IGEC2 COLLABORATION: A NETWORK OF RESONANT BAR DETECTORS SEARCHING FOR GRAVITATIONAL WAVES

1. IGEC2 COLLABORATION: A NETWORK OF RESONANT BAR DETECTORS SEARCHING FOR GRAVITATIONAL WAVES Massimo Visco on behalf of the IGEC2 Collaboration

2. OUTLINE OF THE TALK • IGEC2 collaboration detectors • Role of the IGEC2 observatory • Data analysis methods • Analysis parameters optimization • Results of first data exchange of IGEC2 • Preliminary analysis of second data exchange of IGEC2 • Summary

3. IGEC2 DETECTORS

4. IGEC2 International Gravitational Events Collaboration ALLEGRO - AURIGA - ROG (EXPLORER-NAUTILUS)

5. A 4-ANTENNAE OBSERVATORY • The four antennae see an identical signal, independently from the source and time

6. SENSITIVITY OF IGEC DETECTORS • The best sensitivity is reached around 900 Hz

7. RESONANT DETECTORS • EXPLORER, is “oldest” among the resonant detectors, started operations about 17 years ago. • Since several years, this kind of detectors has reached a high level of reliability. • The present duty factor is close to 90% . • In the past the data of the different experiments, alone or in coincidence, have been used in searches for different kind of sources: continuous, stochastic background and burst signals. Many upper limits were calculated.

8. PRESENT ROLE OF RESONANT DETECTORS • Nowadays interferometric detectors have reached a sensitivity at least one order of magnitude better than bar detectors. • Interferometric detectors have scheduled up-grades in the near future and an important increase in sensitivity is expected • The bar detectors guarantee unattended and low cost operations. on the other hand The present role of resonant detectors is a continuous observation of the sky with specific attention to the periods not covered by interferometers.

9. IGEC – search for burst signals • IGEC1 1997-2000 – First experience using the data of 5 bar detectors: ALLEGRO, AURIGA, EXPLORER, NAUTILUS and NIOBE. In four years 29 days of four-fold coincidences- 178 days of three-fold coincidences - 713 days of two-fold coincidences • After the last upgrades the resonant detectors have resumed the operations at different time: EXPLORER in 2000, AURIGA in 2003, NAUTILUS in 2003, ALLEGRO in 2004. NIOBE no more in operation. • IGEC2 2005-….. -. • First analysis on May-November 2005 when no other observatory was operating. No detection • Second analysis on the period November 2005 – December 2006 – the data exchange is next to be completed.

10. DATA ANALYSIS METHODS

11. DATA ANALYSIS METHODS • The analysis is based on time coincidence among candidate events selected in each detector. • Each group produces a list of candidate events using filters matched for  signals • The data are exchanged after adding a “secret time shift” (within ± 10s) to the time of the candidate events. • A statistical distribution of the accidental time coincidences number is calculated using lists of candidate events obtained from the original ones adding many different time shifts. • The analysis parameters (search threshold, coincidence window) are fixed “a priori” using the accidental coincidences analysis. • Finally the groups exchange the secret times and the search for real coincidences is performed.

12. TUNING OF ANALYSIS PARAMETER -1 • False dismissal must be low enough not to miss events • False alarm must be kept under control in order to obtain results having statistical significance signal (shape and energy) detector (noise, SNR thresh, coincidence time window) The R factor must be maximized. It depends on shape and energy of the different signals

13. TUNING OF ANALYSIS PARAMETER -2 • The parameters to be tuned are: • SNRthreshold used to select the events • time coincidence windows • on the base of: • detectors noise • expected signals – energy, rate and shape • to obtain: • a false alarm low enough to classify candidates as good ones • a reasonable detection efficiency for the searched signals The efficiency of the observatory can be calculated for different classes of signals by software injection . The expected rateof incoming signals is obviously an unknown parameter but one can reasonably imagine that it rapidly decreases with energy.

14. IGEC2 FIRST ANALYZED PERIOD

15. IGEC 2 1st period: May 20 –Nov 15, 2005 • AURIGA-EXPLORER-NAUTILUS data exchanged • ALLEGRO data available only for follow-up investigation • IGEC2 was the only GW observatory in operation • The analysis was limited to triple coincidences. • The analysis was tuned to a possible detection requiring an identification of possible candidates with high confidence fixing the false alarm to 1 coincidence over 100 years • No special analysis was done on the response to different signals, so we adopted quite conservative choices in the analysis: • A time window of about 60 ms, was chosen to ensure an efficiency of about 85 % for  signals • The false alarm was brought at the required level rising the SNR thresholds.

16. SNR Threshold adopted • We decided to perform one composite search made by the OR among 3 different data selections: • A - SNR > 4.95 for AU, EX e NA: 0.396 false alarm/century • target signals centered in the EX-NA peaks: • B -SNR > 7.00 for AU, SNR>4.25 for EX , NA:0.572 false alarm/century • target -like signals: • C - common thresholds IGEC1-style: 0.134 false alarm/century • thresholds a 1.3, 1.4, 1.5, ..., 3.0 x 10-21/Hz • targets -like signals: 3.6310-3 over 130 days 1.0 false alarm/century If one candidate had been found we could reject the hypothesis of only accidental coincidences with 99.637%  0.006% (3) of confidence.

