Guesswork in the gap: the impact of uncertainty in the compact binary population on source classification

TL;DR

This paper investigates how uncertainties in the compact binary population and measurement noise affect the classification of neutron stars versus black holes in gravitational wave observations, revealing significant variability in classification probabilities.

Contribution

It quantifies the impact of population models and measurement noise on neutron star probability estimates, highlighting the dominant factors influencing source classification.

Findings

P(NS) varies widely across events, from 1% to 67%.

Neutron star pairing preferences and spin distributions are key drivers.

EOS constraints influence P(NS) mainly through maximum neutron star mass.

Abstract

The nature of the compact objects within the supposed "lower mass gap" remains uncertain. Observations of GW190814 and GW230529 highlight the challenges gravitational waves face in distinguishing neutron stars from black holes. Interpreting these systems is especially difficult because classifications depend simultaneously on measurement noise, compact binary population models, and equation of state (EOS) constraints on the maximum neutron star mass. We analyze 66 confident events from GWTC-3 to quantify how the probability of a component being a neutron star, P(NS), varies across the population. The effects are substantial, the dominant drivers of classification are the pairing preferences of neutron stars with other compact objects, and the neutron star spin distributions. The data reveals that P(NS) varies between 1% - 67% for GW230529's primary and between 51% - 100% for GW190425's primary. By contrast, P(NS) for GW190814's secondary varies by <10%, demonstrating robustness from its high signal-to-noise ratio and small mass ratio. Analysis using EOS information tends to affect P(NS) through the inferred maximum neutron star mass rather than the maximum spin. As it stands, P(NS) remains sensitive to numerous population parameters, limiting its reliability and potentially leading to ambiguous classifications of future GW events.