Identity Crisis: Untangling Mass-Ordering Artifacts from Compact-Object Pairing in Gravitational-Wave Populations
Abstract
Mass distributions of binary black hole mergers inferred from gravitational-wave observations offer clues to how binaries form. These distributions conventionally label the heavier component as the primary, with mass m1, and the lighter as the secondary, with mass m2, such that m1 ≥ m2. This ordering folds the physically labeled mass distribution along the equal-mass line, potentially reshaping the underlying population structure. In this Letter, we determine which population features can arise from mass ordering and which require genuine astrophysical structure. We show that ordering alone can produce (1) an m2 distribution that declines more rapidly than the m1 distribution, without intrinsic differences between the physically labeled components, and (2) an apparent concentration along equal masses, without correlations that preferentially pair black holes of equal mass. Moreover, different physically labeled populations can produce the same ordered mass distribution. Crucially, however, a preferred companion mass that remains constant as m1 increases requires a characteristic mass scale already present in the physically labeled population. Applied to binary black hole mergers in the fifth Gravitational-Wave Transient Catalog, our model recovers such a scale near m2 ≃ 50 M⊙. Around this scale, the merger-rate density declines much more rapidly toward higher m2 than toward lower m2, ruling out an equal or weaker high-m2 decline at 99.45% credibility. This structure therefore requires a characteristic mass scale in at least one physically labeled component, but masses alone cannot determine which component carries it. Any formation model reproducing the observed high-mass binary black hole merger population must contain such a scale.
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BibTeX
@misc{roy2026identity,
title = {{Identity Crisis: Untangling Mass-Ordering Artifacts from Compact-Object Pairing in Gravitational-Wave Populations}},
author = {Soumendra Kishore Roy and Asad Hussain and L. A. C. van Son and Maya Fishbach and Carl L. Rodriguez and Will M. Farr},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.18962},
eprint = {2610.18962},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.18962},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Identity Crisis: Untangling Mass-Ordering Artifacts from Compact-Object Pairing in Gravitational-Wave Populations
AU - Soumendra Kishore Roy
AU - Asad Hussain
AU - L. A. C. van Son
AU - Maya Fishbach
AU - Carl L. Rodriguez
AU - Will M. Farr
PY - 2026
DA - 2026/10/13
PB - arXiv
DO - 10.48550/arxiv.2610.18962
UR - https://arxiv.org/abs/2610.18962
AB - Mass distributions of binary black hole mergers inferred from gravitational-wave observations offer clues to how binaries form. These distributions conventionally label the heavier component as the primary, with mass m_1, and the lighter as the secondary, with mass m_2, such that m_1 ≥ m_2. This ordering folds the physically labeled mass distribution along the equal-mass line, potentially reshaping the underlying population structure. In this Letter, we determine which population features can arise from mass ordering and which require genuine astrophysical structure. We show that ordering alone can produce (1) an m_2 distribution that declines more rapidly than the m_1 distribution, without intrinsic differences between the physically labeled components, and (2) an apparent concentration along equal masses, without correlations that preferentially pair black holes of equal mass. Moreover, different physically labeled populations can produce the same ordered mass distribution. Crucially, however, a preferred companion mass that remains constant as m_1 increases requires a characteristic mass scale already present in the physically labeled population. Applied to binary black hole mergers in the fifth Gravitational-Wave Transient Catalog, our model recovers such a scale near m_2 ≃ 50 M_⊙. Around this scale, the merger-rate density declines much more rapidly toward higher m_2 than toward lower m_2, ruling out an equal or weaker high-m_2 decline at 99.45% credibility. This structure therefore requires a characteristic mass scale in at least one physically labeled component, but masses alone cannot determine which component carries it. Any formation model reproducing the observed high-mass binary black hole merger population must contain such a scale.
ER -