The Black Hole Mass Distribution in the Galaxy

TL;DR

This study analyzes dynamical measurements of 16 black holes to determine their mass distribution, revealing a narrow peak around 7.8 solar masses and discussing implications for supernova physics and gravitational wave event rates.

Contribution

It provides the first detailed inference of the stellar black hole mass distribution from dynamical measurements in low-mass X-ray binaries, accounting for selection effects.

Findings

Black hole masses are narrowly distributed around 7.8 solar masses.

There is a notable absence of black holes in the 2-5 solar mass range.

The results impact predictions for gravitational wave detection rates.

Abstract

We use dynamical mass measurements of 16 black holes in transient low-mass X-ray binaries to infer the stellar black hole mass distribution in the parent population. We find that the observations are best described by a narrow mass distribution at 7.8 +/- 1.2 Msolar. We identify a selection effect related to the choice of targets for optical follow-ups that results in a flux-limited sample. We demonstrate, however, that this selection effect does not introduce a bias in the observed distribution and cannot explain the absence of black holes in the 2-5 solar mass range. On the high mass end, we argue that the rapid decline in the inferred distribution may be the result of the particular evolutionary channel followed by low-mass X-ray binaries. This is consistent with the presence of high-mass black holes in the persistent, high-mass X-ray binary sources. If the paucity of low-mass black holes is caused by a sudden decrease of the supernova explosion energy with increasing progenitor mass, this would have observable implications for ongoing transient surveys that target core-collapse supernovae. Our results also have significant implications for the calculation of event rates from the coalescence of black hole binaries for gravitational wave detectors.