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Co-Located Interferometers: Overlap Reduction Function

Toward Enabling Co-Located Interferometric Detectors to Provide Upper Limits on the Stochastic Gravitational Wave Background Nick Fotopoulos, MIT On Behalf of the LIGO Scientific Collaboration 2005-12-15 GWDAW-10 @ UT Brownsville. Co-Located Interferometers: Overlap Reduction Function.

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Co-Located Interferometers: Overlap Reduction Function

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  1. Toward Enabling Co-Located Interferometric Detectors to Provide Upper Limits on the Stochastic Gravitational Wave Background Nick Fotopoulos, MIT On Behalf of the LIGO Scientific Collaboration 2005-12-15 GWDAW-10 @ UT Brownsville

  2. Co-Located Interferometers: Overlap Reduction Function H1-H2 promises significantly enhanced sensitivity over H1-L1, especially at higher frequencies BUT Co-located detectors are subject to environmental correlation

  3. H1-H2 Coherence (S4) Coherence squared f (Hz) Very coherent in the instrument’s “sweet spot”. N = 109821 1/N ≈ 9.1·10-6 S1-S4 measurements of eff not consistent with zero

  4. S1 Results S1

  5. Squaring Coherence Sensitivity Theorem: For all Z{PEM channels}, coh(H1,H2)≥coh(H1,Z)•coh(H2,Z) Corollary: coh(H1,H2)≥ coh(H1,Z)•coh(H2,Z) max Z 1/N 1/N 1/N2

  6. Tracking Environmental Coupling in H1-H2 S4 had 107PEM channels in RDS_R_L1 S5 will have roughly the same

  7. Maximum of PEM Coherence Products: Frequency Veto Vetoed regions (H1-H2 1/N ~ 10-5, PEM-IFO 1/N ~10-4 due to resolution choices) Threshold10-5.5 1/N2 • Maximum of PEM coherence products follows H1-H2 measured • coherence very closely (within error) • With this (semi-arbitrary) threshold, 56% bins lost in [50,350]Hz • and 48% bins lost in [50,500]Hz, 30% in [50,1024]Hz

  8. coh(H1,H2) post-vetohistogram  exp(-N2) Success! • This (unreviewed) pipeline results in noticeably reduced • significance for the point estimate • We have physical basis for veto We are one step closer to setting upper limits with the H1-H2 pair!

  9. Detector Characterization Have determined environmental coupling out to 1kHz. Can identify strongest sources at each frequency!

  10. A Few Words on instr eff = instr+ GW • We must estimate or bound instr • Attempts to take this into account in S3 resulted in an GW upper limit a few times worse than the H1-L1 upper limit • As we have flagged and eliminated the major sources of instrumental correlation, instr is greatly reduced • Other sources: Incomplete PEM coverage, non-linear environmental couplings…

  11. H1-H2 and the Future • The new technique: Take maximum across coherence products coh(H1,Z)*coh(H2,Z). • The new capability: H1-H2 can provide upper limits at high frequencies, which are inaccessible to H1-L1. • S4 was playground and proof of concept; will not publish upper limit from H1-H2. • S5 will have “blind” frequency vetoes and the resulting point estimate is planned for publication. • With some confidence in H1-H2, we can begin looking for astrophysical sources of stochastic radiation, which is expected to peak at frequencies >200Hz.

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