On Bimodality in the Eccentricity Distribution of Galactic Double Neutron Stars

Abstract

The detection of Galactic double neutron stars (DNSs) through pulsar timing offers a unique opportunity to probe massive stellar and binary evolution. The observed DNS population exhibits an apparently bimodal eccentricity distribution, with an absence of systems at measured intermediate eccentricities, $0.4 \lesssim e_{\rm m} \lesssim 0.58$, whose origin remains unclear. We propose that this possible gap can arise naturally if the relationship between the progenitor masses and neutron star (NS) masses is non-monotonic, provided that the second-born NS receives a sufficiently small natal kick. We illustrate this scenario using the population synthesis code COMPAS, and find that our DNS population model can reproduce the observed orbital period-eccentricity distribution relatively well, including the apparent bimodality. Although a larger observed sample is required to draw more robust conclusions, our results suggest that this model provides a natural pathway for explaining current observations of Galactic DNSs through isolated binary evolution.