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Implications for PBH Dark Matter from a single Sub-Solar$\unicode{x2013}$GW Detection in LVK O1$\unicode{x2013}$O4

Alberto Magaraggia, Nico Cappelluti

Abstract

The detection of sub-solar mass black holes is a milestone of modern astrophysics as it would open a window either onto new stellar physics or could potentially unveil the nature of Dark Matter as Primordial Black Holes. On November 12, 2025, the LIGO-Virgo-KAGRA (LVK) collaboration reported the compact binary merger candidate S251112cm, a system with no obvious EM counterpart, consistent with binary black hole merger with a chirp mass in the range $0.1-0.87 \, M_\odot$. The probability that at least one component has mass $<$1 $M_{\odot}$ is $>99\%$. Inspired by this trigger, we tested if a population of PBHs formed at Quantum Chromodynamics epoch with a broad mass function could account for a signal of this type. Our results, corresponding to a predicted event rate of $0.8 \,\text{yr}^{-1}$ as seen by LVK O3b, suggest that the observed merger rate of $0.23^{+0.86}_{-0.218}\,\text{yr}^{-1}\;(95\%\;\text{C.L.})$ if the trigger is confirmed as an astrophysical event would be compatible with such a model. Our predicted detection rate is also in agreement with current LVK expectations for stellar-mass binaries, remaining consistent with a scenario in which a non-negligible fraction of the $3-200 \;M_\odot$ mergers observed by LVK originate from Primordial Black Holes. If confirmed, this detection would place a lower limit to the PBH abundance $f_{PBH}>0.04$ for our adopted model.

Implications for PBH Dark Matter from a single Sub-Solar$\unicode{x2013}$GW Detection in LVK O1$\unicode{x2013}$O4

Abstract

The detection of sub-solar mass black holes is a milestone of modern astrophysics as it would open a window either onto new stellar physics or could potentially unveil the nature of Dark Matter as Primordial Black Holes. On November 12, 2025, the LIGO-Virgo-KAGRA (LVK) collaboration reported the compact binary merger candidate S251112cm, a system with no obvious EM counterpart, consistent with binary black hole merger with a chirp mass in the range . The probability that at least one component has mass 1 is . Inspired by this trigger, we tested if a population of PBHs formed at Quantum Chromodynamics epoch with a broad mass function could account for a signal of this type. Our results, corresponding to a predicted event rate of as seen by LVK O3b, suggest that the observed merger rate of if the trigger is confirmed as an astrophysical event would be compatible with such a model. Our predicted detection rate is also in agreement with current LVK expectations for stellar-mass binaries, remaining consistent with a scenario in which a non-negligible fraction of the mergers observed by LVK originate from Primordial Black Holes. If confirmed, this detection would place a lower limit to the PBH abundance for our adopted model.
Paper Structure (9 sections, 9 equations, 2 figures)

This paper contains 9 sections, 9 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Extended PBH mass function
  3. Constraints on Primordial Black Holes
  4. Merger rate estimate
  5. Formation of PBH binaries
  6. Late PBH binaries merger rate density
  7. Detected event rate
  8. Discussion
  9. Summary

Figures (2)

  • Figure 1: Left panel: Mass function expressed as $d f_{\rm PBH}/d\ln M$. The orange dashed line shows the distribution from Hasinger2020, while the blue solid line represents the same model modified by Bodeker2021-hz to account for lepton--flavour asymmetries. The three black arrows highlight the main changes introduced by these asymmetries. Right panel: rough comparison between the abundances $f_{\rm PBH}$ associated with the two mass functions and the current monochromatic constraints (red: CMB-accretion limits, green: microlensing bounds, purple: gravitational-wave limits).
  • Figure 2: Left panel: region in the $(M_1,M_2)$ plane satisfying $0.1 \leq \mathcal{M}_c(M_1,M_2) \leq 0.87$. Central panel: subset of this region further restricted to mass ratios $0.05 \leq q(M_1,M_2) \leq 1$. Right panel: expected detectable merger rate across the resulting allowed parameter space. Note that in this figure we did not enforce $M_1 > M_2$; however, when computing the total rate $R_{\rm tot}$ we summed only the contributions with $M_1 > M_2$, i.e. over the half--plane corresponding to ordered pairs.