G. Modestino

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.

Search for correlation between GRB's detected by BeppoSAX and gravitational wave detectors EXPLORER and NAUTILUS

Data obtained during five months of 2001 with the gravitational wave ~GW! detectors EXPLORER and NAUTILUS, operating with a bandwidth of a few Hz at frequencies near 900 Hz, were studied in correlation with the gamma ray burst data ~GRB! obtained with the BeppoSAX satellite. During this period BeppoSAX was the only GRB satellite in operation, while EXPLORER and NAUTILUS were the only GW detectors in operation. No correlation between the GW data and the GRB bursts was found. The analysis, performed over 47 GRB’s, excludes the presence of signals of amplitude h>6.5310 219 , with 95% probability, if we allow a time delay between GW bursts and GRB within 65 s, and h>1.2310 218 , if the time delay is within 6400 s. The result is also provided in the form of scaled likelihood for unbiased interpretation and easier use for further analysis.

A cosmic-ray veto system for the gravitational wave detector NAUTILUS

The ultracryogenic resonant gravitational wave antenna NAUTILUS now operating at Frascati INFN National Laboratory has been provided with a cosmic-ray veto system consisting of layers of streamer tubes. The experimental setup and performance of the system are shown. Preliminary results on the data collected during the calibration operations of the antenna are also presented together with the expected number of events from the interactions of high-energy hadrons and muons and multihadron showers with the gravitational antenna.

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.

All-sky search of NAUTILUS data

A search for periodic gravitational-wave signals from isolated neutron stars in the NAUTILUS detector data is presented. We have analyzed half a year of data over the frequency band � 922.2; 923.2� Hz, the spindown range �− 1.463 × 10 −8 ; 0� Hz/s and over the entire sky. We have divided the data into two day stretches and we have analyzed each stretch coherently using matched filtering. We have imposed a low threshold for the optimal detection statistic to obtain a set of candidates that are further examined for coincidences among various data stretches. For some candidates we have also investigated the change of the signal-to-noise ratio when we increase the observation time from 2 to 4 days. Our analysis has not revealed any gravitational-wave signals. Therefore we have imposed upper limits on the dimensionless gravitationalwave amplitude over the parameter space that we have searched. Depending on frequency, our upper limit ranges from 3.4 × 10 −23 to 1.3 × 10 −22 .W e haveA search for periodic gravitational-wave signals from isolated neutron stars in the NAUTILUS detector data is presented. We have analyzed half a year of data over the frequency band � 922.2; 923.2� Hz, the spindown range �− 1.463 × 10 −8 ; 0� Hz/s and over the entire sky. We have divided the data into two day stretches and we have analyzed each stretch coherently using matched filtering. We have imposed a low threshold for the optimal detection statistic to obtain a set of candidates that are further examined for coincidences among various data stretches. For some candidates we have also investigated the change of the signal-to-noise ratio when we increase the observation time from 2 to 4 days. Our analysis has not revealed any gravitational-wave signals. Therefore we have imposed upper limits on the dimensionless gravitationalwave amplitude over the parameter space that we have searched. Depending on frequency, our upper limit ranges from 3.4 × 10 −23 to 1.3 × 10 −22 .W e have

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.

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.