G. Cavallari

Study of the coincidences between the gravitational wave detectors EXPLORER and NAUTILUS in 2001

We report the result from a search for bursts of gravitational waves using data collected by the cryogenic resonant detectors EXPLORER and NAUTILUS during 2001 for a total measuring time of 90 days. With these data we repeated the coincidence search performed on the 1998 data (which showed a small coincidence excess) applying data analysis algorithms based on known physical characteristics of the detectors. With the 2001 data, a new interesting coincidence excess is found when the detectors are favourably oriented with respect to the galactic disc.

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.

First joint gravitational wave search by the AURIGA-EXPLORER-NAUTILUS-Virgo Collaboration

We present a methodology of network data analysis applied to the search for coincident burst excitations over a 24 h long data set collected by AURIGA, EXPLORER, NAUTILUS and Virgo detectors during September 2005. The search of candidate triggers was performed independently on each of the data sets from single detectors. We looked for two-fold time coincidences between these candidates using an algorithm optimized for a given population of sources and we calculated the efficiency of detection through injections of templated signal waveforms into the streams of data. To this end we have considered the case of signals shaped as damped sinusoids coming from the galactic center direction. Our method targets an optimal balance between high efficiency and low false alarm rate, aiming at setting confidence intervals as stringent as possible in terms of the rate of the selected source models.

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.

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.

Data analysis for a gravitational wave antenna with resonant capacitive transducer

SummaryWe present here a new approach which simplifies considerably the data analysis for gravitational-wave antennas equipped with resonant transducer, based on the representation of the antenna as two independent oscillators. In fact, we can apply to each of the two modes of these antennas the same data analysis procedures already in use for antennas with nonresonant transducer and then compute the coincidences between the outputs of the two modes. The results deduced by such a procedure are in good agreement with the experimental results from the data collected in March 1984 with our 2270 kg 5056 Al bar cooled at liquid-helium temperature (T=4.2 K). The performances of the algorithms are presented in terms of the effective noise temperatures and sensitivity to short bursts of gravitational waves. With our experimental values, obtained by using a FET amplifier, we get an instrumental sensitivityF(v) of 6 J/m2 Hz. We have also evaluated the sensitivity of the detector for monochromatic gravitational waves with frequencies in bandwidths of ≏0.4 Hz around the frequenciesv− andv+, for one month of observation:h0≏3·10−23.RiassuntoSi presenta un nuovo approccio che semplifica notevolmente l’analisi dei dati per antenne gravitazionali con trasduttore risonante, basato sulla rappresentazione dell’antenna come due oscillatori indipendenti. A ciascuno dei modi di tali antenne si applicano le procedure di analisi dei dati già usati per le antenne con trasduttore risonante e si calcolano quindi le coincidenze tra le uscite dei due modi. I risultati dedotti mediante tale procedura sono in buon accordo con i risultati sperimentali relativi ai dati raccolti nel marzo 1984 con la nostra sbarra di Al 5056 da 2270 kg raffreddata alla temperatura dell’elio liquido (T=4.2 K). Si presentano le prestazioni degli algoritmi in termini delle temperature efficaci di rumore e della sensibilità per brevi fiotti di onde gravitazionali. Con i nostri valori sperimentali, ottenuti usando un amplificatore a FET, si ha una sensibilità strumentaleF(v) di 6 J/m2 Hz. Si è anche calcolata la sensibilità del rivelatore per onde gravitazionali monocromatiche in bande di frequenza di circa 0.4 Hz attorno alle frequenzev− ev+, per un mese di ossevazione:h0≏3.·10−23.РезюмеПредлагается новый подход, котрый значительно упрощает анализ данных для антенн гравитационных волн с резонансным емкостным датчиком, основанный на представлении антенны в виде двух независимых осцилляторов. Мы можем применить к каждой из двух мод этих антенн процедуры анализа данных, которые ыже использовались для антенн с нерезонансными датчиками, а затем вычислить совпадения между результатами на выходе для двух мод. Результаты, полученные с помощью такой процедуры, хорошо согласуются с экспериментальными результатами, собранными в марте 1984 года с помощью 2270 кг 5056 Al антенны, охлажденной до температуры жидкого гелия (T=4.2 К). Описываются алгоритмы анализа, исходя из эффективных температур шума и чувствительности к коротким вспышкам гравитационных волн. Используя экспериментальные значения, полученные с помощью FET усилителя, мы определяем инструментальную чувствительностьF(v)≈6Дж/M2 Гц. Мы также оцениваем чувствительность детектора для монохроматических гравитационных волн в полосе частот 0.4 Гц вблизи частот,v− иv+ для одного месяца наблюдения:h0≏3·10−23.

Vibrational excitation induced by electron beam and cosmic rays in normal and superconductive aluminum bars

We report new measurements of the acoustic excitation of an Al5056 superconductive bar when hit by an electron beam, in a previously unexplored temperature range, down to 0.35 K. These data, analyzed together with previous results of a dedicated experiment obtained for T 4 0:54 K, show a vibrational response enhanced by a factor � 4:9 with respect to that measured in the normal state. This enhancement explains the anomalous large signals due to cosmic rays previously detected in the NAUTILUS gravitational wave detector.

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.

Comments on the 2001 run of the EXPLORER/NAUTILUS gravitational wave experiment

The recently published analysis of the coincidences between the EXPLORER and NAUTILUS gravitational wave detectors in the year 2001 (Astone P et al 2002 Class. Quantum Grav. 19 5449) has drawn some criticism, reported at this workshop. We do not hold with these objections, even if we agree that no claim can be made with our data. The paper we published reports data of unprecedented quality and sets a new procedure for the coincidence search, which can be repeated by us and by other groups in order to search for the signature of possible signals. As for the reported coincidence excess, we remark that it is not destined to remain an intriguing observation for long: it will be confirmed or denied soon by interferometers and bars operating at their expected sensitivity.