S. Vitale

Towards a generic test of the strong field dynamics of general relativity using compact binary coalescence

Coalescences of binary neutron stars and/or black holes are amongst the most likely gravitational-wave signals to be observed in ground based interferometric detectors. Apart from the astrophysical importance of their detection, they will also provide us with our very first empirical access to the genuinely strong-field dynamics of General Relativity (GR). We present a new framework based on Bayesian model selection aimed at detecting deviations from GR, subject to the constraints of the Advanced Virgo and LIGO detectors. The method tests the consistency of coefficients appearing in the waveform with the predictions made by GR, without relying on any specific alternative theory of gravity. The framework is suitable for low signal-to-noise ratio events through the construction of multiple subtests, most of which involve only a limited number of coefficients. It also naturally allows for the combination of information from multiple sources to increase ones confidence in GR or a violation thereof. We expect it to be capable of finding a wide range of possible deviations from GR, including ones which in principle cannot be accommodated by the model waveforms, on condition that the induced change in phase at frequencies where the detectors are the most sensitive is comparable to the effect of a few percent change in one or more of the low-order post-Newtonian phase coefficients. In principle the framework can be used with any GR waveform approximant, with arbitrary parameterized deformations, to serve as model waveforms. In order to illustrate the workings of the method, we perform a range of numerical experiments in which simulated gravitational waves modeled in the restricted post-Newtonian, stationary phase approximation are added to Gaussian and stationary noise that follows the expected Advanced LIGO/Virgo noise curves.

Feedback Cooling of the Normal Modes of a Massive Electromechanical System to Submillikelvin Temperature

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.

3-Mode Detection for Widening the Bandwidth of Resonant Gravitational Wave Detectors

Along with peak sensitivity, an important parameter of a resonant gravitational wave detector is its bandwidth. In addition to the obvious advantage of making the detector more sensitive to short bursts, a wider bandwidth would allow, for instance, details of the signal emitted during a supernova gravitational collapse or the merger of compact binaries to be resolved [1]. Moreover, a wider bandwidth reduces the uncertainty in the burst arrival time [2] and consequently, with a detector network, permits a more precise source location and a higher efficiency of spurious events rejection [3]. The introduction of a mechanically resonant transducer, a standard practice in actual resonant detectors, has greatly improved the coupling between the bar and the amplifier, but the bandwidth is intrinsically limited [4], and in practice, according to the full width at half maximum (FWHM) definition applied to the two minima of the Shh strain noise spectra, values of a few Hz have been achieved [5]. The use of multimode resonant transducers should permit further improvements of the detector bandwidth [6]. This approach has been studied [7] in depth and a few 2-mode transducer prototypes have been realized [8] or are under development [9] to obtain 3mode operation of the resonant mass detectors. This Letter describes how a wider detection bandwidth can be obtained with an alternative 2-mode transduction system in which the resonant amplification is realized by means of a resonant mechanical mode plus a resonant electrical matching network. It also describes the key tests performed on the components of the transduction system in order to verify the achievement of the requirements set by analysis of the detector model. Figure 1 shows the electromechanical scheme of a cryogenic detector with a resonant capacitive transducer read by a SQUID amplifier. The matching transformer couples the output impedance of the transducer (a capacitance of a few nF) to the input impedance of the SQUID (a small

Sensitivity enhancement of Quantum Design dc superconducting quantum interference devices in two-stage configuration

The energy sensitivity of a direct current (dc) superconducting quantum interference device (SQUID) can be improved if it is operated in a two-stage configuration. Employing this technique, a commercial dc SQUID system was modified and made competitive with other sensors especially designed for very low noise applications. We report the noise measurements performed in the temperature range 4.2 K–25 mK. At 4.2 K, the coupled energy sensitivity obtained with the two-stage dc SQUID was approximately ten times better than with a conventional readout electronics. The noise energy decreases linearly until approximately 300 mK, in good agreement with theoretical previsions. At lower temperature the hot-electron effect produces a saturation and the best energy sensitivity measured with open input coil is 35 ℏ.

Superconducting susceptometer for high‐accuracy routine operation

We present a new version of a SQUID susceptometer with the following performance. The overall accuracy is δχ≃3×10−9 (SI units). Within this figure is allowed the reproducibility in interchanging samples, the thermal expansion of the samples up to 10%, and the constancy of the calibration over a time scale of months. The sample can be interchanged in minutes and the overall time taken by the actual measurement procedure, to give results within the accuracy quoted above, is of the order of 5 min for one sample at one temperature. The temperature regulation is precise within 0.2 K between 40 and 300; the accuracy in the sample temperature has been tested to be within 0.5 K in the room temperature range. This performance is of great relevance for studies of biological molecules in solution, when different samples in different solution conditions must be compared with accuracies better than parts in a thousand of the magnetic susceptibility of the solution.

OPTICAL TRANSDUCTION CHAIN FOR GRAVITATIONAL WAVE BAR DETECTORS

A new signal extraction chain for gravitational wave bar detectors is proposed. Signal transduction is made by means of a Fabry–Perot cavity. Using a reference cavity for frequency comparison a sensitivity of hmin=3×10−20 with a bandwidth of about 50 Hz is estimated for an ultracryogenic bar detector. The first prototype implementation as well as room-temperature measurements on it are reported.

Gravitational-wave stochastic background detection with resonant-mass detectors

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 ’10 249 Hz 21 , that converts to an energy density per unit logarithmic frequency of ’8310 25 3r c , with r c’1.9310 226 kg/m 3 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 ’6310 25 3r 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 ’2310 25 3r c while a pair of such spherical detectors at the same sites of AURIGA and NAUTILUS can

A high inductance kHz resonator with a quality factor larger than 106

Electrical LC resonators with superconducting coils of ≊3.5 H inductance, operating in the frequency range 250–1500 Hz with quality factors up to Q≊1.6×106 are presented here. The coil has a reduced, <100 pF, stray capacitance and is housed in a superconducting case. Measurements are made with a low coupling SQUID readout. Some possible applications of the device are briefly discussed.

Gravitational compensation for the LISA pathfinder

This paper addresses the problem of compensating self-gravity for the LISA Technology Package. Massive components onboard the spacecraft produce a gravitational imbalance on the free-falling test masses. We present here a compensation scheme to reduce the gravitational forces, torques, stiffness and cross-talk to values within requirements. Gravitational analysis and subsequent compensation are needed to limit the gravitational imbalances in order to reduce the force noise and force gradients associated with the electrostatic actuation that must compensate any residual gravitational imbalances. Starting from an educated guess based on simple Newtonian arguments, we present the approximate shapes of a compensation block solution which minimizes the residual gravitational imbalance and stiffness while adding a minimum of mass.

Status report of the gravitational wave detector AURIGA

We present the status of the ultracryogenic gravitational wave detector AURIGA, which is taking data since may 1997 with an energy sensitivity in the mK range and bandwidth greater than 1 Hz. The typical detector output is summarized in daily reports which are important tools for detector diagnostic and for checking the vetoes of periods of unsatisfactory operation of the detector.