Bridging the Gap: Categorizing Gravitational-wave Events at the Transition between Neutron Stars and Black Holes

We search for features in the mass distribution of detected compact binary coalescences which signify the transition between neutron stars (NSs) and black holes (BHs). We analyze all gravitational-wave (GW) detections by the LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration (LVK) made through the end of the first half of the third observing run, and find clear evidence for two different populations of compact objects based solely on GW data. We confidently (99.3%) find a steepening relative to a single power law describing NSs and low-mass BHs below 2.4(-0.5)(+0.5)M(circle dot), which is consistent with many predictions for the maximum NS mass. We find suggestions of the purported lower mass gap between the most massive NSs and the least massive BHs, but are unable to conclusively resolve it with current data. If it exists, we find the lower mass gap's edges to lie at 2.2(-0.5)(+0.7)M(circle dot) and 6.0(-1.4)(+2.4)M(circle dot). We reexamine events that have been deemed exceptional by the LVK collaborations in the context of these features. We analyze GW190814 self-consistently in the context of the full population of compact binaries, finding support for its secondary to be either a NS or a lower mass gap object, consistent with previous claims. Our models are the first to accommodate this event, which is an outlier with respect to the binary BH population. We find that GW200105 and GW200115 probe the edges of, and may have components within, the lower mass gap. As future data improve global population models, the classification of these events will also improve.

Automated summary

We analyze gravitational wave data and find evidence for two populations of compact objects, indicating the transition between neutron stars and black holes. We also uncover hints of a lower mass gap between the most massive neutron stars and the least massive black holes. This research has significant implications for understanding the properties and evolution of compact objects.