Observing binary neutron star subpopulations with the Einstein Telescope

TL;DR

This paper explores how the Einstein Telescope can identify and characterize different subpopulations of binary neutron stars, revealing their formation channels and evolution through hierarchical Bayesian analysis of simulated data.

Contribution

It demonstrates that next-generation gravitational wave detectors can distinguish multiple BNS subpopulations and analyze their redshift evolution, advancing understanding of their formation mechanisms.

Findings

The total mass distribution of BNSs is bimodal with hundreds of detections.

A few thousand events allow disentangling redshift distributions for certain delay models.

Detection of multiple BNS subpopulations provides insights into their formation pathways.

Abstract

The formation channels of binary neutron stars (BNSs) remain uncertain. The detection of GW190425 by LIGO/Virgo/KAGRA (LVK) suggests a subpopulation of massive BNSs, possibly formed through unstable "case BB" mass transfer with short merger delays. We investigate whether next-generation detectors such as the Einstein Telescope (ET) can identify and characterise such subpopulations. Using the latest LVK constraints on the BNS merger rate, we generate mock ET catalogues containing a mixture of light and heavy subpopulations. The redshift distribution of each subpopulation is modeled as the convolution of the cosmic star formation rate with a time-delay distribution: heavy BNSs have fixed short delays, while light BNSs follow power-law delays with indices -0.5,-1,-1.5. Hierarchical Bayesian analyses are then performed on catalogues of 100-5,000 events. With hundreds of detections from ET, we will be able to establish that the total mass distribution is bimodal. A few thousand events are sufficient to disentangle the redshift distributions of the two subpopulations for moderate time-delay indices (-0.5 or -1). For steeper indices (-1.5), the differences are more subtle, requiring larger catalogues, beyond what we could explore given our computational resources. Next-generation detectors should enable the detection of multiple BNS subpopulations and their redshift evolution, providing valuable insight into their formation pathways.