Black Hole Mass Function of Coalescing Binary Black Hole Systems: Is There a Pulsational Pair Instability Mass Cutoff?

TL;DR

This study investigates the primary mass distribution of merging black holes from LIGO/Virgo data, finding a cutoff around 50 solar masses consistent with pulsational pair instability models, and exploring the origins of very massive black holes.

Contribution

It introduces two empirical mass functions and provides evidence for a mass cutoff near 50 solar masses, supporting pulsational pair instability theories.

Findings

Mass cutoff at ~50 M_sun supports pulsational pair instability.

Two empirical models fit the data well.

Very massive black holes may form via hierarchical mergers.

Abstract

We analyze the LIGO/Virgo GWTC-2 catalog to study the primary mass distribution of the merging black holes. We perform hierarchical Bayesian analysis, and examine whether the mass distribution has a sharp cutoff for primary black hole masses below $65 M_\odot$, as predicted in pulsational pair instability supernova model. We construct two empirical mass functions. One is a piece-wise function with two power-law segments jointed by a sudden drop. The other consists of a main truncated power-law component, a Gaussian component, and a third very massive component. Both models can reasonably fit the data and a sharp drop of the mass distribution is found at $\sim 50M_\odot$, suggesting that the majority of the observed black holes can be explained by the stellar evolution scenarios in which the pulsational pair-instability process takes place. On the other hand, the very massive sub-population, which accounts for at most several percents of the total, may be formed through hierarchical mergers or other processes.