Mass measurements from low-mass black hole X-ray binaries (LMXBs) and radio
pulsars have been used to identify a gap between the most massive neutron stars
(NSs) and the least massive black holes (BHs). BH mass measurements in LMXBs
are typically only possible for transient systems: outburst periods enable
detection via all-sky X-ray monitors, while quiescent periods enable
radial-velocity measurements of the low-mass donor. We quantitatively study
selection biases due to the requirement of transient behavior for BH mass
measurements. Using rapid population synthesis simulations (COSMIC), detailed
binary stellar-evolution models (MESA), and the disk instability model of
transient behavior, we demonstrate that transient-LMXB selection effects
introduce observational biases, and can suppress mass-gap BHs in the observed
sample. However, we find a population of transient LMXBs with mass-gap BHs form
through accretion-induced collapse of a NS during the LMXB phase, which is
inconsistent with observations. These results are robust against variations of
binary evolution prescriptions. The significance of this accretion-induced
collapse population depends upon the maximum NS birth mass MNS,birthβmaxβ. To reflect the observed dearth of low-mass BHs, COSMIC and MESA
models favor MNS,birthβmaxββ²2Mββ. In the absence of
further observational biases against LMXBs with mass-gap BHs, our results
indicate the need for additional physics connected to the modeling of LMXB
formation and evolution.Comment: 21 pages, accepted to Ap