STEEL

Abstract Each optical component of the interferometric gravitational wave detector VIRGO is suspended from a cascade of mechanical filters designed to suppress the transmission of seismic vibrations. Each mechanical filter supports the weight of the filters below it by means of a set of steel cantilever blade springs. The stress from the load acting on the blades was found to induce a drooping of the blade tips of several microns per day due to a series of microscopic yielding events (micro-creep). This process induces a mechanical displacement shot-noise on the optical component which can dominate the small displacements produced by gravitational waves. The use of a special precipitation hardened steel (Maraging C250), instead of common spring steel, allows the construction of blades that show an acceptable stability under stress.

Measurement of the transfer function of the steering filter of the Virgo super attenuator suspension

The optical elements of the Virgo antenna are supported and isolated from seismic noise disturbances by super attenuator (SA) suspensions. The steering filter (SF) is a component of the SA, designed for the mirror control. The dynamical properties of the SF are described by transfer functions, which have been measured in order to define the control strategy; the results have made it possible to set and tune the parameters of a simulation of the SA. The measuring devices were linear voltage differential transducers: they were found to be quite effective and flexible in usage. An auto-regressive model was used to fit the experimental data, implementing the linear relation between the input forces and the resulting motion. The ability of the model to reproduce the experimental behavior was a clear indication of the good data quality, showing that the contaminating noise was under control. The simulation was able to reproduce the qualitative behavior, and the simulation parameters were estimated, with 10% and...

First joint gravitational wave search by the AURIGA-EXPLORER-NAUTILUS-Virgo Collaboration

We present a methodology of network data analysis applied to the search for coincident burst excitations over a 24 h long data set collected by AURIGA, EXPLORER, NAUTILUS and Virgo detectors during September 2005. The search of candidate triggers was performed independently on each of the data sets from single detectors. We looked for two-fold time coincidences between these candidates using an algorithm optimized for a given population of sources and we calculated the efficiency of detection through injections of templated signal waveforms into the streams of data. To this end we have considered the case of signals shaped as damped sinusoids coming from the galactic center direction. Our method targets an optimal balance between high efficiency and low false alarm rate, aiming at setting confidence intervals as stringent as possible in terms of the rate of the selected source models.

Experimental upper limit on the estimated thermal noise at low frequencies in a gravitational wave detector

A. Di Virgilio, L. Barsotti, S. Braccini, C. Bradaschia, G. Cella, V. Dattilo , M. Del Prete, I. Ferrante, F. Fidecaro, I. Fiori, F. Frasconi, A. Gennai, A. Giazotto, P. La Penna, G.Losurdo , E. Majorana, M. Mantovani , F. Paoletti, R. Passaquieti, D. Passuello, F. Piergiovanni, A. Porzio, P. Puppo, F. Raffaelli, P. Rapagnani, F. Ricci, S. Solimeno, G. Vajente, F. Vetrano INFN, Sez. di Pisa, Pisa, Italy 2 EGO, European Gravitational Observatory, Cascina (Pi) 3 Universita’ di Pisa, Italy 4 INFN Sezione di Firenze, Sesto Fiorentino, Italy 5 Università di Roma1, and INFN-Roma1, Roma Italy 6 Universita’ di Siena, Italy 7 Università di Urbino, Urbino, Italy 8 Coherentia, CNR-INFM, and CNISM Unitá di Napoli 9 INFN, Sez. di Napoli, Università di Napoli 10 Scuola Normale Superiore, Pisa

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.

First results of the low frequency facility experiment

The first low frequency facility (LFF) data are presented; the main purpose of the LFF is to study the pendulum thermal noise around 10 Hz. The displacement noise floor is at the level of 10−13 m Hz−1x/2 at 10 Hz, and at about 11 Hz enters the 10−14 m Hz−1/2 sensitivity region; this level is compatible with the electronic noise of the drivers of the actuators used to lock the apparatus. The calibration method and the seismic noise contribution are discussed.