Discovery of a likely supermassive neutron star in a remarkably wide binary.

A wide, low-eccentricity red-giant binary with an invisible companion is linked to a compact object candidate with a best-fit mass of [Formula: see text].

• Why it matters: Supermassive neutron stars constrain the dense-matter equation of state, but systems that could host them are rare and challenging to model, especially when orbits are wide and nearly circular.
• What they did: The team combined radial-velocity data from LAMOST and APOGEE with Gaia DR2/DR3 astrometry to fit a Keplerian orbit, then used spectral disentangling of high-resolution near-infrared APOGEE spectra to search for extra absorption.
• The result: The companion’s mass spans [Formula: see text] with no additional spectral features detected, implying either a supermassive neutron star or a low-mass black hole and motivating improved binary-evolution theory.

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