Transcript International Gravitational Event Collaboration Observation Summary

INTERNATIONAL GRAVITATIONAL EVENT
COLLABORATION:
OBSERVATION SUMMARY
Giovanni Andrea Prodi
University of Trento and INFN Italy
2nd GWPW, Penn State, Nov.6th, 2003
IGEC observations in 1997-2000:
• exchanged data
• multiple detector analysis
• background of accidental coincidences
• upper limits and their interpretation
International Gravitational Event Collaboration
http://igec.lnl.infn.it
ALLEGRO group:
ALLEGRO (LSU)
http://gravity.phys.lsu.edu
Louisiana State University, Baton Rouge - Louisiana
AURIGA group:
AURIGA (INFN-LNL)
http://www.auriga.lnl.infn.it
INFN of Padova, Trento, Ferrara, Firenze, LNL
Universities of Padova, Trento, Ferrara, Firenze
IFN- CNR, Trento – Italia
NIOBE group:
NIOBE (UWA)
http://www.gravity.pd.uwa.edu.au
University of Western Australia, Perth, Australia
ROG group:
EXPLORER (CERN)
http://www.roma1.infn.it/rog/rogmain.html
NAUTILUS (INFN-LNF)
INFN of Roma and LNF
Universities of Roma, L’Aquila
CNR IFSI and IESS, Roma - Italia
PRD 68 (2003) 022001
astro-ph/0302482
DETECTORS
almost parallel detectors
Resonant Bars
The planar gravitational wave impinging
on the bar with an angle  excites its
longitudinal mechanical mode, with
amplitude proportional to sin2()
The detector is sensitive in a narrow
frequency range near the resonance
(~900Hz)
Typical 1997-2000 bandwidths ~ 1 Hz
vibration insulation
f
low T
pre-amplifier

Bar
L
electromechanical
transducer tuned
to the lowest
longitudinal mode
DIRECTIONAL SENSITIVITY
The achieved sensitivity of bar detectors limits the observation range to
sources in the Milky Way. The almost parallel orientation of the
detectors guarantees a good coverage of the Galactic Center
 ALLEGRO
AURIGA -EXPLORER –NAUTILUS
 NIOBE
amplitude directional sensitivity factor
vs sideral time (hours)
TARGET GW SIGNALS
Fourier amplitude of burst gw
h( t )  H   ( t  t0 )
Detectable signals:
transients with flat Fourier amplitude
at the detector frequencies (900 Hz)
each detector applies
arrival time
an exchange threshold on measured H
OBSERVATION
TIME 1997-2000
(days)
threshold on burst gw
EXCHANGED PERIODS of OBSERVATION 1997-2000
ALLEGRO
AURIGA
EXPLORER
NAUTILUS
NIOBE
fraction of time in monthly bins
threshold on burst gw
 6  1021 Hz 1
3  6  1021 Hz 1
 3  1021 Hz 1
MULTIPLE DETECTOR ANALYSIS
network is needed to estimate (and reduce) the false alarms
time coincidence search among exchanged triggers
time window is set according to timing uncertainties by requiring a
conservative false dismissal
ti  t j  k  i2   2j  false dismissal 
1
k2
by Tchebyscheff inequality
false alarms  k
maximize the chances of detection i.e. the ratio
efficiency of detection
fluctuations of false alarms
measure the false alarms:
time shifts  resampling the stochastic processes so that:
• gw sources are off (as well as any correlated noise)
• statistical properties are preserved (max shift ~ 1 h)
• independent samples (min shift > largest time window ~ few s)
AMPLITUDE DISTRIBUTIONS of EXCHANGED EVENTS
normalized to each detector threshold for trigger search
1
1
-1
relative counts
10
-2
-2
10
10
-3
-3
10
10
-4
-4
10
10
-5
-5
10
10
1
10 AMP/THR
ALLEGRO
1
10 AMP/THR
AURIGA
1
10 AMP/THR
EXPLORER
1
10 AMP/THR
NAUTILUS
1
10 AMP/THR
NIOBE

typical trigger search thresholds:
SNR 3 ALLEGRO, NIOBE
SNR 5 AURIGA, EXPLORER, NAUTILUS
The amplitude range is much wider than expected:
non modeled outliers dominate at high SNR
relative counts
-1
10
amplitude (Hz-1·10-21)
DIRECTIONAL SEARCH: sensitivity modulation
10
9
8
7
6
5
4
3
2
1
0
0
6
12
18
24
30
36
42
48
54
60
amplitude (Hz-1·10-21)
time (hours)
1.0
10
0.9
9
amplitude
directional
sensitivity
0.8
8
7
0.7
6
0.6
sin 2 GC
0.5
5
sin 2 GC
0.4
4
0.3
3
2
0.2
1
0.1
0
0.0
0
6
12
18
24
30
36
42
48
54
60
time (hours)
amplitude (Hz-1·10-21)
Resampling statistics by time shifts
10
9
8
7
6
5
4
3
2
1
0
0
6
12
18
24
30
36
42
48
54
60
time (hours)
We can approximately resample the stochastic process by time shift.
in the shifted data the gw sources are off, along with any correlated noise
Ergodicity holds at least up to timescales of the order of one hour.
