Introduction

Black holes and neutron stars are among the most powerful phenomena in the universe. When these compact objects merge with other stars, matter falls from one component, the “donor” star, to the black hole or neutron star releasing gravitational potential energy in X-rays. The mass of a compact object plays a pivotal role in its detection and, in turn, our understanding of its formation process. We are particularly interested in low-mass black holes as their existence remains a mystery. Known as the lower mass gap, astronomers have noticed a discrepancy between theoretical models and observational data where the latter indicates a deficiency of compact objects whose mass lies between the maximum mass of a neutron star (∼ 2.5M⊙ ) and the minimum measured mass of a black hole (∼ 5M⊙ ) (Zevin et al., 2020). It’s worth noting that a black hole is generally known to emerge from a supernova explosion after a star’s death; nonetheless, our simulations show that low-mass black holes can also originate from heavy neutron stars that accrete mass from their donor stars during their evolutionary track. For this reason, scientists are even more perplexed by the existence of the lower mass gap.

This paper investigates two potential explanations to the lower mass gap mystery. The first is that there is an observation bias against low-mass black holes, hence, we cannot detect events whose components lie within the gap. The alternative explanation is that low-mass black holes and heavy neutron stars do not form and that’s why we do not see them. To determine which answer is more plausible, we need to recall how we detect black holes in the first place. As mentioned above, a black hole’s mass is the key factor; however, to measure its mass, the X-ray binary system must be transient (Farr et al., 2011). Namely, the binary has to undergo many fluctuations in its luminosity for us to be able to observe it. For this to occur, the mass transfer rate in the system must be below a critical value, which we determine from theoretical models (Dubus et al., 1999). On the other hand, if the binary is not transient, the mass transfer rate surpasses the critical value causing the system to be obscured by constantly high luminosities; as a result, we are not able to see the low-mass black holes even if they formed (observation bias).