The gravitational wave (GW) event S250206dm, as the first well-localized neutron star 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-meter Wide Field Survey Telescope (WFST), covering about 64% of the localization region up to a 5-sigma limiting magnitude of 23 mag. In total, 12 potential candidates have been identified while none of them are likely related to S250206dm. This non-detection provides the most stringent constraint to date on any associated kilonova. Crucially, an AT 2017gfo-like event at 269 Mpc can be excluded by WFST observations alone. Based on ejecta mass limits, a neutron star-black hole with a large mass ratio (Q >= 3.2) is disfavored. This optical-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 events 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.
@article{arxiv.2602.17267,
title = {Illuminating the Mass Gap Through Deep Optical Constraint on a Neutron Star Merger Candidate S250206dm},
author = {Zhengyan Liu and Zelin Xu and Ji-an Jiang and Wen Zhao and Zhiping Jin and Zigao Dai and Dezheng Meng and Xuefeng Wu and Daming Wei and Runduo Liang and Lei He and Minxuan Cai and Lulu Fan and Weiyu Wu and Junhan Zhao and Ziqing Jia and Kexin Yu and Jinjun Geng and Di Xiao and Feng Li and Jinlong Tang and Yingxi Zuo and Xiaoling Zhang and Hao Liu and Jian Wang and Hongfei Zhang and Ming Liang and Hairen Wang and Dazhi Yao and Lei Hu and Xu Kong and Bin Li and Ning Jiang and Tinggui Wang and Zhen Wan and Yongquan Xue and Qingfeng Zhu and Xianzhong Zheng},
journal= {arXiv preprint arXiv:2602.17267},
year = {2026}
}