Results of the IGEC-2 search for gravitational wave bursts during 2005

The network of resonant bar detectors of gravitational waves resumed coordinated observations within the International Gravitational Event Collaboration (IGEC-2). Four detectors are taking part in this Collaboration: ALLEGRO, AURIGA, EXPLORER and NAUTILUS. We present here the results of the search for gravitational wave bursts over 6 months during 2005, when IGEC-2 was the only gravitational wave observatory in operation. The implemented network data analysis is based on a time coincidence search among AURIGA, EXPLORER and NAUTILUS; ALLEGRO data was reserved for follow-up studies. The network amplitude sensitivity to bursts improved by a factor $\approx 3$ over the 1997-2000 IGEC observations; the wider sensitive band also allowed the analysis to be tuned over a larger class of waveforms. Given the higher single-detector duty factors, the analysis was based on threefold coincidence, to ensure the identification of any single candidate of gravitational waves with high statistical confidence. The false detection rate was as low as 1 per century. No candidates were found.

Figure 1

Typical strain noise spectral density (single-sided) curves of IGEC-2 detectors in 2005. From light gray to black: ALLEGRO, EXPLORER, NAUTILUS, and AURIGA. All detectors share comparable minimum levels of noise spectra. The wider bandwidth of AURIGA covers the bandwidths of the other detectors.

Figure 2

Noise vs time of AURIGA (black), EXPLORER (light gray), and NAUTILUS (dark gray) detectors. The ordinate is the $1\sigma$ noise in terms of equivalent Fourier component $H$ of the strain waveform $h(t)$ of a millisecond GW pulse.

Figure 3

Amplitude distribution of the exchanged candidate events above the minimal thresholds: AURIGA (darker gray) $\mathrm{SNR}>4.5$, EXPLORER (lighter gray) $\mathrm{SNR}>4.0$, and NAUTILUS (gray) $\mathrm{SNR}>4.0$. The amplitude is the Fourier component $H$ of the $h(t)$ waveform of a millisecond GW pulse.

Figure 4

Histogram of the number of accidental coincidences in each resampling (black continuous line), using exchanged events with $\mathrm{SNR}>4.0$ for EXPLORER and NAUTILUS and $\mathrm{SNR}>4.5$ for AURIGA. 19’355’600 independent off-source resamplings of the experiment have been computed by shifting the time of two detectors within $\pm 11000\mathrm{s}$ in 5 s steps, excluding the central region around the nominal zero lag (see Sec. 3). The histogram is well in agreement with the Poisson distribution with mean equal to 2.16 (gray dotted line and shaded area), as ${\chi }^2=11.3$ with 12 degrees of freedom corresponding to a $p$ value of 50.3%.

Figure 5

Histogram of the number of accidentals in each off-source resampling from the composite search, union of three data selection procedures (see Sec. 4b). The histogram (black continuous line) agrees with a Poisson distribution with mean 0.003 64 (gray dotted line and shaded area, ${\chi }^2=0.06$ with 1 degree of freedom), the reference distribution for the coincidences assuming that only accidental coincidences are present. The false detection probability is 0.003 63, corresponding to 1.01 false alarms per century.

Figure 6

Comparison between the present upper limit and the one set in the 1997–2000 campaign. These uninterpreted upper limits were computed with the same methodology. The new result, however, uses only a subset (i.e. the C data selection, see Sec. 3) of the composite search performed on 2005 data.

Figure 7

P.d.f. of the variable $x=t_2-t_3$. The area of the part of the trapezium inside $\pm w_{23}$ (gray area) gives the probability of $|x|\le w_{23}$.