Results by the IGEC2 collaboration on 2005 data

GWDAW11 – Potsdam 21.12.2006

Results by the IGEC2 collaboration on Results by the IGEC2 collaboration on 2005 data2005 data

Gabriele Vedovatofor the IGEC2 collaboration


HighLightsHighLights

• 6 month of data [May 20 - Nov 15, 2005]

– AURIGA-EXPLORER-NAUTILUS - data exchanged– ALLEGRO - data available only for a follow-up investigation

• IGEC2 was the only gw observatory in operation

• Search for GWB signals (wider target signals than IGEC1)was tuned to identify single candidates with high confidence :

– only triple coincidences – false alarm rate = 1/century

• NO CANDIDATES FOUND !!!


DATA ANALYSIS PIPELINE : DATA ANALYSIS PIPELINE : BASED ON TIME COINCIDENCEBASED ON TIME COINCIDENCE

Fourier amplitude of gwb

0 ( ) ( )h tH tt

arrival time

• Exchange of candidate events using a protocol analogous to the one used in IGEC1 analysis

Candidate events lists are produced by each group using a search optimized for -like signals the amplitude H [Hz-1] exchanged is the fourier component of h, gw strain each group added a “secret time shift” to the time of the candidate events no detection efficiency is used !!!

• Cross check of the filter pipeline on a sample day of raw data.

• Tuning of the analysis to a possible detection requiring an identification of the candidates with high confidence fixing the three-fold false alarm to 1 over 100 years (an upper limit, at the present sensitivity of the bar detectors, is not an interesting scientific result)

• “a priori” tuning of the parameters (search thresholds, coincidence window and working periods) by a blind analysis of the background characteristics

• Exchange of the secret time shifts and search for possible candidates and follow-up investigations.


THE DETECTORSTHE DETECTORS

AURIGA

NAUTILUS

EXPLORER

ALLEGRO


IGEC 1

1997-2000

IGEC 2

2005

FROM IGEC1 TO IGEC2FROM IGEC1 TO IGEC2


SENSITIVITY OF IGEC2 DETECTORSSENSITIVITY OF IGEC2 DETECTORS


Detector AcceptanceDetector Acceptance

• The four antennas are coalligned and the received signals are projected onto detectors in the same way regardless of the source direction


DATA EXCHANGEDATA EXCHANGE

AURIGA- EXPLORER- NAUTILUSAURIGA- EXPLORER- NAUTILUS

MAY 20 –NOV 15, 2005MAY 20 –NOV 15, 2005


OPERATION TIME – OPERATION TIME – MAY 20 –NOV 15, 2005MAY 20 –NOV 15, 2005(AURIGA- EXPLORER- NAUTILUS) (AURIGA- EXPLORER- NAUTILUS)

• no detector 0.6 days

• Single 3.6 days

• Double 45.0 days

• Triple 130.8 days

AL AU EX NA

AL 0

AU 0 172.9

EX 0 151.8 158.0

NA 0 150.2 135.3 155.0

96 %

87 %

86 %

180 days

Data from ALLEGRO to be added

HIGH DUTY FACTOR

days of operation


AURIGANAUTILUSEXPLORER

IGEC 1

1997-2000

typical values

EXPLORER

Noise of Detectors vs TimeNoise of Detectors vs Timein terms of Fourier component Hin terms of Fourier component H

AURIGA

NAUTILUS


AMPLITUDE vs TIMEAMPLITUDE vs TIMEOF THE EXCHANGED EVENTS OF THE EXCHANGED EVENTS

EXPLORERSNR > 4.0

AURIGASNR > 4.5

NAUTILUSSNR > 4.0


AMPLITUDE DISTRIBUTIONAMPLITUDE DISTRIBUTIONOF THE EXCHANGED EVENTS OF THE EXCHANGED EVENTS

EXPLORERSNR > 4.0

AURIGASNR > 4.5

NAUTILUSSNR > 4.0


EXPLORER

AURIGA

NAUTILUS

Integrated time during which the detector Integrated time during which the detector exchange threshold has been lower than Hexchange threshold has been lower than Htt


FALSE ALARMS FALSE ALARMS ESTIMATIONESTIMATION


• Coincidence window IGEC1 style

|t1 –t2| < bt * sqrt [2(t1) + 2(t2)]

for the 3 detector pairs bt=4.47 ensure a contribution to false dismissal < 15% according to Byenaimé-Tchebychev

• For EXPLORER and NAUTILUS the t was fixed at 10 ms

COINCIDENCE WINDOWCOINCIDENCE WINDOW

t AURIGA


THREE-FOLD COINCIDENCES BACKGROUNDTHREE-FOLD COINCIDENCES BACKGROUNDALL THE EXCHANGED DATA ALL THE EXCHANGED DATA SNRSNR4.5 AU SNR4.5 AU SNR4.0 EX-NA4.0 EX-NA

+/- 2200 Time Shift (5 sec) EX-AU & NA-AU +/- 2200 Time Shift (5 sec) EX-AU & NA-AU

Mean number of accidental

triples on 131 days

Total number of Trials


DATA SELECTIONDATA SELECTIONTo set the background to 1 false alarm per century the

background must be reduced by a factor 600 : we decided to perform one composite search made by the OR among 3

different procedures data selection

Selection A : based on false alarms statistic & data quality (detection efficiency not available)

Selection B : -like signals (-like signals are detected with the same amplitude H by each

detector)

Selection C : -like signals IGEC1-Style (for comparison)


