The Black Hole Mass Distribution in the Galaxy

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.

• The paper reports a narrow Gaussian mass distribution centered at 7.8 ± 1.2 M☉ based on dynamical measurements of 16 black holes in transient low-mass X-ray binaries.
• The paper identifies a striking absence of black holes in the 2–5 M☉ range, challenging the expectation of a continuous mass distribution.
• The paper examines selection effects and contrasts its findings with theoretical models, prompting a reevaluation of supernova explosion and binary evolution theories.

Insights into the Black Hole Mass Distribution in the Galaxy

The paper "The Black Hole Mass Distribution in the Galaxy" presents a comprehensive analysis of the mass distribution of stellar-mass black holes within the Galaxy. Utilizing dynamical mass measurements of 16 black holes observed in transient low-mass X-ray binaries, the study aims to elucidate the underlying mass distribution in the larger parent population of Galactic black holes. The investigation corroborates the presence of a restricted range in observed black hole masses, approximately centered around $7.8 \pm 1.2~M_\odot$, and notably identifies an absence of black holes within the $2-5~M_\odot$ mass range.

Key Findings

1. Narrow Mass Distribution: The study identifies a narrow mass distribution for black holes in the observed sample. The best fit for the distribution is a Gaussian centered at $7.8 \pm 1.2~M_\odot$. This observation aligns with previous limited studies suggesting a low-mass gap but improves upon them with a larger data set.
2. Absence of Low-Mass Black Holes: The study finds no evidence of black holes with masses between $2-5~M_\odot$, which contradicts basic theoretical expectations for a continuous distribution of black hole masses resulting from supernova explosions of varying progenitor stars.
3. Selection Effect Analysis: The research addresses possible selection effects stemming from an optical follow-up strategy that favors bright X-ray transient sources. While identified, this selection does not account for the observed lack of low-mass black holes.
4. Comparison with Theoretical Models: The observed minimal mass cutoff and rapid decrease in the number of higher mass black holes may point to the particular evolutionary trajectory of low-mass X-ray binaries, differing from pathways that lead to high-mass X-ray binary systems where more massive black holes have been detected.

Implications

The results have significant implications for the understanding of stellar evolution, supernova energetics, and black hole formation. The findings suggest that mass transfer dynamics in low-mass X-ray binaries might extensively affect the observed masses but not enough to form the observed gap. The constrained mass distribution challenges models that predict a more uniform distribution across the mass range, calling for a reevaluation of supernova explosion models and potential environmental influences on binary evolution.

Moreover, the apparent mass gap in black holes poses questions about the physics governing the endpoint of massive stellar evolution and the mechanisms of supernova explosions. Ongoing surveys and observations may help to elucidate whether abrupt changes in supernova explosion energy based on progenitor mass contribute to the mass distribution observed.

Speculation on Future Developments

Future work might include expanding the dataset of dynamically measured black holes, incorporating more advanced techniques like the dynamical Bowen method for persistent systems. Expanding transient surveys to include more potential candidates will also be critical. For gravitational wave astronomy, a refined understanding of these mass distributions is pivotal in improving rate calculations for black hole mergers, impacting both observational strategies and theoretical predictions significantly.

This investigation into the black hole mass distribution showcases the complexities of stellar evolution and the multifaceted nature of observational astrophysics. It underscores the need for continued advancements in observational techniques and theoretical modeling to further clarify the processes that lead to the formation of black holes.