G Z Searches for bursts of gravitational waves with LIGO, GEO and Virgo E. Katsavounidis MIT for the LIGO Scientific Collaboration and the Virgo Collaboration APS Meeting St. Louis, MO, April 13, 2008

G Z 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/

G Z 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 LIGO, GEO and Virgo interferometers

G Z 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 The running of the instruments

G Z Instrument sensitivities

G Z 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)

G Z Burst searches in a nutshell Basic assumption: multi-interferometer response 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) frequency time 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 Instument 1 Instument 2

G Z 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) H1 L1

G Z How do we identify candidates? the LIGO S4 search example Signal candidate region No event passed all analysis cuts Background 3 events out of ~77 effective S4 runs LIGO S4 search: CQG 24 (2007) 5343 Zg : combined significance of excess power  combined significance of waveform correlation

G Z 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) = h 0 sin(2  ft) exp(-2(  ft/Q) 2 ), linearly polarized; random sky position & polarization angle Measure detection efficiency in terms of LIGO S4 search: CQG 24 (2007) 5343

G Z Results from previous all-sky searches 4 science runs of the LIGO-GEO instruments in »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

G Z 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 f 0 and quality factor Q: Assume for a sine-Gaussian-like signal, 153 Hz, Q=8.9, h rss 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

G Z Status of the S5 and VSR1 burst searches 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 Year-1 of S5

G Z Coherent analysis methods Coherent sum: Find linear combinations of detector data that maximize signal to noise ratio Null sum: Linear combinations of detector data that cancel the signal provide useful consistency tests. data = response x signal + noise coherent sum N-2 dimensional null space detector data coherent null 2 dimensional signal space Naturally handles arbitrary networks of detectors Analysis repeated as a function of frequency and sky position Produces significance and consistency sky maps Gursel and Tinto, PRD 40 (1989) 3884 Klimenko, et al., PRD 72 (2005) Rakhmanov, CQG 23 (2006) S673 Wen and Schutz, CQG 22 (2005) S1321 Chatterji, et al. PRD 74 (2006)

G Z An example of coherent network method: 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) L1 H1 H2 time +  + frequency

G Z Some features of S5 data 380 days since Nov.15, Hz-2048 Hz 1 year of coherent WaveBurst triggers (100 time-lags) preliminary GPS days (since Jan 6, 1980) Effective correlated SNR

G Z S5 data: frequency character From Q-pipeline: excess w/r/t gaussian of unclustered triggers with SNR>5 A random H1 day, no data quality flags applied preliminary

G Z S5 detection efficiency Instantaneous energy flux: Assume isotropic emission to get rough estimates For a sine-Gaussian with Q>>1 and frequency f 0 : Detection Probability preliminary A factor ~2 improvement in sensitivity with respect to LIGO’s S4

G Z The LIGO-GEO-Virgo network during 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

G Z Searches triggered by GRBs 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 –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 Gareth Jones for the LSC and Virgo, on “Coherent Network Searches for Gravitational Waves associated with Gamma-Ray Bursts” in session E8 (yesterday!)

G Z 180 seconds LIGO detector 1 LIGO detector 2 Cross-correlate output of two detectors look for largest cross-correlation within 180-second on-source segment use 180-second LIGO on-source data surrounding GRB trigger 21 Search for model-independent bursts associated with GRB The prototypical GRB search in LIGO is based on the cross-correlation statistic between 2 instruments correlated signal in two detectors  large crosscorr GRB trigger time from satellite

G Z Burst search results: probability of largest 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: 22 p = 0.58 for 25-ms cc p = 0.96 for 100-ms cc  consistent with null hypothesis LIGO Scientific Collaboration: arXiv: accepted by ApJarXiv: Measured max on-source cc

G Z Sensitivity of unmodelled burst search associated with GRB Corresponding upper limit in energy emitted in GW, assuming isotropic emission, with source at D = 770 kpc: Inject simulated sine-Gaussians into data to estimate search sensitivity Take into account antenna response of interferometers 50% efficiency LIGO noise spectral densities

G Z LSC and Virgo results from GRB searches No gravitational wave burst signals found associated with GRBs analyzed by LIGO and Virgo so far: »GRB [LIGO Scientific Collaboration: arXiv: accepted by ApJ]arXiv: »39 GRBs in S2, S3, S4 runs [LIGO Scientific Collaboration: Phys. Rev. D 77, (2008), Phys. Rev. D 72, (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

G Z 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]

G Z 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?

G Z 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 GRB only a first glimpse of what is to come Preparing analyses for the next step in gravitational wave astronomy: the enhanced detectors in the horizon and the increase in science reach O(10) that they will bring Stay tuned! Conclusions