Astronomers report an unseen companion in the system J0008+0128 that they detected through its gravitational effect on a red-giant star and that they say is likely a rare supermassive neutron star, 404 Media reported Wednesday, citing a study in Science Advances¹.

404 Media said the object's mass could be as high as about 2.8 times the Sun's, which the study said could place it in the so-called mass gap between typical neutron stars and black holes, often described as a scarcity of remnants between about 2.5 and five solar masses¹. The orbit lasts 1,218 days, which the report called unusually wide for how such objects are thought to gain mass¹.

Song Wang, an associate researcher at the National Astronomical Observatories of the Chinese Academy of Sciences and a co-author, told 404 Media the team searches spectroscopic surveys for compact objects and has found more than 10 neutron stars, with J0008+0128 the most massive among them¹. "The equation of state of neutron stars is a long-standing unsolved problem in astrophysics," Wang said¹.

404 Media said the team ran thousands of simulations that did not reproduce the system's physical and orbital properties¹. Standard mass transfer from the red giant is a poor fit at this separation, Wang said, and many recently found compact objects also sit outside standard binary-evolution models¹. Proposed alternatives include rare supernovae that form massive neutron stars, or a past triple system in which two objects merged¹.

The team used sky surveys including astrometry from Europe's Gaia spacecraft, 404 Media said, and Wang said a Gaia data release due next month may tighten the mass estimate¹. The report did not independently confirm the mass, the merger scenario, or whether the object is a neutron star rather than another compact remnant¹.