• Data Selection A – Same SNR ThresholdsData Selection A – Same SNR Thresholds– SNR > 4.95 for AU, EX & NA

• Equal data quality • Target signals : better detected by NA-EX than AU• 0.396 false alarm/century


• Data Selection B – Different SNR ThresholdsData Selection B – Different SNR Thresholds– AU-SNR > 7.00 : EX-SNR & NA-SNR > 4.25

Target signals : -like 0.572 false alarm/century


AURIGA

NAUTILUS

EXPLORER

AURIGA

NAUTILUS

EXPLORER

AURIGA

NAUTILUS

EXPLORER

• Data Selection Data Selection C – Common Thresholds IGEC1-styleC – Common Thresholds IGEC1-style– search thresholds @ H=1.2, 1.3, 1.4, 1.5, …, 2.4, …, 3.0*10-21/Hz

• target -like signals• 0.134 false alarm/century


Integrated time during which the 3 detectors exchange threshold has been lower than search threshold Ht

• Data Selection Data Selection C – Common Thresholds IGEC1-styleC – Common Thresholds IGEC1-style

OBSERVATION TIMESOBSERVATION TIMES

[Hz-1]

t


AU-SNR=4.95

NA-SNR=4.95

EX-SNR=4.95

AU-SNR=7.00

NA-SNR=4.25

EX-SNR=4.25

COMMON SEARCH

THRESHOLS

AU-SNR=4.95

NA-SNR=4.95

EX-SNR=4.95

27368[0.396 FA/C]

515 147

AU-SNR=7.00

NA-SNR=4.25

EX-SNR=4.25515 39507

[0.573 FA/C]5177

COMMON SEARCH

THRESHOLS147 5177 9280

[0.134 FA/C]

TOTAL # TRIALS = TOTAL # TRIALS = 19.355.60019.355.600# ACCIDENTAL TRIPLE COINCIDENCES# ACCIDENTAL TRIPLE COINCIDENCES = = 17.121.18917.121.189

1 False Alarm/Century1 False Alarm/Century

Correlationbetween 3 data

selections


CUMULATIVE FALSE ALARM DISTRIBUTION CUMULATIVE FALSE ALARM DISTRIBUTION of the 3 data selectionsof the 3 data selections

The accidental coincidences are the “union” of the accidental coincidences for each data selection counting only once the repeated accidental coincidences.

Empirical estimate of the probability of a false detection:3.63 10-3 over 130 days 1.0 false alarm / century

statistical uncertainty:1 0.02 10-3

systematic uncertainty below 0.1 10-3

tested with independent pipelines & different choices of time shifts


STATISTICAL DATA STATISTICAL DATA ANALYSIS ANALYSIS

&&FOLLOW-UPFOLLOW-UP


• 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 to correlated signal or noise in the 3 detectors

– a follow-up a posteriori analysis is necessary to characterize the coincidences

TEST THE NULL HYPOTHESISTEST THE NULL HYPOTHESIS


CONFIDENCE INTERVALSCONFIDENCE INTERVALS

Confidence belt construction:

– Based on Feldman&Cousins construction

– the noise model is a Poisson distribution fitting the empirical estimates of the accidental coincidences with mean noise

b = 0.00364 0.00006 (3) events

observed = 0Average signals = 0 to 3.09

Set a confidence interval on the mean number of coincidences due to any source of correlated noise or signal assuming a poisson model.

observed = 1Average signals = 0.05 to 5.14


• 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 used to implement network coherent tests based on cross-correlation

• Data from ALLEGRO will be added, in particular h(t) and list of candidates.

• IGEC2 will investigate on simultaneous observations by other kind of detectors (neutrinos, gamma, x …)

FOLLOW-UP PLANNED ANALYSISFOLLOW-UP PLANNED ANALYSIS


RESULTS RESULTS

1,00

10,00

100,00

1000,00

1,00E-21 1,00E-20

IGEC2

IGEC

95%

coverage

Burst amplitude H at detector [Hz-1]

Burst Rate

detectable

[year-1]

• 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 has been used to calculate the uninterpreted upper limit to be compared with the previous one of IGEC1


Strengths:

• 3 bar detectors easily survey with high duty cycle and very low false alarms: possible identification of single candidates at low SNR with very high confidence

• IGEC2 searches for a broader class of signals than IGEC1

• The blind search in the statistical sense makes the statistical interpretation non-controversial

Weaknesses:

• IGEC2 (as IGEC1) lacks a measurement of detection efficiency

– Interpretation of the confidence interval in terms of equivalent gravitational waves from a selected source model was NOT possible

– The tuning of the analysis considered only generic predictions of the detection efficiency

• Poor sensitivity with respect to LIGO IGEC2 upper limits not interesting any more

Short term opportunities:

• improved methodologies of network data analysis have already been tried (coherent data analyses)

• IGEC2 has been requested to collaborate with LIGO during its current scientific run

FINAL REMARKS FINAL REMARKS


CUMULATIVE FALSE ALARM CUMULATIVE FALSE ALARM AU-NA/EX TIME SHIFT DISTRIBUTION AU-NA/EX TIME SHIFT DISTRIBUTION

of the 3 data selectionsof the 3 data selections

False alarms are uniformely distributed regardeless of the time shift of

EX & NA respect to AU

Measurement of statistical uncertainty

on mean accidental coincidences

EX Time Shift

NA

Tim

e S

hift

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Results by the IGEC2 collaboration on 2005 data