Search Results (32 total)

We apply a feedback cooling technique to simultaneously cool the three electromechanical normal modes of the ton-scale resonant-bar gravitational wave detector AURIGA. The measuring system is based on a dc superconducting quantum interference device (SQUID) amplifier, and the feedback cooling is applied electronically to the input circuit of the SQUID. Starting from a bath temperature of 4.2 K, we achieve a minimum temperature of 0.17 mK for the coolest normal mode. The same technique, implemented in a dedicated experiment at subkelvin bath temperature and with a quantum limited SQUID, could allow to approach the quantum ground state of a kilogram-scale mechanical resonator.

The low frequency sensitivity of space-borne gravitational wave observatories will depend critically on the geodesic purity of the trajectories of orbiting test masses. Fluctuations in the temperature difference across the enclosure surrounding the free-falling test mass can produce noisy forces through several processes, including the radiometric effect, radiation pressure, and outgassing. We present here a detailed experimental investigation of thermal gradient-induced forces for the Laser Interferometer Space Antenna (LISA) gravitational wave mission and the LISA Pathfinder, employing high resolution torsion pendulum measurements of the torque on a LISA-like test mass suspended inside a prototype of the LISA gravitational reference sensor that will surround the test mass in orbit. The measurement campaign, accompanied by numerical simulations of the radiometric and radiation pressure effects, allows a more accurate and representative characterization of thermal-gradient forces in the specific geometry and environment relevant to LISA free-fall. The pressure dependence of the measured torques allows clear identification of the radiometric effect, in quantitative agreement with the model developed. In the limit of zero gas pressure, the measurements are most likely dominated by outgassing, but at a low level that does not threaten the current LISA noise estimate, which assumes a maximum net force per degree of temperature difference of 100pN / K for the overall thermal gradient-induced effects.

The network of resonant bar detectors of gravitational waves resumed coordinated observations within the International Gravitational Event Collaboration (IGEC-2). Four detectors are taking part in this Collaboration: ALLEGRO, AURIGA, EXPLORER and NAUTILUS. We present here the results of the search for gravitational wave bursts over 6 months during 2005, when IGEC-2 was the only gravitational wave observatory in operation. The implemented network data analysis is based on a time coincidence search among AURIGA, EXPLORER and NAUTILUS; ALLEGRO data was reserved for follow-up studies. The network amplitude sensitivity to bursts improved by a factor ≈3 over the 1997-2000 IGEC observations; the wider sensitive band also allowed the analysis to be tuned over a larger class of waveforms. Given the higher single-detector duty factors, the analysis was based on threefold coincidence, to ensure the identification of any single candidate of gravitational waves with high statistical confidence. The false detection rate was as low as 1 per century. No candidates were found.

We have measured surface-force noise on a hollow replica of a LISA proof mass surrounded by its capacitive motion sensor. Forces are detected through the torque exerted on the proof mass by means of a torsion pendulum in the 0.1–30 mHz range. The sensor and electronics have the same design as for the flight hardware, including 4 mm gaps around the proof mass. The measured upper limit for forces would allow detection of a number of galactic binaries signals with signal-to-noise ratio up to ≈40 for 1 yr integration. We also discuss how LISA Pathfinder will substantially improve this limit, approaching the LISA performance.

At the time when the giant flare of SGR1806-20 occurred, the AURIGA “bar” gravitational-wave (GW) detector was on the air with a noise performance close to stationary Gaussian. This allows us to set relevant upper limits, at a number of frequencies in the vicinities of 900 Hz, on the amplitude of the damped GW wave trains, which, according to current models, could have been emitted, due to the excitation of normal modes of the star associated with the peak in x-ray luminosity.

We have implemented a novel scheme of signal readout for resonant gravitational wave detectors. For the first time, a capacitive resonant transducer has been matched to the signal amplifier by means of a tuned high Q electrical resonator. The resulting 3-mode detection scheme widens significantly the bandwidth of the detector. We present here the results achieved by this signal readout equipped with a two-stage SQUID amplifier. Once installed on the AURIGA detector, the one-sided spectral sensitivity obtained with the detector operated at 4.5 K is better than 10^-20  Hz^-1/2 over 110 Hz, and in good agreement with the expectations.

The low-frequency resolution of space-based gravitational wave observatories such as LISA (Laser Interferometry Space Antenna) hinges on the orbital purity of a free-falling reference test mass inside a satellite shield. We present here a torsion pendulum study of the forces that will disturb an orbiting test mass inside a LISA capacitive position sensor. The pendulum, with a measured torque noise floor below 10  fN m/sqrt[Hz] from 0.6 to 10 mHz, has allowed placement of an upper limit on sensor force noise contributions, measurement of the sensor electrostatic stiffness at the 5% level, and detection and compensation of stray dc electrostatic biases at the millivolt level.

