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IGEC

IGEC. IGEC1 1997-2000 – First experience using the data of 5 bar detectors: ALLEGRO, AURIGA, EXPLORER NAUTILUS and NIOBE. 1460 days over 4 years 1322 days single antenna 713 days two-fold coincidences 178 days three-fold coincidences 29 days four-fold coincidences

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IGEC

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  1. IGEC • IGEC1 1997-2000 – First experience using the data of 5 bar detectors: ALLEGRO, AURIGA, EXPLORER NAUTILUS and NIOBE. • 1460 days over 4 years • 1322 days single antenna • 713 days two-fold coincidences • 178 days three-fold coincidences • 29 days four-fold coincidences • IGEC2 from May 2004  …NIOBE no more in operation. First exchanged period May-November 2005 when no other observatory was operating. A larger amount of data is available for further analysis.

  2. A DIRECTIONAL 4-ANTENNAE OBSERVATORY • The four antennas receive an identical signal, independently from the source

  3. SENSITIVITY OF PRESENT DETECTORS

  4. FROM IGEC1 TO IGEC2 IGEC 1 1997-2000 IGEC 2 2004 - ….

  5. DATA ANALYSIS METHODS • The analysis uses lists of candidate events selected (SNR>4-4.5) using filters matched for  signals. Auxiliary information connected to the data quality are also exchanged among the groups (the exchange protocol is analogous to the one used in IGEC1 analysis) • The search parameters (search threshold, coincidence window and working periods) are fixed “a priori” by a blind analysis of the background characteristics • The preliminary analysis is blind as each group add a “secret time shift” to the time of the candidate events. • An upper limit at the present sensitivity of the bar detectors is not an interesting scientific result therefore the analysis was tuned to a possible detection requiring an identification of possible candidates with high confidence fixing the three-fold false alarm to 1 over 100 years • To cross check the filter pipeline we also exchanged raw data.

  6. FIRST ANALIZED PERIOD • The first exchanged data set starts on Friday May20 and ends Tuesday Nov 15, 2005. • The collaborations have exchanged the data of AURIGA EXPLORER and NAUTILUS in the period April-July 2006. ALLEGRO had difficulties in producing data: we agreed to proceed anyway and use ALLEGRO data for a follow-up investigation of any candidate found. • The analysis results were public at the end of September 2006

  7. 96 % 87 % 86 % OPERATION TIME – MAY 20 –NOV 15, 2005(AURIGA- EXPLORER- NAUTILUS) 180 days HIGH DUTY FACTOR Data from ALLEGRO to be added • no detector 0.6 days • Single 3.6 days • Double 45.0 days • Triple130.8days days of operation

  8. AMPLITUDE OF THE EXCHANGED DATA MAY – DEC 2005 • SNR  4.5 per AURIGA • SNR  4.0 per EXPLORER E NAUTILUS

  9. AMPLITUDE OF THE EXCHANGED DATA • SNR  4.5 per AURIGA • SNR  4.0 per EXPLORER E NAUTILUS

  10. SEARCH THRESHOLD Percentage of time with the adaptive threshold T smaller than H • The sensitivity is stationary • There is an enhancement of about a factor 3 respect to IGEC 1

  11. THREE-FOLD PERIOD VS AMPLITUDE THRESHOLD Amplitude Ht[10-21 Hz-1]

  12. TIME UNCERTAINTY AURIGA • For EXPLORER and NAUTILUS the  t was fixed at 10 ms • Coincidence window IGEC 1 style • |t1 –t2| < 4.47sqrt [2(t1) + 2(t2)]using Byenaimé-Tchebychev false dismissal < 5%

  13. TWO_FOLD COINCIDENCE DISTRIBUTIONexample AU -EX • The distribution of the coincidence is poissonian

  14. THREE-FOLD COINCIDENCES BACKGROUNDALL THE EXCHANGED DATA SNR4.5 AU SNR4.0 EX-NA • Time shift AURIGA=0

  15. THREE-FOLD COINCIDENCES BACKGROUNDALL THE EXCHANGED DATA SNR4.5 AU SNR4.0 EX-NA Entries 1879417 Mean 2.415 2/ndf 12.6 /12

  16. CHOICES IN THE ANALYSIS • We decided to use in the analysis only three-fold coincidences • All the exchange periods was considered for analysis • We decided to perform one composite search made by the OR among 3 different data selections: • SNR > 4.95 for AU, EX e NA • 0.396 false alarm/century • target signals centered in the EX-NA peaks • SNR > 7.00 for AU, SNR>4.25 per EX e NA • 0.572 false alarm/century • target signals barely detectable by EX and NA • common thresholds IGEC1-style: • thresholds a 1.3, 1.4, 1.5, ..., 3.0 x 10-21/Hz • 0.134 false alarm/century • targets -like signals

  17. DATA SELECTION 1: Common SNR threshold DATA SELECTION 2: Different SNR threshold

  18. CUMULATIVE FALSE ALARM DISTRIBUTION of the 3 data selections 3.63 10-3 over 130 days 1.0 false alarm/century 1  0.02 10-3 Systematic errors  0.1 10-3 • The accidental coincidences are the union of the accidental coincidences for each data selection counting only once the repeated accidental coincidences.

  19. TEST THE NULL HYPOTHESIS • The null hypothesis will be rejected if at least one coincidence is found in zero lag analysis. • If null hypothesis is not confirmed: • the coincidence is not explained by accidental coincidences with 99.637%  0.006% (3); • the coincidence can be due by correlated signal/noise in the two detectors • a follow-up a posteriori analysis is necessary to characterize the coincidences

  20. FOLLOW-UP PLANNED ANALYSIS • Additional checks for mistakes in the network analysis; • it will not affect the confidence of the rejection of the null; • the follow-up results will be interpreted in terms of likelihood or “degree of belief” (subjective confidence) by the collaboration; • investigation on h(t) data will try to discriminate among known possible sources (gravitational waves, electromagnetic or seismic disturbances, …). The h(t) data will be filtered to implement network searches based on cross-correlation and wavelet transform; • Data from ALLEGRO will be added, in particular h(t) and list of candidates. The improvement in false alarm rate will be estimated, but it is difficult to take into account the efficiency of detection; • IGEC2 will investigate on simultaneous observations by other kind of detectors (neutrinos, gamma, x …).

  21. ANALYSIS RESULTS • On September 25, once defined all the analysis parameters, the secret time shifts were exchanged • No candidate event was found and the null hypothesis was not rejected • One of the data subset can be used to calculate an upper limit to be compared with the previous one of IGEC 1 Burst Rate [year-1] 95% coverage Burst amplitude [Hz-1]

  22. FINAL REMARKS • An observatory made of 3 resonant detectors is presently capable of high duty cycle and low false alarm. • Single candidates with very high level of confidences can be identify at low SNR level. • The blind analysis used made the statistical interpretation non-controversial • Periods of data, longer then the one analyzed, are available and IGEC2 was requested to collaborate with LIGO in S5 run

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