For decades, electromagnetic observations of Galactic X-ray binaries revealed an apparent empty mass gap between the most massive neutron star (~2.2 M☉) and the lightest known black hole (~5 M☉). Whether this hiatus arose from supernova explosion physics (rapid vs. delayed fallback) or observational selection bias remained unresolved until the LIGO-Virgo-KAGRA gravitational wave network discovered compact objects squarely inside this gap: GW190814 (secondary mass 2.59 M☉) and GW230529 (primary mass 2.5–4.5 M☉), forcing revisions to core-collapse supernovae models.
| Parameter / Probe | Observational Value | Survey / Instrument Anchor | State |
|---|---|---|---|
| Maximum Stable Neutron Star (TOV) | M_TOV ≈ 2.14 – 2.30 M☉ | PSR J0740+6620 (NANOGrav / NICER) | Nuclear Physics Ceiling |
| GW190814 Secondary Mass | 2.59 (+0.08 / -0.09) M☉ | LIGO-Virgo O3 Run | First Definitive Mass Gap Object |
| GW230529 Primary Mass | 3.6 (+0.8 / -1.2) M☉ | LIGO Livingston O4 Run | Squarely Inside Lower Gap |
| Electromagnetic X-ray Binary Minimum BH | 4.9 ± 0.3 M☉ (GRO J0422+32) | Optical/NIR Radial Velocity Surveys | Classical Lower Boundary |
The maximum mass of a non-rotating cold neutron star is governed by the Tolman-Oppenheimer-Volkoff (TOV) equation and the equation of state (EoS) of supranuclear matter. When the central core density exceeds several times nuclear saturation density (ρ_0 ≈ 2.7 × 10¹⁴ g/cm³), the exact pressure-density relation is unknown due to uncertainties in quantum chromodynamics (QCD) at finite baryon density.
Relativistic Shapiro delay measurements of massive millisecond pulsars, notably PSR J0740+6620 (M = 2.08 ± 0.07 M☉) and PSR J0952-0607 (M = 2.35 ± 0.17 M☉), set an empirical lower bound on M_TOV: any viable nuclear EoS must support at least ~2.1 M☉. Beyond M_TOV, thermal and degeneracy pressure cannot resist gravitational collapse, and the object must collapse into a black hole.
Conversely, dynamical mass measurements of Galactic stellar-mass black holes in low-mass X-ray binaries consistently placed the lowest black hole mass around 5 M☉. This created a conspicuous ~2.2 to 5 M☉ "mass gap."
Theoretical astrophysicists debated whether this hiatus was real. In 2012, Fryer et al. demonstrated that supernova explosion engines play a decisive role: if the bounce shock is revived rapidly by neutrino heating (< 100–200 ms), the star explodes before substantial fallback can occur, creating a clean bimodal mass distribution. If the explosion is delayed (> 500 ms), fallback accretion occurs continuously, filling the gap.
The debate was transformed with the advent of gravitational wave astronomy. In August 2019, the LIGO and Virgo detectors recorded GW190814: the coalescence of a 23 M☉ black hole with a mysterious 2.6 M☉ compact object. At 2.59 M☉, the secondary was either the most massive neutron star ever observed (requiring an exceptionally stiff nuclear EoS and extreme differential rotation) or the lightest black hole ever discovered.
In May 2023, during the early days of the O4 observing run, LIGO Livingston detected GW230529: a merger involving a 1.2–2.0 M☉ neutron star and a 2.5–4.5 M☉ object. This detection demonstrated that compact objects in the mass gap not only exist, but merge at an estimated astrophysical rate of tens of events per cubic gigaparsec per year. The lower mass gap is not empty; it is a vital window into the terminal collapse of massive stars and the true equation of state of nuclear matter.