Upper Bound Set on Non-Stationary Gravitational Wave Background

This work presents the first spectral covariance analysis of the Stochastic Gravitational-Wave Background (SGWB) using data from the LIGO-Virgo-KAGRA collaboration during the third and the first part of the fourth observing runs. The analysis aims to constrain the properties of the underlying source population, specifically distinguishing between astrophysical black holes (ABHs) and primordial black holes (PBHs).

Conventional analyses based on the power spectrum are limited in constraining these properties under the assumption of a non-sporadic Gaussian distribution. However, recent studies suggest that SGWB is expected to be sporadic and non-Gaussian, giving rise to non-zero spectral correlation. The current analysis indicates that the spectral correlation is consistent with non-stationary noise, yielding no detection and providing only upper bounds over the frequency range of 20 Hz to 100 Hz.

This upper bound on the spectral correlation translates into a mass-distribution-dependent upper bound on the merger rate of PBHs. This provides a stringent upper bound on the PBH merger rate at high redshift and hence puts constraints on the PBH formation scenarios. Future detection of this signal will provide a new avenue to probe the high-redshift black hole population using gravitational waves.

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