Surprisingly Similar: The Mass Function of Gaia Neutron Stars and Firstborn Double Neutron Stars

Abstract The mass distribution of neutron stars (NSs) encodes information about their formation and binary evolution. We compare the masses of two distinct populations: (I) the recently identified Gaia NSs in wide orbits with solar-like companions and (II) the assumed firstborn recycled pulsar in Galactic double NS systems. Naively, one would expect their masses to differ due to both the presumed differences in their evolutionary histories, as well as astrophysical selection effects that can filter out configurations that would merge or be disrupted. Yet we find that their mass distributions are strikingly similar. Using a two-component Gaussian model, we find that both populations exhibit a narrow component centred near 1.3 M ⊙ , accompanied by a broader, higher-mass component that extends the distribution toward larger masses. The highest density regions of their fitted parameter posteriors coincide by over 91.6%. Statistical tests further confirm the agreement between these distributions with a Jensen–Shannon divergence JS < 0.08 and an Earth mover’s distance of W < 0.063 M ⊙ at 90% credibility. This finding seems to imply that both mass functions reflect the natal mass distribution of firstborn NSs in binary systems, supporting the hypothesis that NSs can be born with high masses. Consequently and perhaps surprisingly, binary evolutionary processes need not impart features on the NS mass distribution.