17. ANALYSIS RESULTS • No candidate event was found and the null hypothesis was not rejected • One data subset was used to calculate an upper limit for comparison with the previous one of IGEC 1 IGEC 95% COVERAGE IGEC2

18. IGEC2 SECOND ANALYZED PERIOD

19. IGEC 2 2nd period: Nov 16, 2005 – Dec 31, 2006 • The analysis will be based on a composite search of four and three folds coincidences fully including ALLEGRO data. • During the same period other observatories (es. LIGO) were taking data. • The data preparation is almost over (data of three detectors available, data from ALLEGRO are going to be distributed in the final version) • Software injections on one day of data to measure efficiency of the observatory .

20. Coming soon 95 % 79 % 81 % OPERATION TIME – NOV 16 2005–DEC 31, 2006(Partial: AURIGA- EXPLORER- NAUTILUS ) 411 days • no detector 0.9 days • Single 19.9 days • Double 140.8 days • Triple249.4days When ALLEGRO data will be available, we expect that triple coincidences will cover the whole considered period

21. AMPLITUDE OF THE EXCHANGED DATA in terms of Fourier amplitude H • SNR > 4.5 for AURIGA • SNR > 4.0 for EXPLORER and NAUTILUS

22. EVENTS AMPLITUDE DISTRIBUTIONS • SNR > 4.5 for AURIGA • SNR > 4.0 for EXPLORER and NAUTILUS

23. Few events/day with SNR>7 Few very large events (SNR>30) on the whole period DATA QUALITY: DISTRIBUTIONS OF EVENTS

24. THREE-FOLD COINCIDENCES BACKGROUND Preliminary: AURIGA - EXPLORER - NAUTILUS 751501 time shifts 4.8 accidental triples on 249 days ALL THE EXCHANGED DATA SNR>4.5 AU SNR>4.0 EX-NAFixed time window= 60 ms ; +/- 500 Time Shift (1 sec) EX-AU & NA-AU

25. THREE-FOLD COINCIDENCES BACKGROUND Preliminary: AURIGA - EXPLORER - NAUTILUS 12006001 time shifts 3.6 10-3 accidental triples on 249 days 0.5 false alarm/century SELECTING DATA SNR>4.95 AU-EX-NAFixed time window= 60 ms ; +/- 2000 Time Shift (1 sec) EX-AU & NA-AU

26. ANALYSIS OF SOFTWARE INJECTED SIGNALSPreliminary • The analysis was made using the data of 15 september 2005 • We injected on experimental data damped sinusoidal signals with elliptical random polarization and Sine-Gaussian linearly polarized of different amplitudes . • We used different shape parameters (frequency of the sinusoid and damping time). • We considered different possible sources distributions: Isotropic, Milk way and Galactic center.

27. DAMPED SINUSOID INJECTED • AURIGA • EXPLORER • NAUTILUS • DS 914Hz-1ms • DS 882Hz-3ms • DS 898Hz-10ms • DS 938Hz-30ms

28. SINE-GAUSSIAN INJECTED • AURIGA • EXPLORER • NAUTILUS • SG 935Hz – Q=3 • SG 935Hz – Q=8.9 • SG 935Hz – Q=100

29. EFFICIENCY VS COINCIDENCE WINDOW • The window chosen in the analysis of the first period analyzed in IGEG2 is quite conservative Damped sinusoids from Galactic Center Hrss • 5 10-20 • 1 10-19 • 2 10-19 • 5 10-19 • 1 10-18

30. EFFICIENCY VS AMPLITUDEcoincidence windows=60 ms Damped Sinusoid Galactic Center Sin Gaussian Isotropic Damped Sinusoid Galactic Disk • DS f:914Hz tau:0.001s • DS f=930Hz tau:0.030s Efficiency Efficiency • SG f:930Hz Q:3 • SG f:930Hz Q:8.9 • SG f:930Hz Q:100 Efficiency • An efficiency of about 50% is reached with different signals at amplitudes hrss of 5·10-20 - 5·10-19

31. PRELIMINARY RESULTS FROM TUNING ANALYSIS • The choices made in the IGEC2 2005 analysis were reasonable, but having learned more on the response of the detectors, we can better optimize the search parameters. • Two approaches for the time window choice are possible: fixed or varying. By tuning the time window with the amplitude of the events forming the coincidences it is possible to maintain almost constant efficiency for signals of different energy. • The SNR thresholds is mainly driven by the desired false alarm level, but we are studying how to preserve the detection efficiency for small energy signals. • We are going to extend the analysis to four folds case

32. SUMMARY • The IGEC observatory is presently capable of high duty cycle and low false alarm. • In the first data exchange of IGEC2 the analysis was tuned on triple coincidences, now we are going to approach the four folds case. • The observatory is ready to fully comply with the role of sentinel of the near sky.