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.