Black hole astrometric binaries in the Roman Galactic Bulge Time Domain Survey

Abstract

The Nancy Grace Roman Space Telescope (Roman), NASA's next flagship mission, is currently scheduled to launch in August 2026. As part of its mission, Roman will conduct the Galactic Bulge Time Domain Survey (GBTDS), which will generate ∼50,000 epochs of high-precision photometric and astrometric data for ∼ 10⁸ sources across 1.7 deg² in the Galactic Bulge. Roman GBTDS astrometry is comparable to Gaia Data Release 4 in terms of number of stars, astrometric precision, and time baseline, and is highly complementary in terms of wavelength and sky location. In this paper, we investigate a synthetic population of GBTDS sources to characterize the detectability of unresolved astrometric binaries, in particular those with compact object companions. Assuming the occurrence rate of black holes (BHs) and neutron stars (NSs) in AU-scale orbits around stars is 10⁻⁷ and 10⁻⁶, respectively, and that Roman achieves an astrometric precision of 1% of a pixel, 𝒪(10) BH+star and 𝒪(10) NS+star detached binaries will be detectable. The BHs will have median mass measurement uncertainties of ∼ 25%, increasing the existing sample of detached astrometric BH binaries by a factor of three. Together with the 𝒪(10²) isolated BHs expected to be discovered by microlensing in the Roman GBTDS and an additional 𝒪(10) detached BH binaries in Gaia DR4, this will provide a representative view of the quiescent Galactic stellar-mass BH population.

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Cite

BibTeX RIS

BibTeX

@misc{lam2026black,
  title = {{Black hole astrometric binaries in the Roman Galactic Bulge Time Domain Survey}},
  author = {Casey Y. Lam and Jessica R. Lu and Kareem El-Badry and Anil Seth and Joshua D. Simon and Natasha S. Abrams and Andrea Bellini and Matthew W. Hosek Jr. and Peter McGill},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2608.24998},
  eprint = {2608.24998},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2608.24998},
  note = {CC BY-NC-SA 4.0},
}

RIS

TY  - UNPB
TI  - Black hole astrometric binaries in the Roman Galactic Bulge Time Domain Survey
AU  - Casey Y. Lam
AU  - Jessica R. Lu
AU  - Kareem El-Badry
AU  - Anil Seth
AU  - Joshua D. Simon
AU  - Natasha S. Abrams
AU  - Andrea Bellini
AU  - Matthew W. Hosek Jr.
AU  - Peter McGill
PY  - 2026
DA  - 2026/09/30
PB  - arXiv
DO  - 10.48550/arxiv.2608.24998
UR  - https://arxiv.org/abs/2608.24998
AB  - The Nancy Grace Roman Space Telescope (Roman), NASA's next flagship mission, is currently scheduled to launch in August 2026. As part of its mission, Roman will conduct the Galactic Bulge Time Domain Survey (GBTDS), which will generate ∼50,000 epochs of high-precision photometric and astrometric data for ∼ 10^8 sources across 1.7 deg^2 in the Galactic Bulge. Roman GBTDS astrometry is comparable to Gaia Data Release 4 in terms of number of stars, astrometric precision, and time baseline, and is highly complementary in terms of wavelength and sky location. In this paper, we investigate a synthetic population of GBTDS sources to characterize the detectability of unresolved astrometric binaries, in particular those with compact object companions. Assuming the occurrence rate of black holes (BHs) and neutron stars (NSs) in AU-scale orbits around stars is 10^−7 and 10^−6, respectively, and that Roman achieves an astrometric precision of 1% of a pixel, 𝒪(10) BH+star and 𝒪(10) NS+star detached binaries will be detectable. The BHs will have median mass measurement uncertainties of ∼ 25%, increasing the existing sample of detached astrometric BH binaries by a factor of three. Together with the 𝒪(10^2) isolated BHs expected to be discovered by microlensing in the Roman GBTDS and an additional 𝒪(10) detached BH binaries in Gaia DR4, this will provide a representative view of the quiescent Galactic stellar-mass BH population.
ER  -

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