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## Gravitational wave bursts

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**Searches for bursts of gravitational waves with LIGO, GEO**and VirgoE. KatsavounidisMITfor the LIGO Scientific Collaborationand the Virgo CollaborationAPS MeetingSt. Louis, MO, April 13, 2008**Gravitational wave bursts**• Short-lived signals, lasting only a few cycles within the frequency band of the instruments • Typically from few milliseconds to few seconds and with frequency content in the 100 Hz - few kHz regime • Unknown or poorly known waveforms • Sources • Core-collapse supernova • Merger phase of binary compact objects • Neutron star instabilities • Cosmic string cusps and kinks • The unexpected! • Need to make minimal assumptions about the candidate signal Dimmelmeier et al., astro-ph/0702305**The LIGO, GEO and Virgo interferometers**• Laser Interferometer Gravitational-wave Observatory (3 instruments) • Hanford, WA: 2 km and 4 km detectors • Livingston, LA: 4 km detector • 3000km/10ms separation • GEO600 • German-UK collaboration, 600 m instrument near Hannover, Germany • Virgo • French-Italian collaboration, 3 km instrument near Pisa, Italy**The 2005-2007 running of the instruments**• LIGO-GEO (LSC) Fifth Science Run (S5) • Nov 4, 2005 – Oct 1, 2007 • >365 days of 3-instrument coincidences • >400 days of 2-site coincidences • Duty cycle: 78% for the H1, 79% for the H2 and 66% for L1 • Virgo Science Run 1 (VSR1) • May 18, 2007 – Oct 1, 2007 • >75 days of 3-site coincidences • >95 days of 2-site coincidences • Duty cycle: 81% for Virgo • LSC-Virgo started data-sharing on May 18, 2007 • All data collected by the instruments after that day are being analyzed jointly • Instruments at or close to their design sensitivities**Burst search goals**• Direct detection of gravitational wave bursts from astrophysical sources, or otherwise upper limits on their flux and energy emitted into gravitational waves • Eyes wide open search for burst-like signals with minimal or no assumption about their waveform details “untriggered” searches performed over the whole sky and over all collected data • Searches guided by astronomical observations in the electromagnetic spectrum “triggered” searches customized around the times of Gamma-Ray Bursts (GRBs) and Soft Gamma Repeaters (SGRs)**Burst searches in a nutshell**frequency time Instument 1 • Basic assumption: multi-interferometerresponse consistent with a plane wave-front incident on network of detectors • Time-frequency decomposition of data • Project data stream on a Fourier or wavelet basis • Normalize to noise, threshold on power and form clusters • Require coincidence (time, frequency) Instument 2 • Require trigger clusters to fall within some frequency band and have excess signal power • Repeat this for O(100) “time-shifts” of all data • Tuning and background studies • Set threshold to yield O(0.01) accidentals LIGO S4 search: CQG 24 (2007) 5343**H1**L1 Burst searches in a nutshell • Data quality and vetoes • high seismic noise, wind, jets, calibration line drop-outs, last 30 seconds of each segment • Consistency checks • Amplitude (h) reconstruction by the two co-located Hanford detectors • Cross-correlation of h(t) data from pairs of detectors (20, 50 and 100ms) • Sign of H1-H2 correlation • End result: coincidence events characterized by a statistical significance, time, frequency, amplitude and waveform similarity among detector sites • For “zero-lag” between the sites (i.e., where astrophysical signal may be present) • For “time-delayed” between the sites (i.e., where astrophysical signal can not be present)**How do we identify candidates?the LIGO S4 search example**LIGO S4 search: CQG 24 (2007) 5343 Signal candidate region G : combined significance of waveform correlation Zg : combined significance of excess power • No event passed all analysis cuts • Background 3 events out of ~77 effective S4 runs**Are we capable of detection?