A High Frequency Burst Search with the LIGO Interferometers Brennan Hughey MIT-LIGO and the LIGO Scientific Collaboration February 15th 2008 MIT Postdoc Symposium

Gravitational waves: In GR, propagate the gravitational force Result from changing quadrupole moment of mass (thus requiring a system that isn't spherically symmetric) Propagate at the speed of light. Distort space itself, stretching in one direction and squeezing in the perpendicular direction, then vice versa. Are extremely weak: only hope of detecting them is via huge gravitational wave generators (astrophysical objects)

The Evidence for Gravitational Waves Radio pulsar B1913+16, discovered in 1974 by Hulse and Taylor as part of a binary system Long-term radio observations have yielded neutron star masses and orbital parameters System shows very gradual orbital decay just as general relativity predicts! Very strong indirect evidence for gravitational radiation

Interferometer Concept Orthogonal arm lengths change in different ways as they interact with a gravitational wave Use laser to measure relative lengths L/L by observing the changes in interference pattern at the anti-symmetric port, for example, for L ~ 4 km and for a hypothetical wave of h ~ 10–21 L ~ 10-18 m ! Power-recycled Michelson interferometer with Fabry-Perot arm cavities

Ground interferometers’ noise budget Best strain sensitivity ~3x10-23 1/Hz1/2 at 200 Hz Displacement Noise Seismic motion Thermal Noise Radiation Pressure Sensing Noise Photon Shot Noise Residual Gas Facilities limits much lower Several ground interferometers are currently operating at or near design sensitivity

Interferometric Detectors TAMA 300m Japan VIRGO 3km Italy LIGO Louisiana 4km USA CLIO 100m Japan GEO 600m Germany LIGO Washington 2km& 4km USA

LIGO Laser Interferometer Gravitational-wave Observatory Hanford, Washington: 2 km and 4 km detectors Livingston, Louisiana: 4 km detector 10 ms light travel time Managed and operated by Caltech and MIT with NSF funding LIGO Scientific Collaboration – 500+ researchers from 45 institutions worldwide run and analyze data from the LIGO and GEO instruments

Sources And Methods Template-less methods Matched filter Short Bursts Stochastic Background Template-less methods Compact Binary Inspirals Pulsars Matched filter Short duration Long

LIGO Science Runs and Sensitivities S1: 23 Aug – 9 Sep ‘02 S2: 14 Feb – 14 Apr ‘03 S3: 31 Oct ‘03 – 9 Jan ‘04 S4: 22 Feb – 23 Mar ‘05 S5: 4 Nov ‘05 – Oct '07 S5 represents a full year livetime of triple-coincident science quality data

High Frequency Burst Search Analyzes data in 1-6.5 kHz range whereas previous burst searches have been limited to region < 2 kHz Currently being performed over 1st calendar year of LIGO's 5th science run and will be extended to 2nd calendar year. A “Burst” search – looking for short transient signal without relying on a specific model of emission Mirrors lower frequency analysis but with needed adjustments for higher frequency Work conducted with Erik Katsavounidis Michele Zanolin and others

Sources of High Frequency Waves Models which have resulted in predictions of high frequency burst signals include: Stellar collapse as predicted by Baiotti, Rezzola et al. Phys.Rev.Lett. 97 (2006) 141101 Class. Quant. Grav. 24 (2006) S187-S206 Burrows-Ott supernovae Ap J 600 (2004) 834-864 Low mass black hole mergers Phys.Rev.Lett. 91 (2003) 021101 Nonaxisymmetric hypermassive neutron stars as predicted by Oeschlin and Janka astro-ph/0702228v

Analysis Pipeline Qpipeline Hanford Postprocessing CorrPower Environmental Vetoes Event Candidates Qpipeline Livingston Next few slides will address these steps individually........

QPipeline The QPipeline is a multi-resolution time-frequency search for statistically significant excess signal energy Targets gravitational wave bursts of unknown waveform Projects whitened data onto an overlapping bank of complex valued sinusoidal Gaussians characterized by central time , central frequency , and Q (ratio of central frequency to bandwidth): Equivalent to a templated matched filter search for waveforms that are sinusoidal Gaussians after whitening Measures the normalized tile energy Z, matched filter SNR , and white noise significance P, where Reports the minimal set of non-overlapping templates that best describes the signal.

Postprocessing Qpipeline generates thousands of triggers, Livingston Observatory 4 km interferometer L1 Hanford Observatory 4 km and 2 km interferometers H1 and H2 Qpipeline generates thousands of triggers, which must then be subjected to postprocessing A coincidence window is applied between sites GW's propagate as plane wave traveling at c We require each Hanford trigger have a matching trigger at Livingston within 20 ms (longer than light travel time) We also require consistent central frequencies The surviving triggers are clustered Triggers are combined into clusters of 1 second with the characteristics of the most dominant trigger, otherwise the same gravitational wave or other disturbance would generally produce multiple triggers Data quality flags are applied to remove time segments wherein we don't trust the data sufficiently Flags include periods with known seismic or wind activity or problems originating within the detectors themselves There is a fundamental set of data quality flags applied to both high and low frequency analyses, but we studied the effects of less obvious flags specifically at high frequencies to decide which to use

CorrPower CorrPower finds correlated power between waveforms in the various interferometers. It produces an output variable (Gamma) based on the maximum observed correlation using Pearson's linear correlation statistic. CorrPower output is normalized so that it doesn't consider overall energy, just degree of correlation, and thus provides a method of background rejection which is largely independent of Qpipeline Unphysical time lags (on order 5 to a few hundred seconds) between the two sites were used to tune the cuts

Environmental Vetoes Elimination of triggers with known external causes Trigger-by-trigger basis rather than specific time periods as in data quality flags Seismic/wind: seismometers, accelerometers, wind monitors Sonic/acoustic: microphones Magnetic fields: magnetometers Line voltage fluctuations: volt meters

Status and Future Plans Finalizing 1st year analysis, in particular environmental vetoes Will soon move on to second year Will incorporate Virgo, which has sensitivities equivalent to that of LIGO at high frequencies and began its 1st science run near the end of LIGO's S5

Advanced LIGO Factor 10 better amplitude sensitivity (Reach)3 = rate Factor 4 lower frequency bound Infrastructure of initial LIGO but replace many detector components with new designs Advanced LIGO Increase laser power in arms. Better seismic isolation. Quadruple pendula for each mass Larger mirrors to suppress thermal noise. Silica wires to suppress suspension thermal noise. “New” noise source due to increased laser power: radiation pressure noise. Signal recycling mirror: Allows tuning sensitivity for a particular frequency range. 18

Enhanced LIGO LIGO today ~2009 Advanced LIGO ~2014 100 million light years Advanced LIGO ~2014 19