A Search For Stellar-mass Black Holes Via Astrometric Microlensing II: 2012-2015 Keck Candidates

@misc{huston2026search,
  title = {{A Search For Stellar-mass Black Holes Via Astrometric Microlensing II: 2012-2015 Keck Candidates}},
  author = {Mace J. Huston and Jessica R. Lu and Casey Y. Lam and J. Nijaid Arredondo and Natasha S. Abrams and Shep Brooke and Sage H. Remulla and Eran Ofek and Michael S. Medford and Fatima Abdurrahman and Shrihan Agarwal and Edward Broadberry and Matthew Freeman and Matthew W. Hosek Jr. and Siyao Jia and Sean K. Terry and The OGLE Collaboration and Andrzej Udalski and Przemek Mroz and Radoslaw Poleski and Jan Skowron and Michal K. Szymanski and Igor Soszynski and Pawel Pietrukowicz and Szymon Kozlowski and Krzysztof Ulaczyk and Krzysztof A. Rybicki and Patryk Iwanek and Marcin Wrona and Mariusz Gromadzki and Mateusz J. Mroz and Michal Malkowski and The MOA Collaboration and Fumio Abe and David P. Bennett and Aparna Bhattacharya and Ian A. Bond and Kotaro Daimon and Ryusei Hamada and Yuki Hirao and Shuma Makida and Stela Ishitani Silva and Shota Miyazaki and Yasushi Muraki and Tutumi Nagai and Kansuke Nunota and Ryo Ogawa and Ryunosuke Oishi and Hideaki Ose and Greg Olmschenk and Clement Ranc and Nicholas J. Rattenbury and Yuki K. Satoh and Takahiro Sumi and Daisuke Suzuki and Takuto Tamaoki and Chihiro Ueda and Paul J. Tristram and Aikaterini Vandorou and Hibiki Yama},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.28420},
  eprint = {2609.28420},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.28420},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - A Search For Stellar-mass Black Holes Via Astrometric Microlensing II: 2012-2015 Keck Candidates
AU  - Mace J. Huston
AU  - Jessica R. Lu
AU  - Casey Y. Lam
AU  - J. Nijaid Arredondo
AU  - Natasha S. Abrams
AU  - Shep Brooke
AU  - Sage H. Remulla
AU  - Eran Ofek
AU  - Michael S. Medford
AU  - Fatima Abdurrahman
AU  - Shrihan Agarwal
AU  - Edward Broadberry
AU  - Matthew Freeman
AU  - Matthew W. Hosek Jr.
AU  - Siyao Jia
AU  - Sean K. Terry
AU  - The OGLE Collaboration
AU  - Andrzej Udalski
AU  - Przemek Mroz
AU  - Radoslaw Poleski
AU  - Jan Skowron
AU  - Michal K. Szymanski
AU  - Igor Soszynski
AU  - Pawel Pietrukowicz
AU  - Szymon Kozlowski
AU  - Krzysztof Ulaczyk
AU  - Krzysztof A. Rybicki
AU  - Patryk Iwanek
AU  - Marcin Wrona
AU  - Mariusz Gromadzki
AU  - Mateusz J. Mroz
AU  - Michal Malkowski
AU  - The MOA Collaboration
AU  - Fumio Abe
AU  - David P. Bennett
AU  - Aparna Bhattacharya
AU  - Ian A. Bond
AU  - Kotaro Daimon
AU  - Ryusei Hamada
AU  - Yuki Hirao
AU  - Shuma Makida
AU  - Stela Ishitani Silva
AU  - Shota Miyazaki
AU  - Yasushi Muraki
AU  - Tutumi Nagai
AU  - Kansuke Nunota
AU  - Ryo Ogawa
AU  - Ryunosuke Oishi
AU  - Hideaki Ose
AU  - Greg Olmschenk
AU  - Clement Ranc
AU  - Nicholas J. Rattenbury
AU  - Yuki K. Satoh
AU  - Takahiro Sumi
AU  - Daisuke Suzuki
AU  - Takuto Tamaoki
AU  - Chihiro Ueda
AU  - Paul J. Tristram
AU  - Aikaterini Vandorou
AU  - Hibiki Yama
PY  - 2026
DA  - 2026/09/23
PB  - arXiv
DO  - 10.48550/arxiv.2609.28420
UR  - https://arxiv.org/abs/2609.28420
AB  - The Milky Way is expected to host ∼10^8 stellar-mass black holes with an uncertain binary fraction. The only proven method to detect isolated stellar mass black holes is gravitational microlensing. Here we report the results of a microlensing search for black holes with photometry and astrometry. By combining 10 years of seeing-limited photometry from OGLE and MOA with diffraction-limited photometry and astrometry from adaptive optics imagers at the W.~M.~Keck Observatory, we constrain lens masses for OGLE-2012-BLG-0169, OGLE-2014-BLG-0613/MOA-2015-BLG-041, OGLE-2015-BLG-0029/MOA-2015-BLG-170, and OGLE-2015-BLG-0211. Of the four long-duration microlensing events monitored, we ruled out black hole lenses in 3 events, which likely have stellar or white dwarf lenses. OGLE-2015-BLG-0211 remains a black hole candidate with a poorly constrained lens mass with a 1σ upper mass limit of 3.2M_⊙ and a 3σ upper mass limit of 21.6M_⊙. This event suffered from poor observing conditions and significant astrometric reference frame uncertainties, but its analysis may benefit from additional astrometric data in the upcoming Gaia Data Release 4. Of the six long-timescale (t_E>100 days) microlensing events from this work and previous studies, one black hole has been confirmed with a second not ruled out. We briefly examine Galactic model simulations and find that our result agrees with current expectations. Ultimately, we need a larger sample of isolated black holes to constrain their formation processes. This will be possible in the coming years with Rubin and Roman, as well as improved astrometry from JWST and large, ground-based telescopes equipped with adaptive optics.
ER  -