**• Signal injections: both in hardware (by shaking the mirrors) and in software provide • the data set for tuning analysis cuts • the ultimate measurement of the sensitivity of the instruments and the search pipeline overall • Example: h(t) = h0 sin(2pft) exp(-2(pft/Q)2), linearly polarized; random sky position & polarization angle • Measure detection efficiency in terms of LIGO S4 search: CQG 24 (2007) 5343**Results from previousall-sky searches**• 4 science runs of the LIGO-GEO instruments in 2002-2005 • Total livetime analyzed: ~35 days, only triple coincidence • No event candidates observed • Upper limit (90% confidence level) on rate of detectable events set • Interpret bound on a rate vs. strength exclusion diagram • LIGO-TAMA, LIGO-AURIGA, LIGO-GEO • First methodological studies with a diverse network of detectors • Search over 5 days in September 2005 from Virgo’s “C7” run is nearing completion • Sensitivity comparable to LIGO’s S2 S1 S2 S4 LIGO S4 search: CQG 24 (2007) 5343**Mass equivalence:order of magnitude analysis**• Instantaneous energy flux: • Integrate over signal duration and over a sphere at radius r assuming a sine-gaussian signal of frequency f0 and quality factor Q: • Assume for a sine-Gaussian-like signal, 153 Hz, Q=8.9,hrss at 50% efficiency is 6.5 x 10–22 Hz–1/2 • 2 x 10–8 M emitted at 10 kpc • 0.05 M emitted at Virgo Cluster**Status of the S5 and VSR1burst searches**Year-1 of S5 • All 2 and 3 LIGO instrument coincidence livetime is being analyzed • GEO: detection follow up instrument • Post-May 2007 data corresponding to coincident LIGO and Virgo data taking are being analyzed jointly • Analyses are well underway, but not yet complete • Multi-methods approach – full scale implementation of coherent network techniques • Today: a first look at methods and science reach during S5-VSR1**coherent null**N-2 dimensionalnull space detector data coherent sum 2 dimensionalsignal space Coherent analysis methods • Naturally handles arbitrary networks of detectors • Analysis repeated as a function of frequency and sky position • Produces significance and consistency sky maps Coherent sum: Find linear combinations ofdetector data that maximizesignal to noise ratio data = response x signal + noise Null sum: Linear combinations ofdetector data that cancelthe signal provide usefulconsistency tests. • Gursel and Tinto, PRD 40 (1989) 3884 • Klimenko, et al., PRD 72 (2005) 122002 • Rakhmanov, CQG 23 (2006) S673 • Wen and Schutz, CQG 22 (2005) S1321 • Chatterji, et al. PRD 74 (2006) 082005**H2**H1 L1 An example of coherent networkmethod: coherent WaveBurst • End-to-end pipeline to search for unmodeled gravitational wave bursts • Coherent statistic – constrained likelihood - is used both for detection and signal reconstruction • Time-frequency analysis is done using wavelets • Analysis of multiple TF resolutions: f=8,16,32,64,128,256 Hz • Reconstruction of source position and waveforms coherent statistic L(t,f) + + frequency time**Some features of S5 data**380 days since Nov.15, 2005 1 year of coherent WaveBurst triggers (100 time-lags) preliminary Effective correlated SNR 64 Hz-2048 Hz GPS days (since Jan 6, 1980)**S5 data: frequency character**preliminary From Q-pipeline: excess w/r/t gaussian of unclustered triggers with SNR>5 A random H1 day, no data quality flags applied**S5 detection efficiency**preliminary A factor ~2 improvement in sensitivity with respect to LIGO’s S4 Instantaneous energy flux: Detection Probability Assume isotropic emission to get rough estimates For a sine-Gaussian with Q>>1 and frequency f0 :**The LIGO-GEO-Virgo networkduring S5 and VSR1**• Materialize the benefits of a gravitational wave detector network • Detection confidence • Reduce false alarm rate • Coherent consistency tests can differentiate between gravitational-wave signals and instrumental anomalies • Improve parameter estimation (source sky position, time-frequency volume, amplitude) • Extract both polarizations of the signal waveforms • Increased sky coverage • Increased coincident observation time • Incoherent (coincidence-based), fully coherent and hierarchical methods are being applied to joint data • Benchmarking of methods on ‘playground’ data currently in progress**Gamma-Ray Bursts (GRBs) occur at cosmological distances at**about 1/day rate Long duration (>2s) ones may be associated with “hypernovae” Short duration (< 2s) may have a binary NS-NS or NS-BH progenitor A network of satellites (Swift, HETE-2, INTEGRAL, IPN, Konus-Wind) provides GRB alerts close to real-time 39 GRBs during S2,S3,S4 with >=2 LIGO instruments online 213 GRB triggers from November 4, 2005 to September 30, 2007 (S5) Among them, GRB 070201 short hard burst with position consistent with M31 Andromeda (~770kpc) the two LIGO Hanford detectors were on at the time of the GRB Time-of-flight between instruments and their antenna response can be accounted for in the analysis given the known sky position (and time) of GRB Searches triggered by GRBs Gareth Jones for the LSC and Virgo, on “Coherent Network Searches for Gravitational Waves associated with Gamma-Ray Bursts” in session E8 (yesterday!)