This paper presents the results of the observations of the detectors participating in the International Gravitational Event Collaboration (IGEC) from 1997 to 2000 and reviews the data analysis methods. The analysis is designed to search for coincident excitations in multiple detectors. The data set analyzed in this article covers a longer period and is more complete than that given in previous reports. The current analysis is more accurate for determining the false dismissal probability for a time coincidence search and it optimizes the search with respect to a target amplitude and direction of the signal. The statistics of the accidental coincidences agrees with the model used for drawing the results. The observations of this IGEC search are consistent with no detection of gravitational wave burst events. A new conservative upper limit has been set on the rate of gravitational wave bursts with a Fourier component H>2×10^-21 Hz^-1, both for searches with and without a filter for the galactic center direction. This study confirms that the false alarm rate of the observation can be negligible when at least three detectors are operating simultaneously.

The cosmological origin of γ-ray bursts (GRB’s) is now commonly accepted and, according to several models for the central engine, GRB sources should also emit at the same time gravitational wave bursts (GWB’s). We have performed two correlation searches between the data of the resonant gravitational wave detector AURIGA and GRB arrival times collected in the BATSE 4B catalogue. No correlation was found and an upper limit h_RMS<~1.5×10^-18 on the averaged amplitude of gravitational waves associated with γ-ray bursts has been set for the first time.

We analyzed the spectrum of the ¹⁸⁷Re beta decay, obtained with a cryogenic microcalorimeter, searching for heavy neutrinos in the mass range 50–1000 eV. No evidence has been found for them and the upper limits on the mixing angle with a zero-mass neutrino are reported. Upper limits of 9×10^-3 at 1000 eV/c², 1.2×10^-2 at 500 eV/c², 4.4×10^-2 at 200 eV/c², and 0.116 at 100 eV/c² at 95% C.L. have been obtained. These upper limits are a factor of 2 to 4 lower than the current limits reported in the literature.

We report the initial results from a search for bursts of gravitational radiation by a network of five cryogenic resonant detectors during 1997 and 1998. This is the first significant search with more than two detectors observing simultaneously. No gravitational wave burst was detected. The false alarm rate was lower than 1 per 10⁴ yr when three or more detectors were operating simultaneously. The typical threshold was H≃4×10^-21 Hz^-1 on the Fourier component at ∼10³ Hz of the gravitational wave strain amplitude. New upper limits for amplitude and rate of gravitational wave bursts have been set.

A high statistics measurement of the β decay of ¹⁸⁷Re with a cryogenic microcalorimeter has been performed. The high accuracy and the calorimetric nature of the measurement allow setting a value for the end-point energy of the decay in a metallic rhenium crystal equal to [2470±1(stat)±4(syst)] eV. A good extrapolation of the spectrum to zero energy has also been possible thanks to the stable and well determined detector response down to the energy threshold of about 420 eV. This allows an estimate of the ¹⁸⁷Re half-life of [4.12±0.02(stat)±0.11(syst)]×10¹⁰ yr. Both results are significantly better than the previous measurements reported in the literature.

We have studied the flow of superfluid ³He-B forced through small apertures. There are unexpectedly large dissipative currents, which can be described by two independent processes. One process involves the creation of quasiparticles within the aperture and their subsequent acceleration in the ambient pressure gradient. The second process involves the dissipative precession of a texture in a geometry-induced anisotropic order parameter. For both mechanisms we make a simple estimate of the relevant effect and find these agree well with the data.

We have implemented likelihood testing of the performance of an optimal filter within the online analysis of AURIGA, a sub-Kelvin resonant-bar gravitational wave detector. We demonstrate the effectiveness of this technique in discriminating between impulsive mechanical excitations of the resonant-bar and other spurious excitations. This technique also ensures the accuracy of the estimated parameters such as the signal-to-noise ratio. The efficiency of the technique to deal with nonstationary noise and its application to data from a network of detectors are also discussed.

We have discovered that the dc mass current through a superfluid ³He weak link is substantially increased when the Josephson frequency matches the resonant frequency of a coupled mechanical oscillator. The phenomenon is the result of homodyne mixing between the Josephson oscillations and the oscillating pressure field associated with the resonant system. The measured sizes of the current enhancements are in excellent agreement with calculations based on this homodyne model. Similar observations in superconducting junctions, in which microwave radiation changes the dc electronic current, were used for the first confirmation of the dynamics of the superconducting Josephson effect.

⁷Be electron capture decay to ⁷Li has been investigated by means of a cryogenic μ calorimeter. A cryogenic particle detector, due to its low energy threshold and good energy resolution, allows us to detect for the first time the complete ⁷Be calorimetric energy spectrum with a peak width of 24 eV full width at half maximum at 112 eV. The observed decrease in the rate of the absorber activity corresponds to the expected ⁷Be half-life of 53 days. This kind of detector has an important application in the environment of a lithium radiochemical solar neutrino experiment which is in progress in Moscow. A discussion about the possible processes which take place inside the BeO absorber during the energy thermalization is reported.

