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Search for Gravitational Wave Transients
Florent RobinetOn behalf of the LSC and Virgo Collaborations
Rencontres de Moriond - March 2011 1
Gravitational WavesGravitational Waves
Black hole merger
Gravitational wavesGravitational waves = "ripples" in spacetime = "ripples" in spacetime
Weak field approximation :Weak field approximation :
Wave equation, speed Wave equation, speed cc
Solution with 2 d.o.f. : Solution with 2 d.o.f. :
Dimensionless amplitude given by Dimensionless amplitude given by hh
Signal strength:Signal strength:
Production of gravitational waves
A good GW source : is compact and massive is asymmetric has a relativistic speed
c5
G
2R s
R 2
vc 6
ℒ =
Lab production : Lab production : h ~ 10 h ~ 10 – 39– 39
Astrophysical sources : Astrophysical sources : h ~ 10 h ~ 10 – 21– 21
g=h ∣h∣≪1
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h= h+hx
hrss=∫-∞
+∞∣h+t ∣
2∣hx t ∣
2dt
Gravitational Wave SourcesGravitational Wave Sources
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?
Pulsars(asymmetric rotations, instabilities)
Compact binary coalescence of neutron stars &/or black holes
Supernovae(asymmetric core bounce)
Cosmic strings
Stochastic background
The unexpected
A Network of DetectorsA Network of Detectors
Virgo (3 km)
Geo (600 m)
Livingston (4 km)
Hanford (4&2 km)
LSC – Virgo collaboration– Full data sharing since May 2007– Common analyses and papers– Common tools
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A Network of DetectorsA Network of Detectors
tLivingston
tHanford
tHanford
tVirgo
SOURCE
SOURCE POINTING● Source location within ~ tens of square degrees● Serious candidates followup (EM, neutrinos...)
GHOST
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A Network of DetectorsA Network of Detectors
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Hanford sky coverage – Antenna pattern
A Network of DetectorsA Network of Detectors
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Livingstone sky coverage – Antenna pattern
A Network of DetectorsA Network of Detectors
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Virgo sky coverage – Antenna pattern
A Network of DetectorsA Network of Detectors
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Network sky coverage – Antenna pattern
LIGO / Virgo Science RunsLIGO / Virgo Science Runs
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2005 2006 2007 2008 2009 2010 2011 2012
S4 S5 S6
VSR2VSR1 VSR3 VSR4?
AdvancedDetectors
Many Publications Analyses in progressPublications in preparation
com
mis
sion
ing
LIGO / Virgo Science RunsLIGO / Virgo Science Runs
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S5 - VSR1
S6 - VSR2
Analysis GroupsAnalysis Groups
Compact Binary Coalescence(CBC)
Short Signals(Bursts)
Continuous Waves Stochastic
?
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See C. Palomba's talk
Analysis GroupsAnalysis Groups
Compact Binary Coalescence(CBC)
Short Signals (Bursts)
?
All sky
GRB-triggered
SGR Flares
EM Follow-upBinary Mergers
PulsarsGlitches
Supernovae
Multi-MessengerAstronomy
Low Mass High Mass
Inspiral-Merger-Ringdown (IMR)
ParameterEstimation
Cosmic Strings
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CBC vs. BurstsCBC vs. Bursts
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Compact Binary Coalescence (CBC)
The Signals● Modeled signals● Inspiral – Merger – Ringdown
The search● Template search (selective)● Waveform parameter estimation
Science goals● Detection● Upper limits on GW emission● Multi-messenger (EM, neutrino...)● Parameter estimation● Study gravity● Study populations● Study dense matter● Study GRBs
Burst Signals
The Signals● Short-duration signals (<1s)● Modeled and unmodeled signals● Large variety of sources
The search● Robust signal detection methods● Excess power (unmodeled)● Template search (modeled)
Science goals● Detection● Upper limits on GW emission● Multi-messenger (EM, neutrino...)● Parameter estimation● Star equation of state● Study populations
Analysis MethodsAnalysis Methods
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Data Detector 1
Data Detector 2
Triggers
Triggers
Coincidence
Selection+
DataQuality
Significance?
Data stream
COINCIDENT PIPELINE
Analysis MethodsAnalysis Methods
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Data Detector 1
Data Detector 2
Triggers
Triggers
Coincidence
Selection+
DataQuality
Significancewrt
background
Data Detector 2time-shifted wrt 1 Data stream
Background stream
DETECTION?NO?→ Upper limits
COINCIDENT PIPELINE
Analysis MethodsAnalysis Methods
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Data Detector 1
Data Detector 2
Triggers
Triggers
Coincidence
Selection+
DataQuality
Significancewrt
background
Signal injections
Data Detector 2time-shifted wrt 1
Search efficiency
Data streamInjection streamBackground stream
DETECTION?
