A crowded peak, then a possible gap: the black-hole masses in gravitational-wave mergers

Gravitational waves let astronomers infer the masses of black holes as two of them spiral together and merge. Across the observed population, one pattern has drawn attention: mergers with a primary black hole near 10 times the Sun’s mass are common, while the inferred rate falls sharply toward about 15 solar masses. Could that peak and dip be clues to how the stars that made these black holes died?

Isaac Legred, Jacob Golomb and Katerina Chatziioannou investigate one possibility: a narrow range of stars may be especially likely to collapse directly into black holes rather than produce successful supernova explosions. The authors call the resulting objects a “failed-supernova” population. This is a proposed explanation, not something the gravitational-wave data directly reveal about any individual progenitor.

Building a population model

The researchers analyze 153 binary-black-hole observations in the GWTC-4.0 catalog, using statistical models to infer the underlying distribution of merger rates. This matters because detectors do not observe every merger equally well: a population analysis must account for how detection sensitivity varies with the sources’ properties.

Their main approach represents the primary-mass distribution as a mixture. One component is a narrow, truncated Gaussian peak intended to capture the proposed failed-supernova group. A second, higher-mass component is described in two alternative ways: as a power law with an additional peak, or as a broken power law. The models allow the two groups to have separate distributions of mass ratios and effective spins, and allow the higher-mass group to start above the low-mass peak. That flexibility lets the analysis test for a gap instead of requiring the higher-mass distribution to continue smoothly through the interval.

The team also checks how their conclusions respond to modeling choices, including an alternative treatment of how the two component masses are paired. Such choices are important: the existence of a gap in primary masses does not automatically imply a gap in secondary masses, and assumptions connecting the two can change what the model permits.

A peak is clear; a gap is less certain

Both main models recover a pronounced primary-mass peak centered near 10 solar masses. In one model, the inferred peak rate density is about 7.36 mergers per solar mass per year per cubic gigaparsec, with substantial uncertainty. The inferred rate then drops by roughly two orders of magnitude between about 10 and 14 solar masses.

For primary masses from 12.0 to 16.1 solar masses, the rate is consistent with being effectively zero at the study’s 90% credibility level under one of the models. “Effectively zero” has a specific, practical meaning here: at the estimated sensitivity of the observations, the rate is so low that the absence of detections in that range would not be surprising. It does not mean the authors proved that no mergers occur there.

The gap is favored by much of the model’s posterior, but it is not required by the data at the study’s stated confidence threshold. The exact inferred interval also shifts between the two high-mass models. The authors report that the data favor a separation between the peak and the higher-mass population, while leaving room for a nonzero rate in the intervening region.

The paper also reports that the low-mass group has a narrower effective-spin distribution than the higher-mass population in its main analysis. That difference is consistent with the groups having different properties, but it does not by itself identify their formation history. The authors note that the mass-ratio inference is affected by two low-mass events, and that their results depend in part on how populations and component masses are modeled.

What the pattern can—and cannot—tell us

The proposed stellar explanation has theoretical motivation: simulations and models discussed in the paper suggest that some progenitor stars may be unusually prone to failed explosions, depending on more than core compactness alone. The authors argue that a distinct channel could contribute to the 10-solar-mass peak and the sparse region above it. But their study fits the observed merger population; it does not simulate the full chain from stellar evolution through binary formation to merger. The mass pattern is therefore consistent with the failed-supernova idea, not proof of it.

The authors say their headline results persist in a later catalog, GWTC-5.0, which they analyze as a check. That strengthens the case that the pattern is not simply a feature of one catalog, but does not remove uncertainty about model assumptions or astrophysical interpretation.

This article is based on an arXiv preprint by Legred, Golomb and Chatziioannou. A preprint is a publicly shared research manuscript and may not have undergone peer review; the supplied source does not establish its publication status. The safest takeaway is narrower than “astronomers found a black-hole mass gap”: the 10-solar-mass peak is prominent in this analysis, and a deep dip nearby is plausible, but its emptiness and physical origin remain open questions.