The samples are independent as long as the shift is longer than the
maximum time window for coincidence search (few seconds)
POISSON STATISTICS of ACCIDENTAL COINCIDENCES
Poisson fits of accidental concidences: 2 test
sample of EX-NA background
one-tail probability = 0.71
agreement with uniform distribution
histogram of one-tail 2
probabilities for
ALL two-fold observations
 coincidence times are random
FALSE ALARM RATES
false
alarm
rate
-1
[yr ]
10
AL-AU
AL-AU-NA
1
dramatic improvement by
increasing the detector number:
3-fold or more would allow
to identify the gw candidate
0.1
0.01
1E-3
1E-4
1E-5
1E-6
2E-21
1E-20
-1
common search threshold [Hz ]
mean
rate of
events
[ yr -1]
mean
 timing
[ms]
Setting confidence intervals
IGEC approach is
frequentistic in that it computes the confidence level or coverage as the
probability that the confidence interval contains the true value
unified in that it prescribes how to set a confidence interval
automatically leading to a gw detection claim or an upper limit
based on maximum likelyhood confidence intervals (different from
Feldman & Cousins)
false dismissal is under control (but detection efficiency is only lowerbounded)
estimation of the probability of false detection (many attempts made
to enhance the chances of detection)
UPPER LIMIT on the RATE of BURST GW
from the GALACTIC CENTER DIRECTION
Poisson
rate of
detected
gw
[year –1]
dashed region excluded
with probability  90%
overcoverage
search threshold
• signal template = -like gw from the Galactic Center direction
signal amplitude HS= FT[hS ] at   2 900 Hz
H S ~ 2  1021 / Hz  0.02 M converted in burst gw at GalacticCenter
UPPER LIMIT on the RATE of BURST GW
from the GALACTIC CENTER DIRECTION (2)
Poisson
rate of
detected
gw
[year –1]
search threshold
• analysis includes all the measured signal amplitudes  search threshold
 result is cumulative for HM  Ht
• systematic search vs threshold Ht  many trials (20 /decade)
almost independent results
TESTING the NULL HYPOTHESIS
18
16
14
12
Ngw
10
8
6
4
2
0
1.0
10.0
100.0
search threshold [10-21/Hz]
many trials !
all upper limits but one:
NULL HYPOTHESIS WELL IN
AGREEMENT WITH THE
OBSERVATIONS
 testing the null hypothesis
overall false alarm probability
33% for 0.95 coverage
56% for 0.90 coverage
at least one detection in the set
in case
NO GW are in the data
UPPER LIMIT on the RATE of BURST GW
from the GALACTIC CENTER DIRECTION (3)
dashed region excluded
with probability  90%
overcoverage
Poisson
rate of
detected
gw
[year –1]
1.8 yr -1
search threshold
• no coincidences found, limited by the observation time
• limited by accidental coincidences
• observation time cuts off: sensitivity cut
HOW to UNFOLD IGEC RESULTS
in terms of GW FLUX at the EARTH
• Take a model for the distribution of events impinging on the
HS  H t
detector
(dashed line)
• Estimate the distribution of measured coincidences HM  Ht (cont.line)
• Compare with IGEC results to set confidence intervals on
gw flux parameters
1,000
coverage
100
0.60
rate
(year –1)
0.80
0.90
0.95
10
1
1E-21
1E-20
search threshold
Ht (Hz -1 )
1E-19
Case of gw flux of constant amplitude:
comparison with LIGO results
• IGEC sets an almost independent result per each tried threshold Ht
• correct each result for the detection efficiency as a function of gw amplitude HS:
e.g.
at HS  Ht
efficiency  0.25 due to 2-fold observations at threshold
at HS  2 Ht
efficiency = 1
Poisson
rate of
detected
gw
[year –1]
search threshold
enough above the threshold
Case of gw flux of constant amplitude:
comparison with LIGO results (2)
• the resulting interpreted upper limit
• convert from
HS= FT[hS ] at   2 900 Hz to template amplitude parameter
e.g. for a sine-gaussian(850 Hz;Q=9)
hrss= 10 Hz 0.5 HS
Poisson
rate of
detected
gw
[year –1]
search threshold
Data selection at work
amplitude (Hz-1·10-21)
Duty time is shortened at each
detector in order to have efficiency
at least 50%
A major false alarm reduction is
achieved by excluding low
amplitude events.
