Unmodelled Detection of Overlapping Gravitational-Wave Signals with the Einstein Telescope

Stijn Heyde, Milan Wils, Francesco Cireddu, Tjonnie Li and Marc Moonen

KU Leuven, Leuven, Belgium

Abstract. The third generation of gravitational-wave detectors, including the Einstein Telescope, will detect hundreds of thousands of compact-binary coalescences per year, so that a rare Galactic core-collapse supernova would arrive in a record likely already convoluted by binary inspirals. Coherent burst pipelines assume a single source per trigger and blend such overlapping signals. This work treats the overlap as a multi-source detection and direction-of-arrival problem on a small, heterogeneous detector network. We first quantify the temporal and time-frequency overlap probability for a realistic one-year inspiral population. For an inspiral cutoff frequency of 3 Hz, overlap is highly likely. We then restructure an existing modular burst pipeline so that every stage carries a list of source hypotheses, and extend the coherent beamforming statistic into a joint maximum-likelihood estimator for two simultaneous sources, with a null-stream criterion that determines how many sources are present per time-frequency bin. For a network consisting of the Einstein Telescope plus Cosmic Explorer plus LIGO-India the method achieves single-digit-degree localisation for single sources and recovers all pairwise two-source combinations, consistently outperforming a strict per-pixel single-source assignment.

Keywords: burst detection, coherent network analysis, gravitational waves, null stream, overlapping signals, sky localisation