**use 180-second**LIGO on-source data surrounding GRB trigger Cross-correlate output of two detectors look for largest cross-correlation within 180-second on-source segment Search for model-independent burstsassociated with GRB 070201 • The prototypical GRB search in LIGO is based on the cross-correlation statistic between 2 instruments 180 seconds LIGO detector 1 correlated signal in two detectors large crosscorr on-source segments GRB trigger time from satellite LIGO detector 2 21**Burst search results: probability oflargest on-source**cross-correlation applied search to off-source segments used three hours of off-source data surrounding on-source segment to estimate background distribution of largest cross-correlation false alarm probability of on-source largest cross-correlation is estimated using this distribution: Measured max on-source cc Measured max on-source cc p = 0.58 for 25-ms cc p = 0.96 for 100-ms cc • consistent with • null hypothesis 22 LIGO Scientific Collaboration: arXiv:0711.1163 accepted by ApJ**Sensitivity of unmodelled burst searchassociated with**GRB070201 Inject simulated sine-Gaussians into data to estimate search sensitivity Take into account antenna response of interferometers 50% efficiency LIGO noise spectral densities Corresponding upper limit in energy emitted in GW, assuming isotropic emission, with source at D = 770 kpc: 23**LSC and Virgo results fromGRB searches**• No gravitational wave burst signals found associated with GRBs analyzed by LIGO and Virgo so far: • GRB070201 [LIGO Scientific Collaboration: arXiv:0711.1163 accepted by ApJ] • 39 GRBs in S2, S3, S4 runs [LIGO Scientific Collaboration: Phys. Rev. D 77, 062004 (2008), Phys. Rev. D 72, 042002 (2005) ] • GRB050915a: a prototypical search using Virgo data [Virgo Collaboration: Class. Quantum Grav. 24 (2007) S671] • Upper limits on the gravitational wave signal amplitude associated with each GRB were set via software injections: • Assume a model for the gravitational wave emission (waveform) • Take into account the antenna response given the GRB position • Translate to energy going into gravitational waves if distance to the GRB is known • GRB population study • Searches for gravitational wave bursts associated with GRBs during S5 and VSR1 are ongoing • Go beyond prototypical searches by exploring the full power of coherent methods and the LIGO-GEO-Virgo network**Other ongoing burst searches reporting at this meeting**• Searches triggered by Soft Gamma Repeaters (SGRs) [Peter Kalmus for the LSC, session E8] • Search for high frequency (>2kHz ~6kHz) bursts [Brennan Hughey for the LSC, poster session]**Burst search outlook**• LIGO and Virgo are currently undergoing upgrades that will provide a 2-3 factor sensitivity improvement in O(1year) from now at which time S6 and VSR2 will commence • An “Astrowatch” run currently collects data with the 2-km LIGO Hanford detector and GEO an insurance policy for nearby supernovae • Data being analyzed primarily for detector monitoring and characterization purposes as close to real time as possible • More in-depth analysis expected to take place for significant events reported by electromagnetic (or particle) observations • Emphasis in setting up infrastructure and defining protocol for real-time burst searches in S6 and VSR2 • Minute-scale latency targetted • Use of global network to reconstruct sky positions • Possibly allow prompt electromagnetic follow-up, • Work out calibration, data quality and veto in real-time • Possible?**Conclusions**• A global network of gravitational wave detectors has been formed and data from them are being analyzed jointly • Burst searches with data collected by LIGO, GEO and Virgo in their most recent S5 and VSR1 runs are well underway • Current analyses are the first step towards gravitational wave astronomy: • establish and mature “detection checklists” for searches • utilize global network for solving inverse problem in gravitational wave detection • first S5 burst result associated with GRB070201 only a first glimpse of what is to come • Preparing analyses for the next step in gravitational wave astronomy: the enhanced detectors in the 2009+ horizon and the increase in science reach O(10) that they will bring • Stay tuned!