A New Approach to Understanding the Lower Mass Gap in X-ray Binaries

The observed gap between the masses of black holes and neutron stars has been a mystery for decades in the field of astronomy. Observational data suggest a deficiency of compact objects with masses between the maximum mass of a neutron star (~ 2.5M_⊙) and the minimum measured mass of a black hole (~ 5M_⊙ ), but theoretical models have yet to fully explain these observations. Here we investigate two possible explanations for the lower mass gap. The first entails an observation bias against low-mass black holes, while the second assumes that low-mass black holes and heavy neutron stars do not form in the first place. To determine which scenario is more plausible, we use MESA (Modules for Experiments in Stellar Astrophysics) to extract binary mass transfer sequences for two types of systems: low-mass black holes formed from birth and accretion-induced collapse of heavy neutron stars. Comparing the accretion rates in the simulations to a theoretical critical accretion rate, below which no stable solution can exist, we show that all our binary models are transient: X-ray outbursts are followed by periods of quiescence allowing the compact object’s detection. Thus, we conclude that low-mass black holes do not form via stellar collapse.