A. Moleti

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.

Upper limit for a gravitational-wave stochastic background with the EXPLORER and NAUTILUS resonant detectors

We discuss the sensitivity of resonant-mass gravitational-wave detectors to a cosmic stochastic background of gravitational waves. We report the experimental upper limits given by the gravitational wave detectors EXPLORER and NAUTILUS.

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.

Energetic cosmic rays observed by the resonant gravitational wave detector NAUTILUS

The passage of cosmic rays has been observed to excite mechanical vibrations in the resonant gravitational wave detector NAUTILUS operating at temperature of 100 mK. A very significant correlation (more than 10 standard deviations) is found.

Search for Periodic Gravitational Wave Sources with the EXPLORER detector

We have developed a procedure for the search of signals from periodic sources in the data of gravitational wave detectors. We report here the analysis of one year of data from the resonant detector Explorer, searching for sources located in the Galactic Center ~GC!. No signals with amplitude greater than h 52.9310 224 ,i n the range 921.32‐921.38 Hz, were observed using data collected over a time period of 95.7 days, for a source located at a517.7060.01 h and d5229.0060.05 deg. Our procedure can be extended for any assumed position in the sky and for a more general all-sky search, with the proper frequency correction to account for the spin-down and Doppler effects.

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.

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

Study of coincidences between resonant gravitational wave detectors

Coincidences are searched for with the cryogenic resonant gravitational wave detectors EXPLORER and NAUTILUS, during a period of about six months (2 June-14 December 1998) for a total measuring time of 94.5 d, with the purpose of studying new analysis algorithms, based on the physical characteristics of the detectors.

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.

Cooling down MiniGRAIL to milli-Kelvin temperatures

The latest developments in the construction of the ultra-cryogenic spherical detector MiniGRAIL are presented. The room temperature part of the vibration isolation system was improved and provided with an attenuation of about 60 dB around 3 kHz. The transfer function of the cryogenic stages gave about 20 dB per stage, at the resonant frequency of the sphere. The latest results of three cryogenic tests at ultra-low temperature of the spherical detector MiniGRAIL, using several thermal anchorings, are presented. Minimum temperatures of 20 mK on the mixing chamber of the dilution refrigerator and 79 mK on the surface of the sphere were reached. During the last cool down, two capacitive transducers were mounted on the sphere. The first was coupled to a room temperature FET amplifier and the second to a transformer and a double stage SQUID amplifier. Unfortunately the SQUID did not work, so only the first resonator could be used. An equivalent temperature of about 20 K was measured during an acquisition run of 7 h, using the first transducer corresponding to the FET white noise.