M. Visco

The gravitational wave detector NAUTILUS operating at T = 0.1 K

Abstract We report on the ultralow-temperature resonant-mass gravitational-wave detector NAUTILUS, operating at the Frascati INFN Laboratories. The present aim of this detector is to achieve a sensitivity sufficient to detect bursts of gravitational radiation from sources located in our Galaxy and in the local group. Progress in transducer technology is likely to lead to sensitivities that will enable us to observe events from sources as far away as the Virgo cluster of galaxies. We describe the cryogenic apparatus, readout system, cosmic-ray veto system, and give first results obtained during one year of continuous operation at T = 0.1 K. In particular the Brownian noise of the detector at T = 0.1 K was measured. The measured strain sensitivity was h ≈ 6 · 10 −22 Hz − 1 2 at the frequencies of the two modes, 908 Hz and 924 Hz, with bandwidths of about 1 Hz.

Increasing the Bandwidth of Resonant Gravitational Antennas: The Case of Explorer

Resonant gravitational wave detectors with an observation bandwidth of tens of hertz are a reality: the antenna Explorer, operated at CERN by the ROG Collaboration, has been upgraded with a new readout. In this new configuration, it exhibits an unprecedented useful bandwidth: in over 55 Hz about its center operating frequency of 919 Hz the spectral sensitivity is better than 10(-20) Hz(-1/2). We describe the detector and its sensitivity and discuss the foreseeable upgrades to even larger bandwidths.

Status report on the EXPLORER and NAUTILUS detectors and the present science run

We report on the present scientific run (04–05) of the two detectors EXPLORER and NAUTILUS. The 04–05 run of the two detectors started in March 2004. The strain sensitivity is about 7 × 10−22 Hz−1/2 and the bandwidth is about 5 Hertz. The sensitivity for 1 ms bursts is h = 3 × 10−19.

Torsion pendulum revisited

We present an analysis of the motion of a simple torsion pendulum and we describe how, with straightforward extensions to the usual basic dynamical model, we succeed in explaining some unexpected features we found in our data, like the modulation of the torsion mode at a higher frequency and the frequency splitting of the swinging motion. Comparison with observed values yields estimates for the misalignment angles and other parameters of the model.

EXPLORER and NAUTILUS gravitational wave detectors: a status report

The two cryogenic resonant bar detectors of the ROG Collaboration, EXPLORER and NAUTILUS, have been taking data continuously with a high duty cycle for several years. We report here on the status of recent analysis of the data and in particular on the results of the burst searches in the year 2004.

“Quasi-complete” mechanical model for a double torsion pendulum

We present a dynamical model for the double torsion pendulum nicknamed PETER, where one torsion pendulum hangs in cascade, but off-axis, from the other. The dynamics of interest in these devices lies around the torsional resonance, that is at very low frequencies (mHz). However, we find that, in order to properly describe the forced motion of the pendulums, also other modes must be considered, namely swinging and bouncing oscillations of the two suspended masses, that resonate at higher frequencies (Hz). Although the system has obviously 6+6 Degrees of Freedom, we find that 8 are sufficient for an accurate description of the observed motion. This model produces reliable estimates of the response to generic external disturbances and actuating forces or torques. In particular, we compute the effect of seismic floor motion (tilt noise) on the low frequency part of the signal spectra and show that it properly accounts for most of the measured low frequency noise.

All-sky incoherent search for periodic signals with Explorer 2005 data

The data collected during 2005 by the resonant bar Explorer are divided into segments and incoherently summed in order to perform an all-sky search for periodic gravitational wave signals. The parameter space of the search spanned about 40 Hz in frequency, over 23 927 positions in the sky. Neither source orbital corrections nor spindown parameters have been included, with the result that the search was sensitive to isolated neutron stars with a frequency drift less than 6 x 10 -11 Hz s -1 . No gravitational wave candidates have been found by means of the present analysis, which led to a best upper limit of 3.1 x 10 -23 for the dimensionless strain amplitude.

All-sky upper limit for gravitational radiation from spinning neutron stars

We present results of the all-sky search for gravitational-wave signals from spinning neutron stars in the data of the EXPLORER resonant bar detector. Our data analysis technique was based on the maximum likelihood detection method. We briefly describe the theoretical methods that we used in our search. The main result of our analysis is an upper limit of 2 × 10−23 for the dimensionless amplitude of the continuous gravitational-wave signals coming from any direction in the sky and in the narrow frequency band from 921.00 Hz to 921.76 Hz.

Approaching Free Fall on Two Degrees of Freedom: Simultaneous Measurement of Residual Force and Torque on a Double Torsion Pendulum

A torsion pendulum with 2 soft degrees of freedom (DOFs), realized by off-axis cascading two torsion fibers, has been built and operated. This instrument helps characterize the geodesic motion of a test mass for LISA Pathfinder or any other free-fall space mission, providing information on cross talk and other effects that cannot be detected when monitoring a single DOF. We show that it is possible to simultaneously measure both the residual force and the residual torque acting on a quasifree test mass. As an example of the investigations that a double pendulum allows, we report the measurement of the force-to-torque cross talk, i.e., the amount of actuation signal, produced by applying a force on the suspended test mass, that leaks into the rotational DOF, detected by measuring the corresponding (unwanted) torque.

Detection of Cosmic Rays by NAUTILUS

The passage of cosmic rays has been observed to excite mechanical vibrations in the resonant gravitational wave detector NAUTILUS operating at temperature of 100 mK. A very significant correlation (more than ten standard deviations) is found.