Searching for stochastic gravitational-wave background with the co-located LIGO interferometers (0801.3429v1)

Abstract: This paper presents techniques developed by the LIGO Scientific Collaboration to search for the stochastic gravitational-wave background using the co-located pair of LIGO interferometers at Hanford, WA. We use correlations between interferometers and environment monitoring instruments, as well as time-shifts between two interferometers (described here for the first time) to identify correlated noise from non-gravitational sources. We veto particularly noisy frequency bands and assess the level of residual non-gravitational coupling that exists in the surviving data.

Overview of Techniques for Searching a Stochastic Gravitational-Wave Background with LIGO Interferometers

This paper presents a detailed analysis and discussion of approaches used by the LIGO Scientific Collaboration to search for the Stochastic Gravitational-Wave Background (SGWB). It focuses on using the co-located interferometers at the Hanford site, termed H1 and H2, to enhance the identification of stochastic signals amidst noise. Particularly, the paper elaborates on the cross-correlation analysis methodology and highlights two significant techniques developed to mitigate non-gravitational noise—the IFO-PEM coherence technique and the time-shift technique.

Cross-Correlation Methodology

The search for an isotropic component of the SGWB at LIGO primarily employs cross-correlation techniques. This involves analyzing the data streams from H1 and H2 interferometers to extract the common astrophysical signal while minimizing the correlated environmental noise. The mathematical models employed include an estimator $Y$ and its variance $\sigma_Y^2$, and the optimal filter $Q(f)$, which maximizes the signal-to-noise ratio. The paper details the utilization of overlap reduction functions $\gamma(f)$, accounting for the geometric and orientation factors of the detectors.

Noise Mitigation Techniques

1. IFO-PEM Coherence Technique: The IFO-PEM coherence method estimates the level of environmental interference by using multiple environmental sensors placed within the LIGO vicinity. This method correlates the environmental disturbances detected by these sensors with noise characteristics in the interferometers to isolate non-gravitational couplings.
2. Time-Shift Technique: The complementary time-shift technique involves artificially introducing time offsets between data streams of the H1 and H2 interferometers. It aims to negate broadband contributions that would disappear under phase-offset conditions exceeding the coherence time expected for gravitational signals.

Implications and Future Directions

The techniques outlined provide a substantial advancement in distinguishing true GW signals from environmental noise. Employing both techniques allows researchers to validate each other's results, offering a consolidated approach to segregating non-gravitational disturbances. The development and application of these strategies underscore critical attention to systemic errors and the systematic estimation of uncertainties tied to SGWB amplitude estimates.

The results discussed and the methodologies evolved lay the groundwork for further enhancing the detection sensitivity of SGWB analysis. Future research stands to benefit from refining these techniques, focusing on eliminating broad-spectrum noise sources effectively. A specific area of further exploration involves increasing environmental coverage and refining the separation process for non-linear coupling phenomena, potentially leading to greater precision in gravitational wave astronomy.

Overall, the paper makes a significant contribution to the sophisticated field of gravitational wave detection, providing expert insights into addressing prominent challenges such as noise isolation and signal clarity, vital for robust astronomical observations.