first results by igec2 6 month of data of auriga-explorer-nautilus may 20 - nov 15, 2005 igec2 was...

First results by IGEC26 month of data of AURIGA-EXPLORER-NAUTILUS

May 20 - Nov 15, 2005IGEC2 was the only gw observatory in operation

search for transient gw signalsto identify single candidates with high confidence :

triple coincidences false alarm rate of 1 per centurywider target signals than IGEC1

no candidates found

G.A.Prodi, WG2 Oct.25, 20062

Samples of target signals

A New Mechanism for the Gravitational Wave Signatures of Core-Collapse Supernovae, C.D. Ott, A. Burrows, L. Dessart, and E. Livne,Phys. Rev. Letters, 96, 201102, 2006.

Dominant gw emission: g-modes of the proto-neutron-star core

sources at 10 kpc


Strain Noise Amplitude of IGEC2 detectors


IGEC2 data preparation• Each group implements a search optimized for -like signals

(amplitude H [Hz-1] = Fourier component of h, gw strain) and validates its observation.

• Cross check by using AURIGA Data Analysis on a sample day of raw data of EX and NA :

agreement on most candidate events with SNR > 4.5 - 5.0• Exchanged data set:

Fri May 20 - Tue Nov 15, 2005,

same protocol as IGEC1 but a time shift is added to the true time (blind analysis)

• AU exchanged on April 19, 2006• ROG exchanged EX and NA data on July 5• ALLEGRO had difficulties in producing data: we agreed to

proceed anyway and use ALLEGRO data for a follow-up investigation of any candidate found.


3-fold OPERATION TIME 3-fold OPERATION TIME AURIGA- EXPLORER- NAUTILUS AURIGA- EXPLORER- NAUTILUS

• no detector 0.6 days

• Single 3.6 days

• Double 45.0 days

• Triple 130.8 days 73%

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 %

88 %

86 %

180 days

HIGH DUTY CYCLE

days of exchanged data


Observation time of the INFN detectors as a functionof amplitude threshold Ht.

Y-axis: integrated time during which the detector exchange threshold has been lower than Ht

Amplitude Ht [10-21 Hz-1]


Three-Fold Observation Timeof AU-EX-NA vs amplitude threshold

Amplitude Ht [10-21 Hz-1]


candidate event amplitudes versus time (blu: NA, green:EX, red:AU)

H[Hz-1]


Event amplitude distributionsAU: SNR>4.5 NA: SNR>4 EX: SNR>4

Amplitude H[Hz-1]

#eve

nts


Auto Correlograms

AU-AU NA-NA

EX-EX

sec sec

sec


Cross Correlograms

NA-EX

AU-NA AU-EX

secsec

sec


Time of arrival uncertainty: AU (Red), NA-EX (Blue)

Conservative estimates of t for a ms pulseSystematic errors for longer signals are likely to be similar in different detectors


IGEC2 search for gw (1)Triple coincidence search AU-EX-NA• blind search: tune analysis before looking for true coincidences.

Criteria: • set overall false alarm rate = 1 per century;• search for -like signals (IGEC1 style: equal H amplitude in

the different detectors)• search for some classes of colored signals

• time window for coincidence search (IGEC1 style): Abs [ta –tb] < bt sqrt [2

ta + 2tb ] for the 3 detector pairs

bt=4.47 to ensure a contribution to false dismissal < 15% according to Byenaimé-Tchebychev

assuming a signal with ms duration• analysis pipeline under responsibility of AURIGA. Independent

checks on results by ROG;


Distribution of accidental coincidencesall exchanged events

AU: SNR4.5 NA: SNR 4 EX: SNR 4

Entries 1879417

Mean 2.415

2/ndf 12.6 /12


perform only ONE composite search made by

the OR among the following data selections:

• SNR > 4.95 for AU, EX and NA0.396 false alarm /centurytargets signals peaked on EX-NA sweet spots

• SNR > 7.00 for AU, SNR>4.25 for EX and NA0.572 false alarm /centurytargets signals barely detectable by EX and NA

• common absolute thresholds IGEC1-style:thresholds 1.3, 1.4, 1.5, ..., 3.0 x 10-21/Hz

0.134 false alarm /centurytargets -like signals

130.8 days of net simultaneous observation time by the three INFN resonant bar detectors

IGEC2 search for gw (2)


Trial #1: same SNR thresholds


Trial #2: different SNR thresholds


target 0.5 false alarm / centuryAU SNR vs EX&NA SNRs

AU-SNR=4.95NA-SNR=4.95 EX-SNR=4.95FA/Century=0.40




AU-SNR=4.95 34598NA-SNR=4.95 42028 EX-SNR=4.95 29217

NA-SNR=4.25 351375

EX-SNR=4.25 245000

AU-SNR=8.00 552

AU-SNR=7.00 790

CUMULATIVE AMPLITUDE DISTRIBUTIONS OF EXCHANGED EVENTS


The accidental coincidences are the “union” of the accidental coincidences for each data selection: it takes into account the correlations between different trials counting only once each accidental coincidence.

