TAUP 2007, Sendai, September 12, 2007 IGEC2 COLLABORATION: A NETWORK OF RESONANT BAR DETECTORS SEARCHING FOR GRAVITATIONAL WAVES Massimo Visco on behalf of the IGEC2 Collaboration

TAUP 2007, Sendai, September 12, 2007 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 OUTLINE OF THE TALK

TAUP 2007, Sendai, September 12, 2007 IGEC2 DETECTORS

TAUP 2007, Sendai, September 12, 2007 IGEC2 International Gravitational Events Collaboration ALLEGRO - AURIGA - ROG (EXPLORER-NAUTILUS)

TAUP 2007, Sendai, September 12, 2007 A 4-ANTENNAE OBSERVATORY The four antennae see an identical signal, independently from the source and time

TAUP 2007, Sendai, September 12, 2007 SENSITIVITY OF IGEC DETECTORS The best sensitivity is reached around 900 Hz

TAUP 2007, Sendai, September 12, 2007 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. RESONANT DETECTORS

TAUP 2007, Sendai, September 12, 2007 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. PRESENT ROLE OF RESONANT DETECTORS 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.

TAUP 2007, Sendai, September 12, 2007 IGEC – 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 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 NIOBE no more in operation. IGEC ….. -. ­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. IGEC – search for burst signals

TAUP 2007, Sendai, September 12, 2007 DATA ANALYSIS METHODS

TAUP 2007, Sendai, September 12, 2007 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. DATA ANALYSIS METHODS The analysis is based on time coincidence among candidate events selected in each detector.

TAUP 2007, Sendai, September 12, 2007 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 TUNING OF ANALYSIS PARAMETER -1 The R factor must be maximized. It depends on shape and energy of the different signals signal (shape and energy) detector (noise, SNR thresh, coincidence time window)

TAUP 2007, Sendai, September 12, 2007 The parameters to be tuned are: SNR threshold 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 TUNING OF ANALYSIS PARAMETER -2 The efficiency of the observatory can be calculated for different classes of signals by software injection. The expected rate of incoming signals is obviously an unknown parameter but one can reasonably imagine that it rapidly decreases with energy.

TAUP 2007, Sendai, September 12, 2007 IGEC2 FIRST ANALYZED PERIOD

TAUP 2007, Sendai, September 12, 2007 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. IGEC 2 1 st period: May 20 –Nov 15, 2005

TAUP 2007, Sendai, September 12, 2007 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: false alarm/century target signals centered in the EX-NA peaks: B -SNR > 7.00 for AU, SNR>4.25 for EX, NA: false alarm/century target  -like signals: C - common thresholds IGEC1-style: false alarm/century thresholds a 1.3, 1.4, 1.5,..., 3.0 x /Hz targets  -like signals: 3.63  over 130 days 1.0 false alarm/century If one candidate had been found we could reject the hypothesis of only accidental coincidences with %  0.006% (  3  ) of confidence.

TAUP 2007, Sendai, September 12, 2007 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 IGEC2 95% COVERAGE

TAUP 2007, Sendai, September 12, 2007 IGEC2 SECOND ANALYZED PERIOD

TAUP 2007, Sendai, September 12, 2007 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. IGEC 2 2 nd period: Nov 16, 2005 – Dec 31, 2006

TAUP 2007, Sendai, September 12, 2007 OPERATION TIME – NOV –DEC 31, 2006 (Partial: AURIGA- EXPLORER- NAUTILUS ) no detector 0.9 days Single 19.9 days Double days Triple days ALAUEXNA ALC.S. AUC.S d EXC.S d326.7 d NAC.S d261.7 d332.2 d 95 %79 %81 % 411 days When ALLEGRO data will be available, we expect that triple coincidences will cover the whole considered period Coming soon

TAUP 2007, Sendai, September 12, 2007 AMPLITUDE OF THE EXCHANGED DATA in terms of Fourier amplitude H SNR > 4.5 for AURIGA SNR > 4.0 for EXPLORER and NAUTILUS

TAUP 2007, Sendai, September 12, 2007 EVENTS AMPLITUDE DISTRIBUTIONS SNR > 4.5 for AURIGA SNR > 4.0 for EXPLORER and NAUTILUS

TAUP 2007, Sendai, September 12, 2007 DATA QUALITY: DISTRIBUTIONS OF EVENTS Few events/day with SNR>7 Few very large events (SNR>30) on the whole period

TAUP 2007, Sendai, September 12, 2007 THREE-FOLD COINCIDENCES BACKGROUND Preliminary: AURIGA - EXPLORER - NAUTILUS time shifts 4.8 accidental triples on 249 days ALL THE EXCHANGED DATA SNR>4.5 AU SNR>4.0 EX-NA Fixed time window= 60 ms ; +/- 500 Time Shift (1 sec) EX-AU & NA-AU

TAUP 2007, Sendai, September 12, 2007 THREE-FOLD COINCIDENCES BACKGROUND Preliminary: AURIGA - EXPLORER - NAUTILUS time shifts accidental triples on 249 days SELECTING DATA SNR>4.95 AU-EX-NA Fixed time window= 60 ms ; +/ Time Shift (1 sec) EX-AU & NA-AU 0.5 false alarm/century

TAUP 2007, Sendai, September 12, 2007 ANALYSIS OF SOFTWARE INJECTED SIGNALS Preliminary 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.

TAUP 2007, Sendai, September 12, 2007 DAMPED SINUSOID INJECTED ─ AURIGA ─ EXPLORER ─ NAUTILUS ─ DS 914Hz-1ms ─ DS 882Hz-3ms ─ DS 898Hz-10ms ─ DS 938Hz-30ms

TAUP 2007, Sendai, September 12, 2007 SINE-GAUSSIAN INJECTED ─ AURIGA ─ EXPLORER ─ NAUTILUS ─ SG 935Hz – Q=3 ─ SG 935Hz – Q=8.9 ─ SG 935Hz – Q=100

TAUP 2007, Sendai, September 12, 2007 EFFICIENCY VS COINCIDENCE WINDOW Hrss ─ ─ ─ ─ ─ The window chosen in the analysis of the first period analyzed in IGEG2 is quite conservative Damped sinusoids from Galactic Center

TAUP 2007, Sendai, September 12, 2007 EFFICIENCY VS AMPLITUDE coincidence windows=60 ms An efficiency of about 50% is reached with different signals at amplitudes hrss of 5· · Efficiency Damped Sinusoid Galactic Center ─ DS f:914Hz tau:0.001s ─ DS f=930Hz tau:0.030s Damped Sinusoid Galactic Disk Efficiency ─ SG f:930Hz Q:3 ─ SG f:930Hz Q:8.9 ─ SG f:930Hz Q:100 Sin Gaussian Isotropic Efficiency

TAUP 2007, Sendai, September 12, 2007 PRELIMINARY RESULTS FROM TUNING ANALYSIS The choices made in the IGEC 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

TAUP 2007, Sendai, September 12, 2007 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.