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

TAUP 2007, Sendai, September 12, 2007

IGEC2 COLLABORATION: A IGEC2 COLLABORATION: A NETWORK OF NETWORK OF RESONANT BAR DETECTORSRESONANT BAR DETECTORS SEARCHING SEARCHING

FOR FOR GRAVITATIONAL WAVESGRAVITATIONAL WAVES

Massimo Visco

on behalf of the IGEC2 Collaboration


• 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 OUTLINE OF THE TALK


IGEC2 DETECTORS


IGEC2IGEC2

International Gravitational Events Collaboration

ALLEGRO - AURIGA - ROG (EXPLORER-NAUTILUS)


A 4-ANTENNAE OBSERVATORY A 4-ANTENNAE OBSERVATORY

• The four antennae see an identical signal, independently from the source and time


SENSITIVITY OF IGEC DETECTORSSENSITIVITY OF IGEC DETECTORS

•The best sensitivity is reached around 900 Hz


• 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 DETECTORSRESONANT 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.

PRESENT ROLE PRESENT ROLE OF RESONANT DETECTORSOF 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.


• 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.

IGEC – search for burst signals IGEC – search for burst signals


DATA ANALYSIS METHODS


• 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 DATA ANALYSIS METHODS The analysis is based on time coincidence among candidate events selected in each detector.


• 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 -1TUNING OF ANALYSIS PARAMETER -1

alarmfalse

sig

alarmfalse

sigdetected

NEfficiencyN

N

NR

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)


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 -2TUNING 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.

alarmfalse

sig

NEfficiencyN

R


IGEC2 FIRST ANALYZED PERIOD


• 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 IGEC 2 11stst period: May 20 –Nov 15, 2005 period: May 20 –Nov 15, 2005


SNR Threshold adoptedSNR 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/Hztargets -like signals: 3.63 10-3 over 130

days1.0 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.


ANALYSIS RESULTS 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

IGEC295%

COVERAGE


IGEC2 SECOND ANALYZED

PERIOD


• 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 IGEC 2 22ndnd period: Nov 16, 2005 – Dec 31, 2006 period: Nov 16, 2005 – Dec 31, 2006


OPERATION TIME – OPERATION TIME – NOV 16 2005–DEC 31, 2006NOV 16 2005–DEC 31, 2006((PartialPartial: AURIGA- EXPLORER- NAUTILUS ) : AURIGA- EXPLORER- NAUTILUS )

• no detector 0.9 days

• Single 19.9 days

• Double 140.8 days

• Triple 249.4 days

AL AU EX NAAL C.S.

AU C.S. 390.9 d

EX C.S. 309.8 d 326.7 d

NA C.S. 317.5 d 261.7 d 332.2 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


AMPLITUDE OF THE EXCHANGED DATA AMPLITUDE OF THE EXCHANGED DATA in terms of Fourier amplitude H in terms of Fourier amplitude H

• SNR > 4.5 for AURIGA

• SNR > 4.0 for EXPLORER and NAUTILUS


EVENTS AMPLITUDE DISTRIBUTIONSEVENTS AMPLITUDE DISTRIBUTIONS

• SNR > 4.5 for AURIGA

• SNR > 4.0 for EXPLORER and NAUTILUS


DATA QUALITY: DISTRIBUTIONS OF EVENTSDATA QUALITY: DISTRIBUTIONS OF EVENTS

•Few events/day with SNR>7•Few very large events (SNR>30) on the whole period


THREE-FOLD COINCIDENCES BACKGROUNDTHREE-FOLD COINCIDENCES BACKGROUND Preliminary: AURIGA - EXPLORER - NAUTILUS Preliminary: AURIGA - EXPLORER - NAUTILUS

751501 time shifts

4.8 accidental triples on 249 days

ALL THE EXCHANGED DATA ALL THE EXCHANGED DATA SNR>4.5 AU SNR>4.0 EX-NASNR>4.5 AU SNR>4.0 EX-NA

Fixed time window= 60 ms ; +/- 500 Time Shift (1 sec) EX-AU & NA-AUFixed time window= 60 ms ; +/- 500 Time Shift (1 sec) EX-AU & NA-AU


THREE-FOLD COINCIDENCES BACKGROUNDTHREE-FOLD COINCIDENCES BACKGROUND Preliminary: AURIGA - EXPLORER - NAUTILUS Preliminary: AURIGA - EXPLORER - NAUTILUS

12006001 time shifts

3.6 10-3 accidental triples on 249 days

SELECTING DATA SELECTING DATA SNR>4.95 AU-EX-NASNR>4.95 AU-EX-NA

Fixed time window= 60 ms ; +/- 2000 Time Shift (1 sec) EX-AU & NA-AUFixed time window= 60 ms ; +/- 2000 Time Shift (1 sec) EX-AU & NA-AU

0.5 false alarm/century


ANALYSIS OF SOFTWARE INJECTED SIGNALSANALYSIS OF SOFTWARE INJECTED SIGNALSPreliminaryPreliminary

• 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.


DAMPED SINUSOID INJECTEDDAMPED SINUSOID INJECTED

─ AURIGA

─ EXPLORER

─ NAUTILUS

─DS 914Hz-1ms

─DS 882Hz-3ms

─ DS 898Hz-10ms

─ DS 938Hz-30ms


SINE-GAUSSIAN INJECTEDSINE-GAUSSIAN INJECTED

─ AURIGA

─ EXPLORER

─ NAUTILUS

─ SG 935Hz – Q=3

─ SG 935Hz – Q=8.9

─ SG 935Hz – Q=100


EFFICIENCY VS COINCIDENCE WINDOWEFFICIENCY VS COINCIDENCE WINDOW

Hrss

─ 5 10-20

─ 1 10-19

─ 2 10-19

─ 5 10-19

─ 1 10-18

•The window chosen in the analysis of the first period analyzed in IGEG2 is quite conservative

Damped sinusoids

from Galactic Center


EFFICIENCY VS AMPLITUDEEFFICIENCY VS AMPLITUDEcoincidence windows=60 mscoincidence windows=60 ms

• An efficiency of about 50% is reached with different signals at amplitudes hrss of 5·10-20 - 5·10-19

Eff

icie

ncy

Damped Sinusoid Galactic Center

─ DS f:914Hz tau:0.001s

─ DS f=930Hz tau:0.030s

Damped Sinusoid Galactic Disk

Effic

ienc

y

─ SG f:930Hz Q:3

─ SG f:930Hz Q:8.9

─ SG f:930Hz Q:100

Sin Gaussian Isotropic

Effic

ienc

y


PRELIMINARY RESULTS FROM TUNING PRELIMINARY RESULTS FROM TUNING ANALYSISANALYSIS

• 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


SUMMARYSUMMARY

• 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.

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