Failed supernovae as a natural explanation for the binary black hole mass distribution

Abstract

Context. As the number of detected gravitational wave sources increases, the better we can understand the mass distribution of binary black holes (BBHs). This “stellar graveyard” shows several features, including an apparent mass gap that makes the distribution bimodal. In turn, the observed chirp mass distribution appears to be trimodal. Aims. We aim to investigate the extent to which we can explain the observed mass distribution based on stellar evolution, specifically with the hypothesis that the mass gap is caused by the difference between successful and failed supernovae (SNe). Methods. We posed a hypothetical remnant function, based on the literature of stellar evolution simulations, which relates initial mass to remnant mass, while including a “black hole island” and producing a bimodal remnant distribution. Moreover, we looked at observed type II SN rates in an attempt to detect the effect of failed SNe. Finally, using a simplified estimation of binary evolution, we determined the remnant distribution resulting from our remnant function and compared it with observations. Results. We find that failed SNe lower type II SN rates by approximately 25%, but the inferred rate from SN surveys is not accurate enough to confirm this. Furthermore, our estimation based on the remnant function produces a mass distribution that matches the general shape of the observed distributions of individual as well as chirp masses. Conclusions. Based on our research, we conclude that the failed SN mechanism and the presence of the black hole island are a natural hypothesis for explaining the individual BBH mass distribution and chirp mass distribution. However, to obtain a firmer conclusion, more detailed simulations are needed.