We measure the time of arrival t₀ of a force signal acting on a room temperature gravitational wave antenna. The antenna has a noise spectral density whose shape is a rescaled replica of that predicted for the two subkelvin antennas located in Italy, once at their sensitivity goal. t₀ is expressed as t₀=t_φ+kT₀ where T₀ is half the natural period of oscillation of the antenna, |t_φ|<~T₀/2, and k is an integer. We measure the phase part t_φ with an accuracy of σ_tφ≈174 μs/SNR, where SNR is the signal to noise ratio for the signal amplitude. We also find that, for SNR>~20, the error on k is δk≪1 so that the total statistical error on the arrival time reduces to the phase error σ_tφ. We discuss how this last result can be achieved even for smaller values of the SNR, by better tuning the modes of the antenna. We finally discuss the relevance of these results for source location and spuria events rejection with the two subkelvin detectors above.

In this paper we discuss how the standard optimal Wiener filter theory can be applied, within a linear approximation, to the detection of an isotropic stochastic gravitational-wave background with two or more detectors. We apply then the method to the AURIGA-NAUTILUS pair of ultralow temperature bar detectors, soon to operate in coincidence in Italy, obtaining an estimate for the sensitivity to the background spectral density of ≈10^-49 Hz^-1, that converts to an energy density per unit logarithmic frequency of ≈8×10^-5×ρ_c, with ρ_c≈1.9×10^-26 kg/m³ the closure density of the Universe. We also show that by adding the VIRGO interferometric detector under construction in Italy to the array, and by properly reorienting the detectors, one can reach a sensitivity of ≈6×10^-5×ρ_c. We then calculate that the pair formed by VIRGO and one large mass spherical detector properly located in one of the nearby available sites in Italy can reach a sensitivity of ≈2×10^-5×ρ_c while a pair of such spherical detectors at the same sites of AURIGA and NAUTILUS can achieve sensitivities of ≈2×10^-6×ρ_c.

Bias current and magnetic field strongly influence the switching rates of random-telegraph signals by stressing the two-level fluctuator energy structure. Symmetric-telegraph noise is observed when the stress due to current flow is compensated by the magnetic-field-induced stress. The dependence of the measured symmetrizing magnetic field on current flow enables one to infer the symmetry characteristics of a fluctuator. The symmetry characteristics in granular films were found to be strongly nonlinear. It has been shown that current flow across the intrinsic Josephson inductance is responsible for the observed nonlinearity. A fit of the experimental data to the proposed model has revealed that a Josephson element enclosed in a superconducting loop is likely involved in the random-telegraph voltage noise generation. The evaluated area of the loop is consistent with the free space between average grains in the sample investigated. © 1996 The American Physical Society.

We discuss a method of data filtering for a resonant gravitational wave detector that allows an optimal reconstruction of the input signal. The method consists of the estimate, by optimal Wiener filtering, of the amplitude of the Karhunen-Loève components of the signal at the antenna input, and only needs the assumption that the signal has a finite duration. After discussing an application of the method to a simplified model for a resonant antenna, we present a practical application to the reduction of the data from a room temperature antenna.

We are developing a superfluid ⁴He analog of the rf SQUID. A thin partition with a micrometric orifice is placed inside a toroidal container filled with liquid helium; ac and dc superflows through the orifice are induced by rotating the whole torus, which is the inertial member of a torsional oscillator. The torus oscillation amplitude vs drive curve shows a small but reproducible staircase pattern, which is detectable between 1.4 and 2.05 K, but is not modulated by a superimposed steady rotation. We discuss this behavior in terms of the occurrence of multiple phase slips.

We show how, combining their outputs, at least six resonant bar antennae give with isotropic sensitivity the amplitude, polarization, and direction of propagation of a burst and test for two distinctive properties of the Riemann tensor, transversality, and tracelessness. If not located at the same site, to exert the two vetoes, the burst arrival times on three antennae must be known: We show how to determine them. Then in addition the propagation of the burst at light velocity is tested.

In this paper we describe how quantum coherence of superfluid helium provides a mechanism by which very small rotations can substantially modify the flow in a toroidal container. The specific modifications to that flow are discussed. For ⁴He, we explain how the rotationally induced flow can be detected by monitoring the apparent phase-slip critical current. The rotational resolution is limited by stochastic processes related to the nucleation of phase slips. This type of superfluid-helium gyroscope (SHEG) is an analog of the rf superconducting quantum interference device (SQUID). We also show how the large coherence length of ³He can be utilized to lead to a rotationally induced interference pattern. Changes in this pattern can permit the detection of very small rotational motion. This type of SHEG is analogous to the dc SQUID. In appendixes, electrical circuits equivalent to the SHEG are described, as are certain constraints on rotational sensitivity imposed by external measuring devices.