The Impact of Spin in Compact Binary Foreground Subtraction for Estimating the Residual Stochastic Gravitational-wave Background in Ground-based Detectors

TL;DR

This paper investigates how spin effects in compact binary coalescences impact the subtraction of foreground signals in ground-based gravitational wave detectors, revealing that spins significantly worsen residuals and affect SGWB detectability.

Contribution

It extends previous studies by incorporating spin parameters into the waveform model, showing that spins lead to more pessimistic residuals and highlighting the importance of extreme CBC events in foreground subtraction.

Findings

Residual energy density of foreground is larger with spins than without.

Spin degeneracy causes significant degradation in foreground subtraction.

Extreme CBC events with high condition numbers impact subtraction and SGWB detection.

Abstract

Stochastic gravitational-wave (GW) background (SGWB) contains information about the early Universe and astrophysical processes. The recent evidence of SGWB by pulsar timing arrays in the nanohertz band is a breakthrough in the GW astronomy. For ground-based GW detectors, while in data analysis, the SGWB can be masked by loud GW events from compact binary coalescences (CBCs). Assuming a next-generation ground-based GW detector network, we investigate the potential for detecting the astrophysical and cosmological SGWB with non-CBC origins by subtracting recovered foreground signals of loud CBC events. The Fisher Information Matrix (FIM) method is adopted for quick calculation. As an extension of the studies by Sachdev {\it et al.} (2020) and Zhou {\it et al.} (2023), two more essential features are considered. Firstly, we incorporate non-zero aligned or anti-aligned spin parameters in our waveform model. Because of the inclusion of spins, we obtain significantly more pessimistic results than the previous work, where the residual energy density of foreground is even larger than the original CBC foreground. For the most extreme case, we observe that the subtraction results are approximately 10 times worse for binary black hole events and 20 times worse for binary neutron star events than the scenarios without accounting for spins. The degeneracy between the spin parameters and the symmetric mass ratio is strong in the parameter estimation process, and it contributes most to the imperfect foreground subtraction. Secondly, in this work, extreme CBC events with condition numbers of FIMs $c_{\rm{\Gamma}}>10^{15}$ are preserved. The impacts of these extreme events on foreground subtraction are discussed. Our results have important implications for assessing the detectability of SGWB from non-CBC origins for ground-based GW detectors.