Unresolved discrepancies in Gaia, BeXB, HMXB, and DNS observations

Determine how to account for the low-metallicity Gaia neutron-star binaries, the high-eccentricity Be X-ray binaries, the high-velocity high-mass X-ray binaries, and the double-neutron-star mass distribution that remain discrepant with the texttt{COMPAS} predictions.

Background

The paper’s preferred kick prescriptions reproduce many observed properties of neutron-star binaries but leave several populations or observables inadequately explained. The remaining issues include the large systemic velocities and halo-like orbits of low-metallicity Gaia neutron-star systems, the tight period–eccentricity relation of high-eccentricity Be X-ray binaries, high-velocity systems among the high-mass X-ray-binary sample, and tensions between the predicted and observed masses of Galactic double neutron stars.

The authors list possible explanations rather than resolving these discrepancies: formation in an accreted metal-poor dwarf galaxy, nonzero pre-supernova eccentricity or merger-in-a-triple evolution, dynamical ejection from clusters, and a different treatment of Case BB mass transfer. These alternatives are explicitly presented as unresolved questions in the paper.

References

Apart from these remaining open questions, the observed NS-harboring binaries can be relatively well explained by a kick model in which NSs that are formed with low-mass companions (i.e., Gaia NSs, LMXBs, and NSWDs) obtain kicks similar to young isolated NSs, and NSs that are formed with high-mass companions (i.e., HMXBs and DNSs) obtain significantly reduced natal kicks, providing a relatively consistent picture for the kick velocities of NSs in binary systems.

— The Kick Velocities of Neutron Stars in Binary Systems  (2608.19690 - Disberg et al., 20 Aug 2026) in Section 9, paragraph immediately preceding the final paragraph

Finally, regarding stellar evolution models, significant uncertainties remain in the treatment of binary interactions. The physics of CE evolution and the efficiency of SN engines are among the most critical unknowns. Furthermore, mass-transfer efficiency and angular momentum loss may be also important to consider.

— Linking high-mass X-ray binaries to binary compact object mergers in Milky Way-like galaxies  (2608.23945 - Cádiz et al., 25 Aug 2026) in Section 4, “Observational and Model Uncertainties”