Transcript IGEC2 - ego

First results by IGEC2
6 month of data of AURIGA-EXPLORER-NAUTILUS
May 20 - Nov 15, 2005
IGEC2 was the only gw observatory in operation
search for transient gw signals
to identify single candidates with high confidence :
triple coincidences
false alarm rate of 1 per century
wider target signals than IGEC1
no candidates found
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
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Strain Noise Amplitude of IGEC2 detectors
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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.
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3-fold OPERATION TIME
AURIGA- EXPLORER- NAUTILUS
180 days
HIGH DUTY CYCLE
AL
AL
0
AU
0
AU 96 %
EX
• no detector 0.6 days
NA
88 %
172.9
EX
0
151.8
158.0
NA
0
150.2
135.3
86 %
155.0
• Single
3.6 days
• Double
45.0 days
• Triple
130.8 days 73%
days of exchanged data
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Observation time of the INFN detectors as a function
of amplitude threshold Ht.
Y-axis: integrated time during which the detector
exchange threshold has been lower than Ht
Amplitude Ht [10-21 Hz-1]
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Three-Fold Observation Time
of AU-EX-NA vs amplitude threshold
Amplitude Ht [10-21 Hz-1]
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candidate event amplitudes versus time
(blu: NA, green:EX, red:AU)
H
[Hz-1]
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#events
Event amplitude distributions
AU: SNR>4.5
NA: SNR>4
EX: SNR>4
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Amplitude H
[Hz-1]
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Auto Correlograms
AU-AU
NA-NA
sec
sec
EX-EX
sec
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Cross Correlograms
AU-NA
AU-EX
sec
sec
NA-EX
sec
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Time of arrival uncertainty: AU (Red), NA-EX (Blue)
Conservative estimates of t for a ms pulse
Systematic errors for longer signals are likely to be similar in different detectors
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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 [2ta + 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;
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Distribution of accidental coincidences
all exchanged events
AU: SNR4.5 NA: SNR  4 EX: SNR  4
Entries
Mean
2/ndf
1879417
2.415
12.6 /12
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IGEC2 search for gw (2)
perform only ONE composite search made by
the OR among the following data selections:
• SNR > 4.95 for AU, EX and NA
0.396 false alarm /century
targets signals peaked on EX-NA sweet spots
• SNR > 7.00 for AU, SNR>4.25 for EX and NA
0.572 false alarm /century
targets 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 /century
targets -like signals
130.8 days of net simultaneous observation time by the three
INFN resonant bar detectors
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Trial #1: same SNR thresholds
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Trial #2: different SNR thresholds
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target 0.5 false alarm / century
AU SNR vs EX&NA SNRs
AU-SNR=4.95
NA-SNR=4.95
EX-SNR=4.95
FA/Century=0.40
AU-SNR=7.00
NA-SNR=4.25
EX-SNR=4.25
FA/Century=0.40
AU-SNR=8.00
NA-SNR=4.25
EX-SNR=4.25
FA/Century=0.38
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CUMULATIVE AMPLITUDE DISTRIBUTIONS OF EXCHANGED EVENTS
NA-SNR=4.25 351375
EX-SNR=4.25
245000
AU-SNR=4.95
NA-SNR=4.95
EX-SNR=4.95
34598
42028
29217
AU-SNR=7.00
790
AU-SNR=8.00
552
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Distribution of accidental coincidences
Empirical estimate of
the probability of a
false detection:
3.63 10-3
1.0 false alarm / century
statistical uncertainty
1  0.02 10-3
systematic uncertainty
below  0.1 10-3
tested with independent
pipelines & choices of
time shifts
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.
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# 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=4.95 AU-SNR=8.00 AU-SNR=7.00
COMMON
SEARCH
NA-SNR=4.95 NA-SNR=4.25 NA-SNR=4.25
THRESHOLS EX-SNR=4.95 EX-SNR=4.25 EX-SNR=4.25
9280
[0.134 FA/C]
147
5153
5177
316
515
26664
26664
27368
[0.396 FA/C]
[0.385 FA/C]
AU-SNR=7.00
NA-SNR=4.25
EX-SNR=4.25
39507
[0.573 FA/C]
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Plan for the statistical data analysis (1)
 Testing the null hypothesis
Null hypothesis will be rejected if at least 1 coincidence is found in the “onsource” 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).
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Plan for the statistical data analysis (2)
 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 …).
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Plan for the statistical data analysis (3)
 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
95% coverage
L
H
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
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Plan for the statistical data analysis (4)


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 1000,00
targeting -like signals
IGEC2
IGEC
100,00
Burst Rate
[year-1] 10,00
IGEC2 upper limit
95%
coverage
1,00
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WG2 Oct.25, 2006
1,00E-21
Burst amplitude [Hz-1]
1,00E-20
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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
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... 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
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Measured H [Hz-1]
AURIGA: response to dampedsinusoids hrss = 1e-19
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Measured H [Hz-1]
NAUTILUS: response to dampedsinusoids hrss = 1e-19
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Measured H [Hz-1]
EXPLORER: response to dampedsinusoids hrss = 1e-19
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