Discovery of a likely supermassive neutron star in a remarkably wide binary
Supermassive neutron stars ( M ≳ 2 M ⊙ ) provide stringent constraints on the equation of state of dense matter. We report the discovery of a wide binary system composed of a red giant star and an invisible companion with an orbital period P ≈ 1218 ± 3 days and a comparatively low eccentricity e ≈ 0.09 ± 0.01 (with 1σ uncertainty). By combining radial velocity measurements from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope and the Apache Point Observatory Galactic Evolution Experiment (APOGEE) with astrometric data from Gaia DR2 and DR3 catalogs, we derive a Keplerian orbit with an inclination angle i ≈ 73 ∘ ± 11 ∘ . The unseen companion has a mass in the range of 1.8 to 2.8 M ⊙ (spanning the full spread of all estimates), with a best-fit value of 2.20 − 0.13 + 0.14 M ⊙ . This potentially places the unseen companion within the mass gap between neutron stars and black holes. Spectral disentangling using high-resolution near-infrared APOGEE spectra shows no absorption features from an additional component, suggesting the presence of a compact object: either a supermassive neutron star or a low-mass black hole. The wide, low-eccentricity orbit is difficult to explain through standard binary evolution channels, pointing to the need for improved theoretical models to account for such systems.
Authors
- Guiling Xiao (ORCID: https://orcid.org/0000-0001-6753-4611)
- Roberto Soria (ORCID: https://orcid.org/0000-0002-4622-796X)
- Yang Huang (ORCID: https://orcid.org/0000-0003-3250-2876)
- Xiao‐Hong Yang (ORCID: https://orcid.org/0000-0002-2419-9590)
- Dan Qiu (ORCID: https://orcid.org/0000-0002-8280-4808)
- Fan Feng (ORCID: https://orcid.org/0000-0001-6039-0555)
- Song Wang (ORCID: https://orcid.org/0000-0003-3116-5038)
- Kyle A. Rocha (ORCID: https://orcid.org/0000-0003-4474-6528)
- Maosheng Xiang (ORCID: https://orcid.org/0000-0002-5818-8769)
- Xue Li (ORCID: https://orcid.org/0009-0007-4501-4376)
- P F Wang (ORCID: https://orcid.org/0000-0002-6437-0487)
- Meng Zhang (ORCID: https://orcid.org/0000-0001-9293-131X)
- Jifeng Liu (ORCID: https://orcid.org/0000-0002-2874-2706)
- Hong-Liang Yan (ORCID: https://orcid.org/0000-0002-8609-3599)
- Xuan Mao (ORCID: https://orcid.org/0009-0009-8313-1842)
- Xinlin Zhao (ORCID: https://orcid.org/0009-0005-5459-7433)
- Xihong Luo
- Shuai Liu
- Meng Sun
- Seth Goggage
Institutions
- Northwestern University (US)
- The University of Sydney (AU)
- Chongqing University (CN)
- Shanghai Jiao Tong University (CN)
- Chinese Academy of Sciences (CN)
- Peking University (CN)
- Beijing Normal University (CN)
- Osservatorio Astrofisico di Torino (IT)
- Osservatorio Astronomico di Palermo (IT)
- National Astronomical Observatories (CN)
- University of Chinese Academy of Sciences (CN)
- NSF-Simons AI Institute for the Sky (US)
- National Institute for Astrophysics (IT)
Publication Details
- Journal
- Science Advances
- Published
- 2026-11-11
- DOI
- https://doi.org/10.1126/sciadv.aeg3307
- Primary Topic
- Stellar, planetary, and galactic studies
- Type
- article
- Field-Weighted Citation Impact
- 0.00