The Virgo detector is a kilometer-length interferometer for gravitational wave detection located near Pisa (Italy). During its second science run (VSR2) in 2009, six months of data were accumulated with a sensitivity close to its design. In this paper, the methods used to determine the parameters for sensitivity estimation and gravitational wave reconstruction are described. The main quantities to be calibrated are the frequency response of the mirror actuation and the sensing of the output power. Focus is also put on their absolute timing. The monitoring of the calibration data as well as the parameter estimation with independent techniques are discussed to provide an estimation of the calibration uncertainties. Finally, the estimation of the Virgo sensitivity in the frequency-domain is described and typical sensitivities measured during VSR2 are shown.

Calibration and sensitivity of the Virgo detector during its second science run

The VIRGO superattenuator (SA) is effective in suppressing seismic noise below the expected thermal noise level above 4 Hz. However, the residual mirror motion associated with the SA normal modes can saturate the interferometer control system. This motion is reduced by implementing a wideband (DC–5 Hz) multidimensional active control (the so-called inertial damping) which makes use of both accelerometers and position sensors and of a digital signal processing (DSP) system. Feedback forces are exerted by coil–magnet actuators on the top of an inverted pendulum pre-isolator stage. The residual root mean square motion of the mirror in 10 s is less than 0.1 µm. PACS number: 0480N (Some figures in this article are in colour only in the electronic version)

/pdf/the-inertial-damping-of-the-virgo-superattenuator-and-the-3b07kk9ruy.pdf

The inertial damping of the VIRGO superattenuator and the residual motion of the mirror

Sensitivity of the Low Frequency Facility experiment around 10 Hz

The low frequency facility Fabry-Perot cavity used as a speed-meter

Status report of the low frequency facility experiment, Virgo R&D

The automatic control of the suspended mirrors is a major task in operating an interferometric gravitational wave antenna. To reach the extreme sensitivity required for this kind of detector, an accurate alignment and a stable locking of the interferometer on its working point are crucial. The solution of this problem is particularly complex in the case of a multistage pendulum, such as the suspension system for seismic isolation adopted in VIRGO. A precise knowledge of the suspension mechanical transfer functions (TFs) for different forces applied in the control servo-loops represents essential information to reach the goal. In this article, we describe the apparatus we developed to measure the VIRGO suspension TF and we report the results thus obtained on full-scale suspensions at the VIRGO site. Preliminary results for the implemented control system of the last suspension stage are also presented.

/pdf/last-stage-control-and-mechanical-transfer-function-3xeaa18eme.pdf

Last stage control and mechanical transfer function measurement of the VIRGO suspensions

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Last stage control and mechanical transfer function measurement of the VIRGO suspensions