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.