Searching for Gravitational-Wave Bursts

1. Searching for Gravitational-Wave Bursts Isabel Leonor University of Oregon for the LIGO Scientific Collaboration (LSC) LIGO-G060528-01-Z

2. Gravitational-Wave Bursts: Sources and Searches SNR E0102-72 NASA, Chandra, SAO, Hubble ATNF, ATCF Astrophysical sources: • possible sources are core-collapse supernovae, coalescing compact binaries, gamma-ray burst (GRB) engines, soft gamma repeaters, ??? • waveforms of gravitational-wave burst signals from these sources are unknown • untriggered search: perform an all-sky search using entire data set • triggered search: analyze data which are contemporaneous with astrophysical events observed by other experiments, e.g. GRBs found by satellite experiments Search methods: GRB credit: Laura Whitlock, GSFC, NASA NSF Annual Review LIGO-G060528-01-Z

3. Untriggered Search NSF Annual Review LIGO-G060528-01-Z

4. time-frequency plane Overview of the Most Mature Search Pipeline WaveBurst algorithm: • measures signal power in time-frequency plane • signal time scale < 1 second • frequency range of 64-1600 Hz • for each interferometer, WaveBurst searches for signals with power in excess of baseline noise • be seen in all three interferometers with a combined, overall significance above a pre-determined WaveBurst threshold • be “coincident” in time at the three interferometers; have consistent amplitudes at H1 and H2; have consistent waveforms at the three interferometers We require a candidate GW signal to: We estimate the background rate by: • shifting the LLO data stream relative to the LHO data streams, and then identifying events which pass the selection requirements We analyze many auxiliary channels to decide on data quality cuts and vetoes NSF Annual Review LIGO-G060528-01-Z

5. S4 Search Sensitivity for Sine-gaussian Waveforms preliminary • sensitivity of search is measured by injecting burst-like waveforms, e.g. sine-gaussians, into data and performing search • hrss: root-sum-square strain amplitude of injected waveform • efficiency curves give a measure of the sensitivity of the search at different frequencies • e.g. at 153 Hz, 50% efficiency at hrss = 1.2e-21 strain/Hz1/2 We also use other types of waveforms. Similar sensitivities expected for different waveforms with similar overall properties. NSF Annual Review LIGO-G060528-01-Z

6. S4 Search Results No event candidates pass all cuts preliminary • set upper limit on rate of gravitational-wave bursts (90% C.L.): paper in preparation WaveBurst significance cut waveform consistency cut NSF Annual Review LIGO-G060528-01-Z

7. Rate exclusion curves for Q=8.9 sine-gaussian waveforms Comparison of S4 Run Results to Published Results from Previous Runs preliminary • S4 rate upper limit: • S2 rate upper limit: • S1 rate upper limit: paper in preparation published published (S3 sensitivity: ~10e-20 strain/Hz1/2) published NSF Annual Review LIGO-G060528-01-Z

8. efficiency S5 Run Preliminary Burst Search Results under internal review Efficiency curves for Q=8.9 sine-gaussian waveforms • using a high threshold for WaveBurst search, no candidate GW events were found in S5 data up to October 8, 2006 • low-threshold search will also be performed • S5 burst search sensitivity for low-threshold search is about a factor of 2 better than S4 sensitivity • e.g. at 153 Hz,50% efficiency ofhrss = 5.8e-22 strain/Hz1/2 Still being tuned. NSF Annual Review LIGO-G060528-01-Z

9. Triggered Searches NSF Annual Review LIGO-G060528-01-Z

10. Triggered Searches • analyze segments of data which coincide with astrophysical events observed by other experiments to look for associated gravitational-wave bursts • one can think of this as “pointing” the detectors at the target astrophysical source for short time intervals –-- this decreases the amount of background noise significantly • cosmic gamma-ray bursts (GRB) detected by satellite experiments • hyperflares from soft gamma repeaters (SGR) or magnetars detected by satellite experiments • crosscorrelate interferometer data streams in pair-wise manner to search for correlated signals • search for excess power in data spectrum We currently use the following astrophysical events as triggers: Analysis methods: NSF Annual Review LIGO-G060528-01-Z

