All-sky LIGO Search for Periodic Gravitational Waves in the Fourth Science Run (S4) Keith Riles University of Michigan For the LIGO Scientific Collaboration.

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The Search For Periodic Gravitational Waves

Presentation transcript:

All-sky LIGO Search for Periodic Gravitational Waves in the Fourth Science Run (S4) Keith Riles University of Michigan For the LIGO Scientific Collaboration APS Meeting, Jacksonville, Florida April 14-17, 2007 LIGO DCC: G Z

Livingston Hanford LIGO Interferometers & S4 Run Sensitivities H1 (4 km) H2 (2 km) L1 (4 km)

Searching for Spinning Neutron Stars (“Pulsars”) with LIGO Signals expected to be weak – need long integration times Coherent all-sky searches over entire data period are computationally intractable because of parameter space explosion: Doppler modulation corrections require ever-finer sky gridding as the observation time increases Alternative method: “Semi-coherent” summing of spectra from many shorter time intervals: Advantage: computationally tractable Disadvantage: unable to exploit full intrinsic sensitivity of detector Courtesy: NASA

Stack-Slide Hough PowerFlux All methods correct for Doppler modulations: (daily and annual) Time Frequency Time Frequency bin Will summarize and present results from three different methods explored within LIGO to carry out a semi-coherent search: Number of sky grid points scales like (frequency) 2

What exactly is summed? StackSlide – Normalized power (power divided by estimated noise)  Averaging gives expectation of 1.0 in absence of signal Hough – Weighted binary counts (0/1 = normalized power below/above SNR), with weighting based on antenna pattern and detector noise PowerFlux – Average strain power with weighting based on antenna pattern and detector noise  Signal estimator is direct excess strain power (circular polarization and 4 linear polarization projections) Comparing the Methods

Hough (2 of 92 sky patches shown) (North Pole)(Equator) Determines limit (highest-SNR patch) Hough count Determines limit StackSlide StackSlide power Sample 0.25-Hz bands Limit PowerFlux Strain limit Counts

What kind of limits are set? StackSlide & Hough Population-based frequentist limits on h 0 Averaged over sky location and pulsar orientation PowerFlux Strict frequentist limits on circular and linear polarization amplitudes h 0 CIRC and h 0 LIN Results interpreted as limits on best-case and worst-case pulsar amplitudes h 0  Limits placed separately on tiny sky patches  Worst limit over fiducial sky is quoted Comparing the Methods (cont.)

R.A. Dec. Simulated Pulsar (A + = A - = 8.4 x  nearly circ. polarized) Sample PowerFlux circular-polarization strain H1 upper limits in Hz band PRELIMINARY

StackSlide & Hough Line Removal: PowerFlux Line Avoidance: Known lines replaced by random noise (Effects included in Monte Carlo simulation) Regions of Doppler stationarity excluded from quoted limits (frequency & spindown dependent)

What frequency & spindown ranges are covered? [ Hz for all] StackSlide & PowerFlux: –1.0 x Hz/s < df/dt < 0 Hough: –2.2 x Hz/s < df/dt < 0 Results What interferometer data is analyzed? StackSlide & PowerFlux – H1 and L1 individually (coincidence checks for high-SNR candidates) Hough – H1, H2, and L1 combined powers (coincidence check for high-SNR candidates; also: sample single-IFO limits produced for comparison)

H1 (Hanford 4-km) and Multi-IFO Results PRELIMINARY

L1 (Livingston 4-km) and Multi-IFO Results PRELIMINARY

Results & Prospects Carried out follow-up coincidence (frequency, spindown, sky location) studies on outliers from individual interferometers No plausible candidates found Now carrying out analysis of data from ongoing S5 data run with PowerFlux as “first look” algorithm (see talk by V. Dergachev) StackSlide & Hough incorporated into distributed-computing project called using longer coherence times and a hierarchical search algorithm (see talk by B. Owen) You can help!  Go to Upper limits improving and now probing interesting astrophysical territory (h < )  Stay tuned…

END OF SLIDES

Comparing the Methods How are instrumental lines handled? StackSlide & Hough Direct removal of known lines from spectrum (replaced with random noise) Allows entire sky to be searched (population-based limits) PowerFlux Spectral lines flagged on the fly and bins marked for avoidance Source occupancy tracked – no limits placed if source would be lost Leads to exclusion of Doppler-stationary skybands (dependent on frequency and spindown)