Comparison of maximum likelihood mapping methods for gravitational-wave backgrounds

TL;DR

This paper compares maximum likelihood intensity mapping and amplitude-phase mapping methods for detecting anisotropic gravitational-wave backgrounds, concluding intensity mapping is more suitable for realistic signals.

Contribution

It provides a comparative analysis of two mapping methods, demonstrating the limitations of amplitude-phase mapping for typical gravitational-wave backgrounds.

Findings

Amplitude-phase mapping is only effective for phase-coherent backgrounds.

Intensity mapping is more robust for realistic, incoherent gravitational-wave signals.

The study guides future gravitational-wave background searches towards intensity mapping.

Abstract

Detection of a stochastic background of gravitational waves is likely to occur in the next few years. Beyond searches for the isotropic component of SGWBs, there have been various mapping methods proposed to target anisotropic backgrounds. Some of these methods have been applied to data taken by the Laser Interferometer Gravitational-wave Observatories (LIGO) and Virgo. Specifically, these directional searches have focused on mapping the intensity of the signal on the sky via maximum likelihood solutions. We compare this intensity mapping approach to a previously proposed, but never employed, amplitude-phase mapping method to understand whether this latter approach may be employed in future searches. We build up our understanding of the differences between these two approaches by analysing simple toy models of time-stream data, and run mock-data mapping tests for the two methods. We find that the amplitude-phase method is only applicable to the case of a background which is phase-coherent on large scales or, at the very least, has an intrinsic coherence scale that is larger than that of the detector. Otherwise, the amplitude-phase mapping method leads to a loss of overall information, with respect to both phase and amplitude. Since we do not expect these phase-coherent properties to hold for any of the gravitational-wave background signals we hope to detect in the near future, we conclude that intensity mapping is the preferred method for such backgrounds.