M. McHugh

Calibration of the ALLEGRO resonant detector

We describe a method for calibrating the ALLEGRO resonant detector. The resulting response function can be used to transform the observed data backwards to gravitational strain data. These data are the input to a cross-correlation analysis to search for stochastic gravitational waves.

Allegro: noise performance and the ongoing search for gravitational waves

The noise performance of Allegro since 1993 is summarized. We show that the noise level of Allegro is, in general, stationary. Non-Gaussian impulse excitations persist despite efforts to isolate the detector from environmental disturbances. Some excitations are caused by seismic activity and flux jumps in the SQUID. Algorithms to identify and automatically veto these events are presented. Also, the contribution of Allegro to collaborations with other resonant-mass detectors via the International Gravitational Event Collaboration and with LIGO is reviewed.

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.

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.

Stochastic background search correlating ALLEGRO with LIGO engineering data

We describe the role of correlation measurements between the LIGO interferometer in Livingston, LA, and the ALLEGRO resonant bar detector in Baton Rouge, LA, in searches for a stochastic background of gravitational waves. Such measurements provide a valuable complement to correlations between interferometers at the two LIGO sites, since they are sensitive in a different, higher, frequency band. Additionally, the variable orientation of the ALLEGRO detector provides a means to distinguish gravitational wave correlations from correlated environmental noise. We describe the analysis underway to set a limit on the strength of a stochastic background at frequencies near 900 Hz using ALLEGRO data and data from LIGOs E7 Engineering Run.

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.

A test of Newton's inverse square law of gravitation using the 300‐m tower at Erie, Colorado

Gravity was measured at eight different heights on a 300-m meteorological tower, using electrostatically nulled LaCoste and Romberg gravimeters. The observed values were corrected for tides, temperature, and gravimeter screw errors, and tested for systematic effects due to (wind-induced) tower motion (no such effects were found). These corrected results were compared with values predicted by means of Newtons inverse square law from surface gravity values. The differences exhibit no systematic trends, and their rms value is only 10 μGal, well within the errors of the experiment, as the estimated measurement errors increase from 9 μ Gal at the lowest platform to 14 μ Gal at the top and those of the predictions from 10 μ Gal to 23 μ Gal. These results set new constraints on the magnitude of any non-Newtonian gravitational force; if such a force is derived from a Yukawa potential, the absolute value of α must be less than 0.001 for λ = 1000 m.

THE ALLEGRO GRAVITATIONAL WAVE DETECTOR

The ALLEGRO gravitational wave detector has been in nearly continuous operation for the past four years with a duty cycle of over 90%. The data is continuously filtered for bursts and has a noise level near 12 mK, or 8×10-22 strain/Hz. An initial data exchange under the IGEC has been completed.

Resonant detectors and interferometers can work together

Resonant gravitational wave detectors are described. Examples are given for signal improvement by combining signals from several independent detectors. The successful test run of Allegro in coincidence with LIGO Livingston during an engineering run (E7) is also described.

A data analysis technique for the LIGO-ALLEGRO stochastic background search

We describe the cross-correlation measurements being carried out on data from the LIGO Livingston Observatory and the ALLEGRO resonant bar detector. The LIGO data are sampled at 16 384 Hz while the ALLEGRO data are base-banded, i.e., heterodyned at 899 Hz and then sampled at 250 Hz. We handle these different sampling parameters by working in the Fourier domain, and demonstrate the approximate equivalence of this measurement to a hypothetical time-domain method in which both data streams are upsampled.