A Census of Stellar-mass Black Holes in the Milky Way with POPKIN. I. Isolated Black Holes
Abstract
Gravitational-wave observations have revealed hundreds of stellar-mass black holes, yet only about two dozen are known in the Milky Way, almost all in binaries. We present POPKIN, a Python framework that couples single- and binary-star evolution with Galactic orbital dynamics to trace black-hole progenitors from the zero-age main sequence to the present-day isolated black-hole (IBH) population. Across ten models varying the supernova (SN) prescription, mass-transfer efficiency, and common-envelope ejection efficiency, the total IBH abundance is controlled primarily by the SN prescription. Our fiducial model, with a recently proposed metallicity- and stripping-history-dependent SN prescription, predicts ∼4×107 IBHs in the Galaxy, including ∼8×104 within 1 kpc of the Sun; alternative SN prescriptions predict ∼(1−2)×108 IBHs. The fiducial model yields a bimodal mass distribution, peaking near 9 M⊙ and 20 M⊙, with a deficit at 13−17 M⊙. This distinguishes it from alternative prescriptions, some of which populate the 2−5 M⊙ mass-gap region. Non-kicked IBHs follow nearly circular orbits near the Galactic plane, with typical peculiar velocities of 20−30 km s−1, whereas kicked systems undergo stronger radial migration and span a broader velocity range. We estimate ∼5×103 accreting IBHs with FX>10−14 erg s−1 cm−2, nearly all non-kicked; this estimate is sensitive to the adopted radiative-efficiency and hot-flow treatments. For a Roman-like bulge survey, our fiducial model predicts ∼360 intrinsic IBH microlensing events over five years in a 1.70 deg2 effective area, before survey-selection effects. We propose that long-timescale microlensing events from IBHs can strongly constrain the SN physics governing stellar-mass black hole formation.
Related
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BibTeX
@misc{he2026census,
title = {{A Census of Stellar-mass Black Holes in the Milky Way with POPKIN. I. Isolated Black Holes}},
author = {Jian-Guo He and Yong Shao and Yu-Dong Nie and Gui-Yu Wang and Xiao-Tian Xu and Chen Wang and Xiao-Jie Xu and Xiang-Dong Li},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.29211},
eprint = {2609.29211},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.29211},
}
RIS
TY - UNPB
TI - A Census of Stellar-mass Black Holes in the Milky Way with POPKIN. I. Isolated Black Holes
AU - Jian-Guo He
AU - Yong Shao
AU - Yu-Dong Nie
AU - Gui-Yu Wang
AU - Xiao-Tian Xu
AU - Chen Wang
AU - Xiao-Jie Xu
AU - Xiang-Dong Li
PY - 2026
DA - 2026/09/24
PB - arXiv
DO - 10.48550/arxiv.2609.29211
UR - https://arxiv.org/abs/2609.29211
AB - Gravitational-wave observations have revealed hundreds of stellar-mass black holes, yet only about two dozen are known in the Milky Way, almost all in binaries. We present POPKIN, a Python framework that couples single- and binary-star evolution with Galactic orbital dynamics to trace black-hole progenitors from the zero-age main sequence to the present-day isolated black-hole (IBH) population. Across ten models varying the supernova (SN) prescription, mass-transfer efficiency, and common-envelope ejection efficiency, the total IBH abundance is controlled primarily by the SN prescription. Our fiducial model, with a recently proposed metallicity- and stripping-history-dependent SN prescription, predicts ∼4×10^7 IBHs in the Galaxy, including ∼8×10^4 within 1 kpc of the Sun; alternative SN prescriptions predict ∼(1−2)×10^8 IBHs. The fiducial model yields a bimodal mass distribution, peaking near 9 M_⊙ and 20 M_⊙, with a deficit at 13−17 M_⊙. This distinguishes it from alternative prescriptions, some of which populate the 2−5 M_⊙ mass-gap region. Non-kicked IBHs follow nearly circular orbits near the Galactic plane, with typical peculiar velocities of 20−30 km s^−1, whereas kicked systems undergo stronger radial migration and span a broader velocity range. We estimate ∼5×10^3 accreting IBHs with F_X>10^−14 erg s^−1 cm^−2, nearly all non-kicked; this estimate is sensitive to the adopted radiative-efficiency and hot-flow treatments. For a Roman-like bulge survey, our fiducial model predicts ∼360 intrinsic IBH microlensing events over five years in a 1.70 deg^2 effective area, before survey-selection effects. We propose that long-timescale microlensing events from IBHs can strongly constrain the SN physics governing stellar-mass black hole formation.
ER -