11. HETE light curve for GRB 030329 Emergence of supernova spectrum Hjorth, et al. 2003, Nature, 423, 847 Search of S2 LIGO data showed no GW burst signal associated with this GRB. The measured hrss upper limits correspond to an energy emitted in GW waves of: published Gamma-ray Burst 030329 / SN2003dh (S2 Run) • bright and “nearby” GRB • z = 0.1685, DL = 800 Mpc We expect to improve on this! NSF Annual Review LIGO-G060528-01-Z

12. 39 GRB Triggers used to Search for GW Bursts in S2, S3, S4 Runs preliminary We found no evidence for gravitational-wave burst signals associated with this GRB sample • S4 best hrss upper limit (150 Hz, linear polarization): • 3.6 x 10-21 strain/Hz1/2 • S3 best hrss upper limit(250 Hz, linear polarization): • 2.5 x 10-20 strain/Hz1/2 • S2 best hrss upper limit(250 Hz, linear polarization): • 2.7 x 10-20 strain/Hz1/2 paper in preparation NSF Annual Review LIGO-G060528-01-Z

13. using data up to Aug. 1, 2006 unreviewed For a short-duration GRB at 100 Mpc, we might expect (for circular polarization): GRB Triggers for S5 Run under internal review • Swift satellite experiment was launched November 2004, started detecting GRBs • 10 GRB triggers per month • 117 GRB triggers for S5 as of October 7, 2006 • no evidence for GW bursts associated with the GRB sample up to Aug. 1, 2006(unreviewed) • best S5 hrss limit so far (250 Hz, linear polarization): • 1.7 x 10-21 strain/Hz1/2 NSF Annual Review LIGO-G060528-01-Z

14. 92.5 Hz 92.7 Hz (RHESSI) 92.5 Hz (RXTE) counts/sec Search for GW Signal Associated with December 2004 Hyperflare from SGR 1806-20 under internal review 25-100 keV (Watts and Strohmayer, 2006, ApJL, 637, L117) • searched LIGO data for GW signal associated with quasi-periodic oscillations at different frequencies; no GW signal found • the best hrss limit of 2.75 x 10-22 strain/Hz1/2at • f = 92.5 Hz corresponds to an isotropic energy release in gravitational waves of: • searches were also made at other frequencies: 150.3 Hz, 626.5 Hz, 1837.0 Hz • galactic neutron star with huge magnetic field • at a distance of 7.5-15 kpc • source of record gamma-ray flare on December 27, 2004 • flares thought to be caused by “starquakes” – magnetic stresses cause crust to break and release energy • had pulsating tail lasting 6 minutes – quasi-periodic oscillations found NSF Annual Review LIGO-G060528-01-Z

15. Other Ongoing Projects Related to Burst Searches In our zeal to dig out gravitational-wave signals from the data, we have developed several analysis projects and search algorithms, in various stages of maturity, among them: • Joint analyses with GEO and Virgo • Network analysis methods • Q Pipeline • KleineWelle • Excess Power • AstroBurst • GRB population study • supernovae • neutrinos • other hyperflare events from soft gamma repeaters For triggered searches, we plan to expand the astrophysical sources used as triggers, e.g., NSF Annual Review LIGO-G060528-01-Z

16. Summary • We have gained considerable experience in searching LIGO data for gravitational-wave bursts  several published papers, more in the works • We perform both an all-sky, untriggered search, and searches triggered by astrophysical events detected by other experiments • There was about an order of magnitude improvement in sensitivity from the S2 to the S4 run • There is about a factor of 2 improvement in sensitivity from the S4 to the S5 run • The burst group is abuzz with activity as we continue to improve our search algorithms and develop methods to search for gravitational-wave bursts! NSF Annual Review LIGO-G060528-01-Z