L. Baggio

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

Status report and near future prospects for the gravitational wave detector AURIGA

We describe the experimental efforts to set up the second AURIGA run. Thanks to the upgraded capacitive readout, fully characterized and optimized in a dedicated facility, we predict an improvement in the detector sensitivity and bandwidth by at least one order of magnitude. In the second run, AURIGA will also benefit from newly designed cryogenic mechanical suspensions and the upgraded data acquisition and data analysis.

First room temperature operation of the AURIGA optical readout

In the frame of the AURIGA collaboration, a readout scheme based on an optical resonant cavity has been implemented on a room temperature resonant bar detector of gravitational waves. The bar equipped with the optical readout has been operating for a few weeks and we report here the first results.

IGEC2: A 17-month search for gravitational wave bursts in 2005-2007

We present here the results of a 515 day search for short bursts of gravitational waves by the IGEC2 observatory. This network included 4 cryogenic resonant-bar detectors: AURIGA, EXPLORER, and NAUTILUS in Europe, and ALLEGRO in America. These results cover the time period from November 6th 2005 until April 15th 2007, partly overlapping the first long term observations by the LIGO interferometric detectors. The observatory operated with high duty cycle, namely, 57% for fourfold coincident observations, and 94% for threefold observations. The sensitivity was the best ever obtained by a bar network: we could detect, with an efficiency >50%, impulsive events with a burst strain amplitude h{sub rss} < or approx. 1x10{sup -19} Hz{sup -1/2}. The network data analysis was based on time coincidence searches over at least three detectors, used a blind search technique, and was tuned to achieve a false alarm rate of 1/century. When the blinding was removed, no gravitational wave candidate was found.

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.

INITIAL OPERATION OF THE INTERNATIONAL GRAVITATIONAL EVENT COLLABORATION

The International Gravitational Event Collaboration, IGEC, is a coordinated effort by research groups operating gravitational wave detectors working towards the detection of millisecond bursts of gravitational waves. Here we report on the current IGEC resonant bar observatory, its data analysis procedures, the main properties of the first exchanged data set. Even though the available data set is not complete, in the years 1997 and 1998 up to four detectors were operating simultaneously. Preliminary results are mentioned.

Search for gravitational wave bursts by the network of resonant detectors

The groups operating cryogenic bar detectors of gravitational waves are performing a coordinated search for short signals within the International Gravitational Event Collaboration (IGEC). We review the most relevant aspects of the data analysis, based on a time-coincidence search among triggers from different detectors, and the properties of the data exchanged by each detector under a recently-upgraded agreement. The IGEC is currently analysing the observations from 1997 to 2000, when up to four detectors were operating simultaneously. 10% and 50% of this time period were covered by simultaneous observations, respectively, of at least three or at least two detectors. Typical signal search thresholds were in the range 2–6 10−21/Hz. The coincidences found are within the estimated background, hence improved upper limits on incoming GW (gravitational wave) bursts have been set.

Correlation between gamma-ray bursts and gravitational waves

Thirty years after their discovery, gamma-ray bursts ~GRB’s ! are still the most mysterious objects in the Universe, as their properties have not yet been associated with any well-known object, while, for instance, pulsars have been almost immediately identified as final objects of the evolution of stars, and quasars have been well framed among galaxy nuclei in particular in the Seyfert class. Unfortunately ‘‘standard’’ astrophysical objects do not exist for GRB’s. The ‘‘nonstandard’’ compact astrophysical objects which might reproduce the experimental characteristics of GRB’s involve black holes ~BH!, neutron stars ~NS!, and massive stars. The wide range of characteristics prevents the systematic classification of GRB’s. Their energy spectrum is continuous and nonthermal, and covers the range between 1 keV and 1 MeV; their emission lasts from 10 ms to 10 3 s. The burst and transient source experiment ~BATSE! has detected GRB’s with an event rate of about one per day between the years 1991 and 2000. One of the most important results of observations is that many GRB’s are at cosmological distances @1#. This fact, recently confirmed also by the satellite BeppoSAX @2#, which has detected some optical counterpart with redshifts close to z51 @3‐6#, implies the emission of a huge amount of electromagnetic radiation: in a few seconds the GRB’s

χ2testing of optimal filters for gravitational wave signals: An experimental implementation

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 non-stationary noise and its application to data from a network of detectors are also discussed.

IGEC toolbox for coincidence search

The standard IGEC approach to detection of gravitational waves with many detectors is a simple time coincidence search. We discuss the problems of false alarm and false dismissal assessment, in the case of both stationary and non-stationary noise. The significance of any cumulative excess of found coincidences over the background is determined by maximum likelihood methods.