Illuminating the Mass Gap through a Deep Optical Constraint on the Neutron Star Merger Candidate S250206dm

Abstract The gravitational wave (GW) event S250206dm, as the first well-localized neutron star (NS) merger candidate potentially located in the mass gap, presented a unique opportunity to probe the electromagnetic signatures from such a system. Here we report a deep, multiband search with the new 2.5 m Wide Field Survey Telescope (WFST), covering ∼64% of the localization region up to a 5 σ limiting magnitude of 23 mag. In total, 12 potential candidates have been identified, but none of them are likely related to S250206dm. This nondetection provides the most stringent constraint to date on any associated kilonova. Crucially, an AT 2017gfo-like event at 269 Mpc can be excluded only by WFST observations. Based on ejecta mass limits, a NS-black hole with a large mass ratio ( Q ≳ 3.2) is disfavored. This optically derived constraint on the mass ratio reaches, for the first time, a precision comparable to that inferred from the GW signal. This work presents the best observation of this type of event until now, and demonstrates the power of rapid, deep follow-up observations to constrain the properties of compact binary progenitors, offering key insights into the constituents of the mass gap.