10
9
8
7
6
5
4
3
2
1
0
0
6
12
18
24
30
36
42
48
54
60
time (hours)
FALSE ALARM REDUCTION
by amplitude selection of events
amplitude
time
consequence:
selected events have consistent amplitudes
Auto- and cross-correlation of time series (clustering)
 Auto-correlation of time of arrival on timescales ~100s
 No cross-correlation
UPGRADE of the AURIGA resonant bar detector
Previous set-up during
1997-1999 observations
current set-up for the
upcoming II run
• beginning cool down phase
• at operating temperature by November
AURIGA II run
LHe4 vessel
Al2081 holder
Electronics
wiring support
Main Attenuator
Thermal
Shield
Sensitive bar
Compression Spring
Transducer
AURIGA II run: upgrades
new mechanical suspensions:
attenuation > 360 dB at 1 kHz
FEM modelled
new capacitive transducer:
two-modes (1 mechanical+1 electrical)
optimized mass
new amplifier:
double stage SQUID
200  energy resolution
new data analysis:
C++ object oriented code
frame data format
initial goal of AURIGA II: improving amplitude
sensitivity by factor 10 over IGEC results
FUTURE PROSPECTS we are aiming at
DUAL detectors estimated sensitivity at SQL:
Science with HF GW
• BH and NS mergers
and ringdown
• NS vibrations and
instabilities
• EoS of superdense
matter
• Exp. Physics of BH
Mo Dual 16.4 ton height 2.3 m
SiC Dual 62.2 ton height 3 m
Ø 0.94m
Ø 2.9m
T~0.1 K ,
Standard Quantum Limit
• Only very few noise resonances in bandwidth.
• Sensitive to high frequency GW in a wide bandwidth.
PRD 68 (2003) 1020XX in press
PRL 87 (2001) 031101
New concepts - new technologies:
• No resonant transducers:
measure differential motion of
massive cylindrical resonators
• Mode selective readout:
measured quantity: X = x1+x2-x3-x4
• High cross section materials
(up to 100 times larger
than Al5056 used in bars)
Dual detector: the concept
2 nested masses:
below both resonances: the masses
are driven in-phase
→ phase difference is null
Intermediate frequency range:
• the outer resonator is driven
above resonance,
• the inner resonator is driven
below resonance
→ phase difference of p
In the differential measurement:
→ the signals sum up
→ the readout back action
noise subtracts
above both resonances: the masses
are driven out-of-phase
→ phase difference is null
Differential measurement strategy
• Average the deformation of the resonant masses over a wide area:
reduce thermal noise contribution from high frequency
resonant modes which do not carry the gravitational signal
• Readout with quadrupolar symmetry: ‘geometrically selective readout’
that rejects the non-quadrupolar modes
bandwidth free from acoustic modes not sensitive to gw.
Example:
- capacitive readout The current is
proportional to:
Dual Detector with √Shh~10-23/√Hz in 1-5 kHz range
Molybdenum
Silicon Carbide (SiC)
• Q/T>2x108 K-1 - Mass = 16 tons
• Q/T > 2x108 K-1 - Mass = 62 tons
• R = 0.47 m
• R = 1.44 m
- height = 2.3 m
- height = 3 m
Feasibility issues
Detector:
Readout:
• Massive resonators ( > 10 tons )
• Selective measurement strategy
• Cooling
• Quantum limited
• Suspensions
• Wide area sensor
• Low loss and high cross-section
materials
• Displacement sensitivity
R&D on readouts: status
• Requirement: ~ 5x10-23 m/√Hz
• Present AURIGA technology:  10-19 m/√Hz
with:
optomechanical readout - based on Fabry-Perot cavities
capacitive readout - based on SQUID amplifiers
Foreseen limits of the readout sensitivity: ~ 5x10-22 m/√Hz.
Critical issues:
optomechanical – push cavity finesse to current technological limit together
with Watts input laser power
capacitive – push bias electric field to the current technological limit
Develop non-resonant devices to amplify the differential
deformation of the massive bodies.
Idea to relax requirements on readout sensitivity:
mechanical amplifiers
• based on the elastic deformation of monolithic devices
• well known for their applications in mechanical engineering.
GOAL:
Amplify the differential deformations of the massive bodies
over a wide frequency range.
Requirements:
* Gain of at least a factor 10.
* Negligible thermal noise with respect to that of the
detector.