A team of astronomers searched for any light that might come from a recent gravitational-wave signal called S251112cm. The signal was reported by the LIGO–Virgo–KAGRA (LVK) network and, according to the LVK, almost certainly included at least one compact object lighter than the Sun. The search turned up 248 candidate transient sources reported by the community, including 67 from the Vera C. Rubin Observatory, but the team found no convincing electromagnetic counterpart.

S251112cm was reported with a false alarm rate of one in 6.2 years and a luminosity distance of 93 ± 27 megaparsecs. The LVK assigned a 100% probability that the system contains a subsolar-mass object (HasSSM = 1.0) and only an 8% chance that it contains a 1–3 solar mass object (the mass range the LVK uses to define a neutron star). The best-fit “chirp mass” of the system falls between 0.10 and 0.87 solar masses. These numbers mean that, if the signal is astrophysical, at least one object in the merging pair would be lighter than known neutron stars or black holes, and forming such objects would require new or speculative astrophysical models.

To look for light from this event, the researchers built a reproducible framework to vet and score candidate counterparts. They considered many types of possible electromagnetic signals, including kilonovae (brief optical/infrared flashes from neutron-rich ejecta), kilonovae embedded in supernovae, unusually bright “super-kilonovae,” and flares associated with active galactic nuclei. The team used a suite of telescopes to tile the sky, target likely host galaxies, and follow promising candidates with photometry and spectroscopy. They also ingested near-real-time candidate reports from other groups, including early Rubin/LSST observations.