UPPER LIMITS
The number of detectors can be increased (up to 4) Various coincidence schemes: union of configurationsIncreasing the number of coincidences enables to be more selective (but less efficient)
COINCIDENT PIPELINE
Analysis MethodsAnalysis Methods
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Data Detector 1
Data Detector 2
TriggersCoherent combination
Selection+
DataQuality
Significancewrt
background
Signal injections
Data Detector 2time-shifted wrt 1
Search efficiency
Data streamInjection streamBackground stream
DETECTION?
UPPER LIMITS
The data of multiple detectors can be combined coherentlySky positions are scanned to take into account the time of arrival and the antenna pattern of each detectors
COHERENT PIPELINE
Analysis Methods: Template SearchesAnalysis Methods: Template Searches
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Detector whitened Strain h(t)Noise + inspiral hardware injection
A template bank covering the parameter space is slid over the data
t ∝∫0
∞ h f f Shf
e−2i f t df
Signal-to-noise ratio time serie:
Template waveformSensitivity
An event is defined when Given by the background estimation(+ additional clustering in time and frequency)
t Threshold
This method is used for:
● CBC searches● Pulsar ringdown search● Cosmic string burst search
Sh
Analysis Methods: Excess Power SearchesAnalysis Methods: Excess Power Searches
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Injected Inspiral Signal
The time-frequency plane is tiled with pixels
An event is defined when the energy of multiple pixels exceeds a given threshold
This method is used for:
● Most of the burst searches
Data QualityData Quality
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The noise of the detector displays a non-Gaussian behavior
Transient glitches removal is crucial to improve the sensitivity of the searches
Noise understanding for each detector have been performed for each science run
Many glitch families have been understood
Specific vetoes based on auxiliary channels have been produced to remove specific glitch families
Veto safety have been carefully checked(we don't want to flag real signals!)
Example: Virgo, VSR2Deadtime ~ 10%Removal efficiency ~ 80% (SNR>8)
SNR10 100
Magnetic sensor
Detection channel
CBC SearchesCBC Searches
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"Realistic" observable BNS coalescence rate ~ 0.02 per year (large uncertainty)
BNSBBHBHNS
S5/VSR1 data have been analyzed and results are published
S6/VSR2-3 analyses are in progressPreliminary results are releasedSee T. Dent's talk
CBC: Low Mass SearchCBC: Low Mass Search
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Description of the search:
● Non-spinning templates● Post-Newtonian up to the innermost stable orbit● Mass region 2 < M
total < 35 M
sun
BNS/BBH Upper limits NS/BH Upper limits
No Detection
Phys. Rev. D 82(2010) 102001
CBC: High Mass SearchesCBC: High Mass Searches
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High-Mass● Effective One Body (EOB) waveforms● Inspiral-Merger-Ringdown is covered● No spin● Mass region 25 < M
total < 100 M
sun
● Uncertainty on the waveforms● LIGO only search (Virgo is not sensitive enough for High mass systems)
Ringdown search ● In progress for S5● LIGO only search● Mass region
75 < MBH
< 750 Msun
● Spin is included● The ringdown contains most of the GW energy● More reliable waveforms
arXiv:1102.3781
CBC & Burst Signals: GRB Triggered SearchesCBC & Burst Signals: GRB Triggered Searches
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Powerful bursts of highly energetic gamma raysTwo populations: short and long duration
Short: possibly produced by the merging of binary objects → CBC colored search
Long: possibly produced by violent stellar collapse (hypernovae)
Short-duration GRBsLong-duration GRBs
Use mainly Swift and Fermi triggers to get a source location a timing and sometimes a distance
Background reductionBetter sensitivity
During S5/VSR1 137 GRBs were analyzed by the burst coherent pipeline (short and long). See M. Was's talk
22 GRBs were analyzed by a CBC pipeline. See N. Christensen's talk
20Num
ber
of B
urst
s
40
60
0.1 1 10T90 (seconds)
CBC & Burst Signals: GRB Triggered SearchesCBC & Burst Signals: GRB Triggered Searches
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GRB 070201
Short and hard GRB detected by 4 satellites in Feb. 2007 in direction of the Andromeda galaxy
The binary merger scenario is excluded with a 99% confidence level!
