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

TL;DR

This study used deep optical observations to constrain the properties of a neutron star merger candidate in the mass gap, providing the most stringent limits to date and demonstrating the effectiveness of rapid follow-up in understanding compact binary systems.

Contribution

The paper presents the first optical constraints on a neutron star merger in the mass gap, achieving precision comparable to gravitational wave data and ruling out certain progenitor models.

Findings

No electromagnetic counterpart was detected for S250206dm.

An AT 2017gfo-like kilonova at 269 Mpc is excluded by WFST observations.

Constraints disfavor a neutron star-black hole system with a large mass ratio (Q >= 3.2).

Abstract

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.