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

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.

Figures (10)

• Figure 1: Skymap coverage and multiband depths of WFST on the first night for S250206dm. Panel (a): the total coverage by WFST for S250206dm, where the Bilby skymap of S250206dm from 2025GCN.39231....1L is adopted, with 50% and 90% probability region contours. Galactic extinction is also considered, with the contour of $E(B-V)=0.3$ for reference Schlafly_2011. Panel (b): The probability coverage by the WFST as a function of limiting absolute magnitudes in $r$, $i$, and $z$ bands. Panel (c), (d), and (e): The zoomed-in covered regions by WFST in $r$, $i$, and $z$ bands, respectively, corresponding to the region delineated by the dashed line in Panel (a). The color map represents the 5$\sigma$ limiting absolute magnitudes after stacking, converted based on the distance estimate for each WFST pointing.
• Figure 2: Skymap coverage of WFST observations for S250206dm after the first night. The Bilby skymap of S250206dm from 2025GCN.39231....1L is adopted, with 50% and 90% probability region contours shown. Galactic foreground is shown as a contour of $E(B-V)=0.3$Schlafly_2011.
• Figure 3: Lightcurves of the six WFST candidates tagged by "Too bright" or "Slow evolution". The luminosity distance of a 2017gfo-like KN is set as the median of 373 Mpc.
• Figure 4: The model-dependent space of peak magnitude and variability in $i$ band. Panel (a) and (b): The KN model POSSIS is adopted for simulating KNe in the BNS and NSBH mergers, respectively. The black dots and red pentacles represent the six candidates tagged by "Too bright" or "Slow evolution" and AT 2017gfo, respectively. To derive the absolute magnitude, the luminosity distance is set as 373 Mpc for the candidates with no spectroscopic redshift available. Three contour lines correspond to 50%, 90%, 99% percentiles of KN model regions.
• Figure 5: Parameters constraints based on the non-detection result, assuming a BNS (left) or a NSBH merger (right) for S250206dm. Two top panels show constraints on KN luminosity derived using the POSSIS model at different distances (corresponding to the median and $\pm1\sigma$ distances from Bilby skymap). The 5$\sigma$ limiting magnitude of each WFST pointing and the median magnitudes are plotted with open and solid inverted triangles, respectively. Galactic extinction with a median $E(B-V) = 0.1$ is applied in all panels. Light/deep blue and gray regions represent excluded and allowed KN scenarios, respectively. For the BNS merger, black dashed and dash-dot lines represent AT 2017gfo-like KNe generated using POSSIS at 269 and 373 Mpc, respectively. Two bottom panels show constraints on ejecta mass. Three viewing angles of $15^\circ$, $45^\circ$ and $75^\circ$ are adopted. The fitted ejecta masses of AT 2017gfo from Dietrich_2020 are plotted as a red star for the BNS merger.