90%
75%
50%
25%
Inspiral Exclusion Zone
99%
Astrophys. J. 681(2008) 1419
Burst Signals: All-Sky SearchBurst Signals: All-Sky Search
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Description of the search● Multiple Algorithms● Broad-band frequency search● Coincident and coherent searches● Very large variety of waveforms● Robustness
Upper limit (sine-gaussian)
Rate (90% C.L.) vs. frequencyNo detection
With a 90% confidence level, the rate of burst signals with 50 < f < 2048 Hz is lower than 2 events per year
Phys. Rev. D 81(2010) 102001
~8e-7 yr-1 Mpc-3
1e-2 yr-1 Mpc-3
100 f(Hz) 1000
(EGW
= Msun
c2)
Burst Signals: Neutron StarsBurst Signals: Neutron Stars
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2006/08/12: timing glitch observed in the radio emission of the Vela pulsar
Quasi-normal mode oscillations GW
Both Hanford detectors were up at that timeRingdown signal is searched → No detectionUpper limits on GW energy released by mono-harmonic modes
Phys. Rev. D83(2011) 042001
Soft Gamma Repeaters (SGR) / Anomalous X-ray Pulsars (AXP)
Neutron stars powered by extreme magnetic fields (magnetars)
6 magnetars have been analyzed by a dedicated pipeline (excess power)
SGR 1900+14SGR 0418+5729SGR 1627-41SGR 1806-20SGR 0501+4516AXP 1E 1547.0-4508
arXiv:1011.407946 52
GW
GW
Burst Signals: Cosmic String CuspsBurst Signals: Cosmic String Cusps
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When 2 cosmic string segments meet they can reconnect and produce loops. The main mechanism for the loop to loose its energy is to radiate gravitationally.
GW radiation is the most promising signature to detect cosmic strings.
Points of the string can acquire a large Lorentz boost and form a "cusp"→ GW burst
S4
S5 Projection
Well-modeled signalTemplate burst searchDedicated pipeline
Upper limits on the cosmic string parameter space:
Gμ: String tensionε: loop size parameterp: reconnection probability
GμPhys Rev D 80(2009) 062002 10 –610 –7
Online SearchesOnline Searches
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Low latency searches took place during S6 / VSR2-3 for both CBC and burst searches
Most significant events were sent to telescopes / satellites(14 events for S6/VSR2-3)
Sky localization is performed with a resolution of ~tens of square degrees for events at threshold
Image analysis is performed within the collaboration
See M. Branchesi's talk
Hanford Livingstone Virgo
Central Location
CBC pipelineMBTA
Burst pipelineOmega
Burst pipelineCoherent Waveburst
CandidateDatabase
Most significantCandidate selection
h(t) h(t) h(t)
h(t)h(t)h(t)
event event event
~ 30 min
Swift Zadko ROTSE
S6/VSR2-3 Science RunS6/VSR2-3 Science Run
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Improved sensitivity
~ 200 days of live-time with at least 2 detectors up
Big challenge: run analyses online– 3 pipelines were running (2 bursts + 1 CBC)– The data quality was performed with low latency (< 1 min)– EM follow-up by partner telescopes
Offline analyses are in progress (some results are released)
Analyses pipelines and data quality tools have been improved for a better sensitivity
Blind hardware injection challenge was successful(we detected it with great confidence). See T. Dent's talk
Preparation for Advanced Detectors EraPreparation for Advanced Detectors Era
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Sensitivity improvement by a factor 10
This translates into a detection rate up to 40 neutron star binary events per year
Science should resume in 2015Design sensitivity achieved by 2019
What are we going to do in the meantime ?
Virgo might run this summer (VSR4) along with the GEO detector Similar sensitivity at high frequencyThis run could be of some interest for external triggered searches
Then GEO will run alone in astrowatch mode during the construction of Adv. detectors (2012-2015)
Some mock data runs are planned to test and improve our searches
A lot of work is required to optimize the EM-followup procedures
Summary - ConclusionsSummary - Conclusions
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A large variety of physical results have been produced from the LIGO-Virgo data
There is no detection yet but upper limits can be used to constrain astrophysical
models
The first generation of detectors is close to the end.
– Analysis pipeline have greatly improved over the last years to perform
optimized and sensitive searches on GW data
– Data quality is a great challenge. Multiple tools have been developed to reject
noise events efficiently.
– GW astronomy is on its way: online searches, Multi-messengers, EM followup
Now, the big challenge is to be fully ready for the Advanced Detectors Era