Existence of a Lower Mass Gap Between Neutron Stars and Black Holes

Establish whether a mass gap exists between the most massive neutron stars and the least massive stellar-mass black holes, by characterizing the true mass distribution of compact objects across the 2.5–5 solar mass range using gravitational-wave and electromagnetic observations.

Background

The purported lower mass gap has been inferred from historical Galactic X-ray binary measurements, but subsequent gravitational-wave detections, including GW190814 and GW230529, suggest compact objects can inhabit this interval.

Despite recent evidence, population-level inference across multiple observing channels has not conclusively determined whether a genuine paucity exists or whether earlier observations were biased. Establishing the presence or absence of a true gap has direct implications for supernova remnant mass distributions, compact-object formation channels, and binary evolution models.

References

Overall, the existence of a mass gap between the most massive neutron stars and least massive black holes still stands as an open question in astrophysics.

— Observation of Gravitational Waves from the Coalescence of a $2.5\text{-}4.5~M_\odot$ Compact Object and a Neutron Star  (2404.04248 - Collaboration et al., 2024) in Section 8: Astrophysical Implications

It remains uncertain whether SN explosions alone can produce mass-gap compact objects.

— A Census of Stellar-mass Black Holes in the Milky Way with POPKIN. I. Isolated Black Holes  (2609.29211 - He et al., 24 Sep 2026) in Section 1.1, “Masses”

However, we leave it to future studies to determine the impact such a sub-population of binaries may affect the derived merger rates using the methodology presented here.

— Empirical determination of the Galactic neutron star--black hole merger rate using StarTrack models  (2609.35723 - Jacques et al., 28 Sep 2026) in Discussion, paragraph beginning “We also comment that this exploratory study does not distinguish…”

Threshold sensitivity does not by itself establish whether the underlying structure is astrophysical: a genuine low-contrast feature may fall below a catalogue cut, while apparent weak structure may also arise from finite-catalogue fluctuations. Additional data or simulation-based investigations will therefore be needed to determine how robustly these structures can be resolved.

— A weakly modelled view of the joint compact-binary mass plane: population structure and spectral-siren cosmology  (2610.10535 - Renzis et al., 7 Oct 2026) in Section 3.2, paragraph 'Emergent intermediate-mass structure'; Section 7, conclusion