Empirical estimate of the probability of a false detection:

3.63 10-3

1.0 false alarm / century

statistical uncertainty1 0.02 10-3

systematic uncertaintybelow 0.1 10-3

tested with independent pipelines & choices of time shifts

Distribution of accidental coincidences


# TRIPLES>0

17121189

# TRIALS

19355600

COMMON SEARCH

THRESHOLS

AU-SNR=4.95

NA-SNR=4.95

EX-SNR=4.95

AU-SNR=8.00

NA-SNR=4.25

EX-SNR=4.25

AU-SNR=7.00

NA-SNR=4.25

EX-SNR=4.25

COMMON SEARCH

THRESHOLS

9280[0.134 FA/C]

147 5153 5177

AU-SNR=4.95

NA-SNR=4.95

EX-SNR=4.95

27368[0.396 FA/C]

316 515

AU-SNR=8.00

NA-SNR=4.25

EX-SNR=4.25

26664[0.385 FA/C]

26664

AU-SNR=7.00

NA-SNR=4.25

EX-SNR=4.25

39507[0.573 FA/C]


Testing the null hypothesisNull hypothesis will be rejected if at least 1 coincidence is found in the “on-

source” analysis

In case the null is not confirmed:

• the coincidence is not explained by the accidental coincidences with 99.637% 0.006% (3) confidence (coverage), i.e. the collaboration excludes it is an accidental coincidence;

• the coincidence can be caused by any source of correlated noise or signal among the detectors outputs, also gravitational waves;

A rejection of the null is a claim for an excess correlation in the observatory at the true time, not taken into account in the measured accidental noise at different time lags. It may NOT be gws, it may be correlated noise, but a paper reporting the null rejection is worthwhile and due. It is useful in pointing to possible problems in the analysis procedures (accidental coincidences estimation) and to hardware problems (instrumental correlations).

Plan for the statistical data analysis (1)


follow-up investigation to characterize any coincidence “a posteriori”:

• 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 (warning: in case the SNR of the candidates is low, we do not expect to get significant information);

• 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 …).



Set a confidence interval on the estimated number of coincidences due to any source of correlated noise or signal. It will be conservative, i.e. ensuring a minimum coverage.

confidence belt construction:• Based on Feldman&Cousins construction;

• the noise model is a Poisson distribution fitting the empirical estimates of the accidental coincidences;

• to take into account uncertainties in the noise model, consider the union of the confidence belts given by the mean noise b 3

b = 0.00364 0.00006 events


nc 90% coverage

L H

95% coverage

L H

0 0 2.44 0 3.09

1 0.11 4.36 0.05 5.14

2 0.53 5.91 0.36 6.72


1,00

10,00

100,00

1000,00

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

IGEC2

IGEC

At last, we “opened the box” on Sept. 25 after exchanging the secret time shifts … …no candidates were found.

…the usual upper limit… Comparison with the upper limits given by the IGEC 1997-2000

observations is possible using a subset of the current analysis:

the IGEC1-style search

targeting -like signals


IGEC2 upper limit

Burst Rate

[year-1]

Burst amplitude [Hz-1]

95%

coverage


Final remarks ...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 is NOT possible (efficiency measurements have been performed in VIRGO-bars)

– 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


... Final remarks

Short term opportunities:• improved methodologies of network data analysis are feasible:

exploit full information of h(t) channels (coherent data analyses)• IGEC2 has been requested to collaborate with LIGO during its

current scientific run: we are now ready to define this joint research program


AURIGA: response to dampedsinusoids hrss = 1e-19

Mea

sure

d H

[H

z-1]


NAUTILUS: response to dampedsinusoids hrss = 1e-19

Mea

sure

d H

[H

z-1]


EXPLORER: response to dampedsinusoids hrss = 1e-19

Mea

sure

d H

[H

z-1]

first results by igec2 6 month of data of auriga-explorer-nautilus may 20 - nov 15, 2005 igec2 was...

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