Surprisingly Similar: The Mass Function of Gaia Neutron Stars and First-Born Double Neutron Stars

TL;DR

This study tests whether the natal mass function of first-born neutron stars is universal across binary environments by comparing Gaia-detected NSs in wide binaries to the first-born recycled NSs in Galactic double neutron star systems. The authors fit a two-component Gaussian model to each population and quantify their similarity using Jensen–Shannon divergence and the Wasserstein distance, finding JS$\leq$0.08 and W$\leq$0.063 M$_\odot$ at 90% credibility. The inferred parameters for both populations show substantial overlap, with a narrow low-mass peak near $\sim 1.3$ M$_\odot$ and a higher-mass tail around $1.5$–$1.6$ M$_\odot$, suggesting a common natal origin rather than differential accretion histories. These results imply a potential universality in the birth masses of first-born NSs, reinforcing the view that binary evolution leaves minimal imprint on the mass distribution; upcoming Gaia data releases are expected to sharpen these conclusions.

Abstract

The mass distribution of neutron stars encodes information about their formation and binary evolution. We compare the masses of two distinct populations: I) the recently identified Gaia neutron stars in wide orbits with solar-like companions and, II) the assumed first-born recycled pulsar in Galactic double neutron star 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 $1.3 \text{ M}_\odot$, 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 $W <0.063 \text{ M}_\odot$ at 90% credibility. This finding seems to imply that both mass functions reflect the natal mass distribution of first-born neutron stars in binary systems, supporting the hypothesis that neutron stars can be born with high masses. Consequently and perhaps surprisingly, binary evolutionary processes need not impart features on the NS mass distribution.

Figures (5)

• Figure 1: A corner plot showing the posteriors of 10,000 samples of the double Gaussian model for each of the Gaia NS population (pink) and the recycled DNS population (blue). The numeric values shown are the median and $90\%$ credible limits (which are also provided in Table \ref{['t:priortable']}). The dotted lines indicate the respective median values and the banded regions show the respective HDIs. The two populations' posteriors overlap indicating their distributions are similar. The HDI for the mean of the second Gaussian, $\mu_2$, extends to larger masses for the Gaia population. The weight parameter, $\omega$, has low support at values of $\omega = 0,1$, indicating preference for a double Gaussian model.
• Figure 2: Left: Posterior predictive distributions of the double Gaussian model fit to the Gaia NSs (pink) and the recycled DNSs (blue). The bands shown depict the $90\%$ credible regions. The second Gaussian is necessary compared to a single Gaussian due to the tail towards high masses, but is not to be mistaken for a peak. Right: Cumulative posterior predictive distributions shown with data points overplotted.
• Figure A.1: The masses of NSs used in this work. The Gaia mass measurement range extends beyond the recycled DNSs, although we are subject to small-number statistics.
• Figure B.2: Left: Posterior predictive distributions for the combined dataset of the Gaia NS and recycled DNS populations. The bands show the 90% credible regions. The $1.3 \text{ M}_\odot$ peak is well defined and there is clear support for a long tail to masses $>1.5 \text{ M}_\odot$, which is beyond the masses accretion could have achieved. Right: Cumulative posterior predictive distributions shown with all combined data overplotted with errorbars.
• Figure B.3: A corner plot showing the posteriors of 10,000 samples of the double Gaussian model for the combination of the Gaia NS and recycled DNS populations. The numeric values shown are the median and 90% credible limits. The dotted lines indicate the median values. These posteriors have more precise ranges than the separate fits (see Figure \ref{['fig:corner']}). The weight parameter is constrained away from the single Gaussian values of $w=0,1$.