1LIGO-G050655-00-D

Toward Enabling Co-Located Interferometric Detectors to Provide Upper Limits on the Stochastic Gravitational Wave

Background

Nick Fotopoulos, MITOn Behalf of the LIGO Scientific Collaboration

2005-12-15GWDAW-10 @ UT Brownsville

2LIGO-G050655-00-D

Co-Located Interferometers: Overlap

Reduction Function

€

SNR ≈3H0

2

10π 2T

γ 2 f( )ΩGW2 f( )

f 6P1 f( )P2 f( )−∞

∞

∫ df ⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

1/ 2

H1-H2 promises significantly enhanced sensitivity over H1-L1, especially at higher frequencies

BUTCo-located detectors are subject to environmental correlation

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H1-H2 Coherence (S4)

N = 1098211/N ≈ 9.1·10-6

Very coherent in the instrument’s “sweet spot”.

Coh

eren

ce s

quar

ed

f (Hz)

S1-S4 measurements of eff not consistent with zero

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S1 Results

Interferometer Pair

effh2 effh2 /

H1-H2 -8.3 0.95 -8.8

H1-L1 32 18 1.8

H2-L1 0.16 18 0.0094

S1

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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

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Tracking Environmental Coupling in H1-H2

S4 had 107

PEM channels in RDS_R_L1

S5 will have roughly the same

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Maximum of PEM Coherence Products: Frequency Veto

1/N2

Threshold10-5.5

•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

Vetoed regions

(H1-H2 1/N ~ 10-5,PEM-IFO 1/N ~ 10-4

due to resolution choices)

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Success!

We are one step closer to setting upper limits with the H1-H2 pair!

coh(H1,H2) post-vetohistogram exp(-N2)

•This (unreviewed) pipeline results in noticeably reducedsignificance for the point estimate•We have physical basis for veto

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Detector Characterization

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

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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…

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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.