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Mass measurements from low-mass black hole X-ray binaries (LMXBs) and radiopulsars 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 LMXBsare typically only possible for transient systems: outburst periods enabledetection via all-sky X-ray monitors, while quiescent periods enableradial-velocity measurements of the low-mass donor. We quantitatively studyselection biases due to the requirement of transient behavior for BH massmeasurements. Using rapid population synthesis simulations (COSMIC), detailedbinary stellar-evolution models (MESA), and the disk instability model oftransient behavior, we demonstrate that transient-LMXB selection effectsintroduce observational biases, and can suppress mass-gap BHs in the observedsample. However, we find a population of transient LMXBs with mass-gap BHs formthrough accretion-induced collapse of a NS during the LMXB phase, which isinconsistent with observations. These results are robust against variations ofbinary evolution prescriptions. The significance of this accretion-inducedcollapse population depends upon the maximum NS birth mass $M_\mathrm{ NS,birth-max}$. To reflect the observed dearth of low-mass BHs, COSMIC and MESAmodels favor $M_\mathrm{ NS, birth-max} \lesssim2M_{\odot}$. In the absence offurther observational biases against LMXBs with mass-gap BHs, our resultsindicate the need for additional physics connected to the modeling of LMXBformation and evolution.

Investigating the Lower Mass Gap with Low Mass X-ray Binary Population Synthesis