Search for the isotropic stochastic background using data from Advanced LIGO's second observing run

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Search for the isotropic stochastic background using data from Advanced LIGO’s second observing run

B. P. Abbott et al. (LIGO Scientific and Virgo Collaboration)

Phys. Rev. D 100, 061101(R) – Published 4 September 2019

Abstract

The stochastic gravitational-wave background is a superposition of sources that are either too weak or too numerous to detect individually. In this study, we present the results from a cross-correlation analysis on data from Advanced LIGO’s second observing run (O2), which we combine with the results of the first observing run (O1). We do not find evidence for a stochastic background, so we place upper limits on the normalized energy density in gravitational waves at the 95% credible level of $Ω_{GW} < 6.0 \times 10^{- 8}$ for a frequency-independent (flat) background and $Ω_{GW} < 4.8 \times 10^{- 8}$ at 25 Hz for a background of compact binary coalescences. The upper limit improves over the O1 result by a factor of 2.8. Additionally, we place upper limits on the energy density in an isotropic background of scalar- and vector-polarized gravitational waves, and we discuss the implication of these results for models of compact binaries and cosmic string backgrounds. Finally, we present a conservative estimate of the correlated broadband noise due to the magnetic Schumann resonances in O2, based on magnetometer measurements at both the LIGO Hanford and LIGO Livingston observatories. We find that correlated noise is well below the O2 sensitivity.

Received 14 April 2019

DOI:https://doi.org/10.1103/PhysRevD.100.061101

Physics Subject Headings (PhySH)

Experimental studies of gravity Gravitational wave detection Gravitational waves

Gravitation, Cosmology & Astrophysics

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Vol. 100, Iss. 6 — 15 September 2019

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Images

Figure 1
The cross-correlation spectrum $\hat{C} (f)$ measured between Advanced LIGO’s Hanford and Livingston detectors during its second observing run. The estimator is normalized so that $⟨ \hat{C} (f) ⟩ = Ω_{GW} (f)$ for tensor-polarized gravitational waves. The black traces mark the $\pm 1 σ$ uncertainties on the measured cross-correlations. Coherent lines that were identified to have an instrumental cause have been removed from the spectrum. The loss in sensitivity visible at approximately 64 Hz is due to a zero in the tensor overlap reduction function $γ_{T} (f)$ .
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Figure 2
Posterior distribution for the amplitude $Ω_{ref}$ and slope $α$ of the stochastic background, using a prior which is uniform in the logarithm of $Ω_{ref}$ , along with contours with 68% and 95% confidence level, using combined O1 and O2 data. There is a small region of increased posterior probability centered around $\log Ω_{ref} = - 8$ and $α = 2$ . This is not statistically significant, and similar-size bumps have appeared in simulations of Gaussian noise. An analogous plot with a prior uniform in $Ω_{ref}$ can be found in the Supplemental Material [87].
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Figure 3
Sensitivity curves for O1, combined $O 1 + O 2$ , and design sensitivity. A power law stochastic background which lies tangent to one of these curves is detectable with $2 σ$ significance. We have used the Advanced LIGO design sensitivity given in Ref. [95], which incorporates improved measurements of coating thermal noise. Design sensitivity assumes that the LIGO noise curve is determined by fundamental noise sources only. The purple line is the median total stochastic background, combining BBHs and BNSs, using the model described in Ref. [59] with updated mass distributions and rates from Refs. [55, 91], and the gray box is the Poisson error region. The dotted gray line is the sum of the upper limit for the $BBH + BNS$ backgrounds with the upper limit on the NSBH background.
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Figure 4
Conservative estimate of correlated magnetic noise. We assume a conservative transfer function (TF) based on measurements as described in the text. The first Schumann resonance at 8 Hz is visible, and higher harmonics are below the noise floor. There is a zero of the overlap function at 64 Hz which leads to an apparent feature in $Ω_{mag}$ . Power line harmonics have been removed, as in the cross-correlation analysis. The two trend lines show power-law fits to the magnetometer spectra, scaled by the O1 (purple dotted) and end-of-O2 (blue dot-dashed) transfer functions. This demonstrates the effect of reducing the magnetic coupling in O2. The trend for the noise budget lies well below the solid black O2 PI curve, which indicates that correlated magnetic noise is negligible in O2. However, magnetic contamination may be an issue in future observing runs.
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