The boundary between the most massive neutron stars and the least massive black holes remains elusive. Electromagnetic surveys detect almost no compact objects with masses from 2 to 5 M⊙ — the lower mass gap. However, gravitational-wave signals recorded by the LIGO-Virgo-KAGRA collaboration suggest the gap may not be empty. Swiftly identifying a merger candidate with a component in this gap helps gauge the chance of an electromagnetic afterglow and organize follow-up observations. To this end, the neural network model GWSkyNet-MassGap was trained to output simultaneously the probabilities P_MassGap (component in the gap) and P_NS (neutron star involvement). The model extracts chirp mass information: for massive systems with chirp mass ≳15 M⊙, predictions are accurate; for lighter ones, accuracy drops because the mass ratio is needed to break degeneracy. On data from the first half of the fourth observing run (O4a), mean errors were 9% for P_MassGap and 6% for P_NS. In the future, the model could be extended to estimate chirp mass on the fly for candidates.
When black holes or neutron stars merge, they strum space, broadcasting gravitational waves—a chirp predicted by Einstein. Detectors like LIGO capture this cosmic ringtone, its pitch encoding the masses. Yet, there's a silent interval in the cosmic scale: objects from 2 to 5 Suns are almost never heard. This lower mass gap lies between the heaviest neutron stars and lightest black holes.
GWSkyNet-MassGap, a neural network, listens to the gravitational wave chirp and judges if a merger includes a neutron star or a gap object—like naming instruments in an orchestra. It's accurate for loud signals but can mishear the mass split in faint ones. If neutron stars collide, telescopes catch a kilonova forging gold; black hole mergers stay dark. Sifting gravitational wave catalogs, it spotlights rare candidates. The gap may hide surprises—perhaps quark-matter ghosts that are neither star nor hole.
🎯 A sugar-cube-sized lump of [tag:neutron_star]neutron star[/tag] matter would outweigh Mount Everest, making it the universe's densest known material outside a black hole.