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 ΩGW < 6.0 × 10−8 for a
frequency-independent (flat) background and ΩGW < 4.8 × 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.

/pdf/search-for-the-isotropic-stochastic-background-using-data-qv8n2pqy4q.pdf

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

We report results of a search for an isotropic gravitational-wave background (GWB) using data from Advanced LIGO’s and Advanced Virgo’s third observing run (O3) combined with upper limits from the earlier O1 and O2 runs. Unlike in previous observing runs in the advanced detector era, we include Virgo in the search for the GWB. The results of the search are consistent with uncorrelated noise, and therefore we place upper limits on the strength of the GWB. We find that the dimensionless energy density 
Ω
GW
≤
5.8
×
10
−
9
 at the 95% credible level for a flat (frequency-independent) GWB, using a prior which is uniform in the log of the strength of the GWB, with 99% of the sensitivity coming from the band 20–76.6 Hz; 
Ω
GW
(
f
)
≤
3.4
×
10
−
9
 at 25 Hz for a power-law GWB with a spectral index of 
2
/
3
 (consistent with expectations for compact binary coalescences), in the band 20–90.6 Hz; and 
Ω
GW
(
f
)
≤
3.9
×
10
−
10
 at 25 Hz for a spectral index of 3, in the band 20–291.6 Hz. These upper limits improve over our previous results by a factor of 6.0 for a flat GWB, 8.8 for a spectral index of 
2
/
3
, and 13.1 for a spectral index of 3. We also search for a GWB arising from scalar and vector modes, which are predicted by alternative theories of gravity; we do not find evidence of these, and place upper limits on the strength of GWBs with these polarizations. We demonstrate that there is no evidence of correlated noise of magnetic origin by performing a Bayesian analysis that allows for the presence of both a GWB and an effective magnetic background arising from geophysical Schumann resonances. We compare our upper limits to a fiducial model for the GWB from the merger of compact binaries, updating the model to use the most recent data-driven population inference from the systems detected during O3a. Finally, we combine our results with observations of individual mergers and show that, at design sensitivity, this joint approach may yield stronger constraints on the merger rate of binary black holes at 
z
≳
2
 than can be achieved with individually resolved mergers alone.

/pdf/upper-limits-on-the-isotropic-gravitational-wave-background-470h2ahls0.pdf

Upper limits on the isotropic gravitational-wave background from Advanced LIGO and Advanced Virgo’s third observing run

The LIGO Scientific Collaboration and the Virgo Collaboration have cataloged eleven confidently detected gravitational-wave events during the first two observing runs of the advanced detector era. All eleven events were consistent with being from well-modeled mergers between compact stellar-mass objects: black holes or neutron stars. The data around the time of each of these events have been made publicly available through the gravitational-wave open science center. The entirety of the gravitational-wave strain data from the first and second observing runs have also now been made publicly available. There is considerable interest among the broad scientific community in understanding the data and methods used in the analyses. In this paper, we provide an overview of the detector noise properties and the data analysis techniques used to detect gravitational-wave signals and infer the source properties. We describe some of the checks that are performed to validate the analyses and results from the observations of gravitational-wave events. We also address concerns that have been raised about various properties of LIGO–Virgo detector noise and the correctness of our analyses as applied to the resulting data.

/pdf/a-guide-to-ligo-virgo-detector-noise-and-extraction-of-4wpytc4fom.pdf

A guide to LIGO–Virgo detector noise and extraction of transient gravitational-wave signals

The LIGO Scientific Collaboration and the Virgo Collaboration have cataloged eleven confidently detected gravitational-wave events during the first two observing runs of the advanced detector era. All eleven events were consistent with being from well-modeled mergers between compact stellar-mass objects: black holes or neutron stars. The data around the time of each of these events have been made publicly available through the gravitational-wave open science center. The entirety of the gravitational-wave strain data from the first and second observing runs have also now been made publicly available. There is considerable interest among the broad scientific community in understanding the data and methods used in the analyses. In this paper, we provide an overview of the detector noise properties and the data analysis techniques used to detect gravitational-wave signals and infer the source properties. We describe some of the checks that are performed to validate the analyses and results from the observations of gravitational-wave events. We also address concerns that have been raised about various properties of LIGO-Virgo detector noise and the correctness of our analyses as applied to the resulting data.

A guide to LIGO-Virgo detector noise and extraction of transient gravitational-wave signals

The Einstein Telescope (ET), the European project for a third-generation gravitational-wave detector, has a reference configuration based on a triangular shape consisting of three nested detectors with 10 km arms, where in each arm there is a `xylophone' configuration made of an interferometer tuned toward high frequencies, and an interferometer tuned toward low frequencies and working at cryogenic temperature. Here, we examine the scientific perspectives under possible variations of this reference design. We perform a detailed evaluation of the science case for a single triangular geometry observatory, and we compare it with the results obtained for a network of two L-shaped detectors (either parallel or misaligned) located in Europe, considering different choices of arm-length for both the triangle and the 2L geometries. We also study how the science output changes in the absence of the low-frequency instrument, both for the triangle and the 2L configurations. We examine a broad class of simple `metrics' that quantify the science output, related to compact binary coalescences, multi-messenger astronomy and stochastic backgrounds, and we then examine the impact of different detector designs on a more specific set of scientific objectives. 

Science with the Einstein Telescope: a comparison of different designs

Correlated magnetic noise from Schumann resonances threatens to contaminate the observation of a stochastic gravitational-wave background in interferometric detectors. In previous work, we reported on the first effort to eliminate global correlated noise from the Schumann resonances using Wiener filtering, demonstrating as much as a factor of two reduction in the coherence between magnetometers on different continents. In this work, we present results from dedicated magnetometer measurements at the Virgo and KAGRA sites, which are the first results for subtraction using data from gravitational-wave detector sites. We compare these measurements to a growing network of permanent magnetometer stations, including at the LIGO sites. We show the effect of mutual magnetometer attraction, arguing that magnetometers should be placed at least one meter from one another. In addition, for the first time, we show how dedicated measurements by magnetometers near to the interferometers can reduce coherence to a level consistent with uncorrelated noise, making a potential detection of a stochastic gravitational-wave background possible.

/pdf/measurement-and-subtraction-of-schumann-resonances-at-m92xa1tgth.pdf

Measurement and subtraction of Schumann resonances at gravitational-wave interferometers

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Measurement and subtraction of Schumann resonances at gravitational-wave interferometers