Bridging the Gap: Categorizing Gravitational-Wave Events at the Transition Between Neutron Stars and Black Holes

Kavli Affiliate: Daniel E. Holz

| First 5 Authors: Amanda M. Farah, Maya Fishbach, Reed Essick, Daniel E. Holz, Shanika Galaudage

| Summary:

We search for features in the mass distribution of detected compact binary
coalescences which signify the transition between neutron stars and black
holes. We analyze all gravitational wave detections by LIGO-Virgo 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 gravitational
wave data. We confidently (99.3%) find a deviation from a single power law
describing both neutron stars and low-mass black holes at $2.4^{+0.5}_{-0.5}
M_{odot}$, which is consistent with many predictions for the maximum neutron
star mass. We find suggestions of the purported lower mass gap between the most
massive neutron stars and the least massive black holes, but are unable to
conclusively resolve it with current data. If it exists, we find the lower mass
gap’s edges to lie at $2.2^{+0.7}_{-0.5} M_{odot}$ and $6.0^{+2.4}_{-1.4}
M_{odot}$. We then re-examine events that have been deemed "exceptional" by
the LIGO-Virgo-KAGRA 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 mass to be either a NS or a
lower mass gap object, consistent with previous claims. Our models are the
first to be able to accommodate this event, which is an outlier with respect to
the binary black hole population. We find that the NSBH events GW200105 and
GW200115 probe the edges of, and may have components within, the lower mass
gap. As future data improves the global population models, the classification
of these events will also become more precise.

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