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Coincidences in gravitational wave experiments. Pia Astone 4 th Amaldi conference Perth July 8-13, 2001. Main coincidence analyses we have done in the past:. Allegro-Explorer : Jun-Dec 1991 (180 days) Phys. Rev D 59, 1999. - PowerPoint PPT Presentation
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Coincidences in
gravitational wave experimentsPia Astone
4th Amaldi conference
Perth July 8-13, 2001
Main coincidence analyses we have done in the past:
Allegro-Explorer : Jun-Dec 1991 (180 days)Phys. Rev D 59, 1999
Explorer-Nautilus-Niobe : Dec 1994-Oct 1996(Explorer –Nautilus: 57 days . Explorer-Niobe: 56 days)
Astrop. Phys. 10, 1999
IGEC 1997-1998Phys. Rev. Letters,85,2000
The IGEC analysis of the data 1997-2000 is now being done The IGEC analysis of the data 1997-2000 is now being done
Some basics figures of the coincidence analysis
• We exchange eventsevents, above given thresholds (depending on the detector sensitivitiesvarying with the time)
• Each group applies vetoing procedures, to the noise and/or to the events, before the analysis is done
• The analysis procedure is based on “the time shift procedure” (see-e.g.-Int. Journal of Modern Physics D,9,2000)
• The sensitivity of each detector varies with time
• The sensitivities of the various detectors are different
• The same signal generates events with energies different for each detector
• The choice of the coincidence window
Main problems
Noiseof
ExplorerAnd
Nautilusin 1998
The y-axis expresses
thesensitivityto burst Days from 1 Jan 1997 Days from 1 Jan 1997
3 10-18
0.5 10-18
The detector sensitivities may be very different..and thus different signals could be detected
and also the event energies may be different,
Explorer -Nautilus 1996
Days from1 Jan 1994
Astrop. Phys., 10, 1999
SIGNALS-EVENTS
What is the solution ?First of all: it should be clear the difference
SIGNALS-EVENTS
Event SNR
Differentialprobability
SNR=signal to noise
ratio of the signal(here: 10, 20, 50)
The x-axis gives thesignal to noise ratio
of the eventThe y-axis gives the
differential probabilityfor the SNRof the event
Signals and events
SNR of the threshold
Probabilityof detection
SNR=signal to noise
ratio of the signal(here: 10, 20, 50)
The x-axis gives thesignal to noise ratio
of the thresholdThe y-axis gives the
probabilityof detection for a given
SNR-t
Probability of detection
Note that this effect does not depend
on thedetector bandwidth
Simulation of the efficiency of detection:delta-like signals applied to the Explorer data,
with various SNRsAstone,D’Antonio,Pizzella PRD 62 (2000)
Simulation of the efficiency of detection:delta-like signals applied to the Explorer data,
with various SNRsAstone,D’Antonio,Pizzella PRD 62 (2000)
The analysis procedure:a new selection algorithm based on the
event energies CQG, 18 (2001)
• Now we know that, for given SNR_s of the signal, there is a chance of obtaining certain SNR_e of the event
• We assume various signal values (h=10^-18-10^-17)• For each h we evaluate SNR_s (different for each detector
and for each event-it is a function of the local noise)
• We accept an event, and thus a coincidence, if the
SNR_s - 1 std < SNR_e < SNR_s + 1 std
Based on an original observation of D. Blair et al.: the distribution ofenergy ratios of the event energies of two detectors
is different for real and accidental coincidences (if non-gaussian noise)Journal of General Relativity and Gravitation (2000)
Explorer and Nautilus 1998 IGEC data
N
events
N
hoursT eff[mK]
Overlap
hoursN of overlap
Events
Ex55070 3415 40.6
227137944
NA37734 3450 19.1 24118
Bursts sensitivity h (SNR=1) : Ex= 1.6 10-18 NA= 1.1 10-18
We have applied this algorithm to theExplorer and Nautilus 1998 IGEC data
Number of
coincidences
Average number of
shifted coincidences
Common hours
of
observation
223 231.7 2271
61 50.5 2271
The use of the energy
selectionalgorithm
has reducedthe number of
accidentalcoincidences by
a factor of 4
Noselect
Energyselect
Another selection criterium:based on the detector orientation
with respect to specific sources, e.g. GC
• Since no extragalactic signals are expected, with the present sensitivity, we can select the events according to the orientation of the detectors, with respect to the GC
(sin)4
This criterium has been applied in CQG,18 (2001)
Based on the same idea, we are now testing a new procedure:coincidences (real or shifted) are weighted according to the
value of sin(teta)4 for given directions(M. Visco)
Experimental probability
Right ascension
Decl The plot represents,for each direction,the experimental
probability that theresult for real(zero delay)
coincidences isdue to noise
A new procedure for evaluation of upper limits(Astone,Pizzella: Astrop. Physics, in press 2001)
• The procedure used in the past (e.g. Allegro-Explorer 1991, IGEC 1997-1998 ) is described in Amaldi et al, A&A, 216 (1989)
Problems Signals-events The energy of the event
is not the energy of the GW
Efficiency of
detection
It is smaller than unity, andthis changes the upper limit
659 d
553 d
852 d
221 d
200 d
ON times for the various detectors 1997-2001
1 detector=609 d
4 detectors=30 d
3 detectors=149 d
2 detectors=535 d
0 detectors=137 d
ON times for the 1997-2000 coincidence analysis2 detectors=714 days3 detectors=179 days
We have applied the GC algorithm to theExplorer and Nautilus 1998 IGEC data
Number of
coincidences
Average number of
shifted coincidences
Common hours
of
observation
223 231.7 2271
61 50.5 2271
19 10 450
All
Energy
GC
Time deviation and the problem of the coincidence window
• We have found that, for signals syncronized with the sampling time, the statistical time uncertainty is expressed by
= 1/(2 f) sqrt(2/SNR)
This suggests to use a
variable coincidence
window (R. Terenzi)
Simulation of the efficiency of detection andtime deviation:
delta-like signals applied to the Explorer data,with various SNRs
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