Long term operation of a niobium resonant bar gravitational wave antenna

Two years of continuous operation of a resonant bar gravitational wave antenna with noise temperatures of about 3 mK and strain sensitivity of h similar to 10(-18) are reported. An average of about 40 candidate events are observed each day, most of which are of non-thermal origin. Coincidence analysis with local environmental monitors shows that only a few percent of the candidate events can be attributed to seismic noise or electromagnetic interference. The remaining events are of unknown but probably mundane origin such as non-stationary noise in the cryogenic microwave readout system or strain relief in the antenna or vibration isolation system, Assuming the same event rate of 40 events per day for all three world-wide antennas, one accidental coincidence in a correlation experiment is expected every 320 years. At its present sensitivity, the antenna is capable of detecting the conversion of a mass of order 10(-2)M(circle dot) to gravitational waves at the galactic centre.